2020-10-21T12:00:40Zhttp://edoc.mpg.de/ac_p_oai.ploai:edoc.mpg.de:108862006-11-1433:1309
Stationary and Transient Divertor Heat Flux Profiles and Extrapolation to ITER
Herrmann, A.
Eich, T.
Jachmich, S.
Laux, M.
Andrew, P.
Bergmann, A.
Loarte, A.
Matthews, G.
Neuhauser, J.
ASDEX Upgrade Team
Contributors to EFDA-JET Work Programme
Experimental results on divertor heat load measurements from ASDEX Upgrade and JET are discussed. Thereby three topics are considered: (i) parameter dependence of steady state heat flux profiles, (ii) spatial distribution of the heat flux profile during type I edge localised modes (ELMs), and (iii) temporal evolution of the energy deposition during type I ELMs. No clear scaling of steady state heat flux profiles with plasma parameters is found. For different data sets a broadening of the heat flux profiles, a constant profile width, as well as a steepening with heating power is found. Extrapolation to ITER requires a review of the data. The heat flux profile is not significantly broadened during type I ELMs. Advantageous is the change of the in/out symmetry. The temporal behaviour of the energy deposition shows a strong increase of the heat flux on time scales of the ion sound speed and an exponential decay with about twice the rise time.
2003
Article
http://edoc.mpg.de/10886
Journal of Nuclear Materials, v.313-316, 759-767 (2003)
en
oai:edoc.mpg.de:155782006-11-1433:1309
"Quasi-local" wave equations in toroidal geometry with applications to fast wave propagation and absorption at high harmonics of the ion cyclotron frequency
Brambilla, M.
The integral constitutive relation for high frequency waves propagating in toroidal axisymmetric plasmas, obtained by formal integration of the linearized Vlasov equation, is simplified assuming the range of spatial dispersion to be small compared to the linear dimensions of the plasma. We propose to call this the 'quasi-local approximation'. A (formally infinite) system of purely differential wave equations is obtained, which should be a good approximation under conditions similar to those which would justify an Eikonal Ansatz for the form of the wave fields. This system is valid to all orders in the Larmor radius, and, in the presence of a poloidal static magnetic field, predicts a different plasma response to each poloidal Fourier component of the h.f. field. Compared to ray tracing based on the Eikonal approximation, these. wave equations have the advantage of allowing to take into account periodicity, boundary conditions, and toroidicity-induced coupling between poloidal Fourier modes. As an example, the quasi-local wave equations are used to model. propagation and absorption of the compressional wave at frequencies higher than the ion cyclotron frequency in the high-P plasma of the National Spherical Tokamak Experiment in Princeton, USA. Because of the low magnetic field and the tight aspect ratio of this device, large Larmor radius effects and toroidicity play an important role in these experiments. This example, therefore, illustrates well the importance of taking into account these effects, and, in particular, the different response of the plasma to each poloidal Fourier mode.
2002
Article
http://edoc.mpg.de/15578
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 2423-2443 (2002)
en
oai:edoc.mpg.de:155812007-05-1633:1309
Electron cyclotron radiative transfer in fusion plasmas
Albajar, F.
Bornatici, M.
Engelmann, F.
The radiative transfer in a system at local thermodynamic equilibrium is investigated on the basis of the solution of the (geometrical optics) transfer equation, accounting for the non- local nature of the radiative process due to both re-absorption of the emitted radiation and reflectivity of the walls of the system. The specific case of the electron cyclotron (EC) radiation in a cylindrical fusion plasma with specularly reflecting walls for which an analytical solution can be derived, is addressed and, in particular, the radial profile of the net power density radiated is evaluated by making use of an improved expression for the EC absorption coefficient. A detailed numerical analysis, carried out by varying both the wall reflection coefficient and the radial profile of the plasma temperature. reveals that a reversal of the net power density profile can occur on the plasma outboard for sufficiently high wall reflectivity. From a comparison with bremsstrahlung radiation profiles it is apparent that a local treatment of EC power emission is needed for sufficiently hot plasmas as expected, e.g. in the so-called advanced regimes of DT tokamak reactors. Furthermore, an exact approach is used to check the accuracy of approximate EC net power density profiles as calculated with the CYTRAN code showing that the latter provides a globally reasonable approximation. Evaluating the total EC radiated power from the exact local approach shows that its scaling with the reflection coefficient is very well described by a scaling following from a recently established global model for the EC radiation. which improves the well- known Trubnikov scaling. The results obtained are discussed in view of their possible relevance to affecting the plasma temperature profile.
2002
Article
http://edoc.mpg.de/15581
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 670-678 (2002)
en
oai:edoc.mpg.de:155832006-11-2333:1309
Chaos in 1D radiative edge Plasmas
Bachmann, P.
Bifurcation and chaos in radiative edge plasmas are investigated on the basis of a periodically driven reaction- diffusion equation which results from the time dependent 1d heat conduction equation including a given periodically time- modulated impurity density. The temporal problem shows the transition to chaos through the Feigenbaum route. In Id and time dependent, plasmas Hopf bifurcation and intermittency phenomena exist. The application to a carbon seeded plasma shows the existence of different oscillation regimes.
2002
Article
http://edoc.mpg.de/15583
urn:ISSN:0863-1042
Contributions to Plasma Physics, v.42, 425-430 (2002)
en
oai:edoc.mpg.de:155852006-11-1433:1309
Bifurcation of temperature and anomalous transport in the boundary region of Wendelstein 7-X
Bachmann, P.
Kisslinger, J.
Sünder, D.
Wobig, H.
This paper discusses some problems of the island divertor in the Wendelstein 7-X configuration. These islands exist on the iota = 1 surface at the plasma boundary and will be utilized for impurity control in the Wendelstein 7-X experiment. The structure of this island region depends on the plasma pressure and the tendency is to become more and more ergodic with rising plasma pressure. Thermal transport in the divertor region is described by the transport equation, which is inherently three- dimensional. By averaging along the helical geometry of the island, this equation can be reduced to a two-dimensional one describing the temperature distribution in the poloidal plane. In this approximation a strong similarity to tokamak geometry exists. Since the plasma currents modify the islands, a finite- beta equilibrium is computed as the basis of a geometric analysis of divertor action, Anomalous transport strongly affects the width of the scrape-off layer and the width of the wetted area on the divertor target plates; therefore it is investigated how anomalous transport modifies the transport equation. The non-linearity of the radiation losses in the divertor region and the non-linearity of the boundary conditions can lead to a bifurcation of the temperature distribution and to multiple solutions. Some numerical examples of one-dimensional temperature profiles and bifurcated solutions are given.
2002
Article
http://edoc.mpg.de/15585
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 83-102 (2002)
en
oai:edoc.mpg.de:155862006-11-1533:1310
Bifurcation of temperature and anomalous transport in the boundary region of Wendelstein 7-X
Bachmann, P.
Kisslinger, J.
Sünder, D.
Wobig, H.
European Physical Society
2002
Conference-Paper
http://edoc.mpg.de/15586
urn:ISSN:0741-3335
Behn, R.; Varandas, C.: 29th European Physical Society Conference on Plasma Physics and Controlled Fusion. Contributed Papers, European Physical Society (2002)
en
oai:edoc.mpg.de:155882006-11-1433:1309
Spatial distribution of turbulence in the Wendelstein 7-AS stellarator
Basse, N. P.
Michaelsen, P. K.
Zoletnik, S.
Saffman, M.
Endler, M.
Hirsch, M.
In this paper measurements of short wavelength electron density fluctuations using collective scattering of infrared light are presented. The Wendelstein 7-AS (W7-AS) stellarator (Renner H et al 1989 Plasma Phys. Control. Fusion 31 1579) and the diagnostic are briefly described. A series of plasma discharges with reproducible confinement transitions was created by ramping up the plasma current. Utilizing the fact that the density fluctuation wavenumber kappa is anisotropic in the directions parallel and perpendicular to the local magnetic field, the diagnostic can provide a radial profile of the turbulence during both normal and degraded confinement. The found profiles display an increase of core turbulence for the reduced confinement state. The results are discussed and compared to similar tokamak measurements.
2002
Article
http://edoc.mpg.de/15588
urn:ISSN:0963-0252
Plasma Sources Science & Technology, v.11, A138-A142 (2002)
en
oai:edoc.mpg.de:155892006-11-1433:1309
Turbulence at the transition to the high density H-mode in Wendelstein 7-AS plasmas
Basse, N. P.
Zoletnik, S.
Bäumel, S.
Endler, M.
Hirsch, M.
McCormick, K.
Werner, A.
Recently a new improved confinement regime was found in the Wendelstein 7-AS (W7-AS) stellarator (Renner H. et al 1989 Plasma Phys. Control. Fusion 31 1579). The discovery of this high density high confinement mode (HDH-mode) was facilitated by the installation of divertor modules. In this paper, measurements of short wavelength density fluctuations in the HDH-mode using collective scattering of infrared light are presented. These measurements will be contrasted to fluctuations during normal confinement operation (NC-mode). The autopower spectra of the measurements show a consistent increase of the fluctuation level associated with the transition from NC- to HDH-mode. Correlation calculations on a 20 mus timescale between magnetic and density fluctuations lead to the result that the fluctuations are correlated in NC- but not in HDH-mode. Finally, a comparative analysis between the enhanced D-alpha H-mode (EDA H-mode) found in the Alcator C-Mod tokamak and the HDH-mode in W7-AS is carried out.
2003
Article
http://edoc.mpg.de/15589
urn:ISSN:0029-5515
Nuclear Fusion, v.43, 40-48 (2003)
en
oai:edoc.mpg.de:155902006-11-1433:1309
Low- and high-mode separation of short wavelength turbulence in dithering Wendelstein 7-AS plasmas
Basse, N. P.
Zoletnik, S.
Saffman, M.
Baldzuhn, J.
Endler, M.
Hirsch, M.
Knauer, J. P.
Kühner, G.
McCormick, K.
Werner, A.
In this article measurements of small scale electron density fluctuations in dithering high confinement (H)-mode plasmas obtained by collective scattering of infrared light are presented. A scan of the fluctuation wavenumber was made in a series of similar discharges in the Wendelstein 7-AS (W7-AS) stellarator [H. Renner , Plasma Phys. Control. Fusion 31, 1579 (1989)]. The experimental setup and discharge properties are described. H-alpha-light observing an inner limiter was used to separate low confinement (L)- and H-mode phases of the plasma; the separated density fluctuations are characterized. It was found that L- (H-) mode fluctuations dominate at high (low) frequencies, respectively, and that they possess well-defined and distinguishable scaling properties. Wavenumber spectra for L- and H-mode measurements are calculated and fitted by power- laws and exponential functions. The separated measurements can be fitted with the same exponents in L- and H-mode. Correlations between the density fluctuations, the H-alpha- signal and magnetic fluctuations as measured by Mirnov coils were analyzed. Correlation calculations using 50 ms time windows (several dithering periods) with time lag steps of 100 mus showed that all the fluctuating quantities are highly correlated and that the maximum correlation occurs for high frequency density fluctuations. Performing separate L- and H- mode correlations on a 20 mus time scale between magnetic and density fluctuations leads to the result that the minimum correlation time scale in L-mode is of order 100 mus, while no correlation exists for H-mode. (C) 2002 American Institute of Physics.
2002
Article
http://edoc.mpg.de/15590
urn:ISSN:1070-664X
Physics of Plasmas, v.9, 3035-3049 (2002)
en
oai:edoc.mpg.de:155932006-11-1433:1309
Elongation and current scalings of local global energy transport
Becker, G.
The scaling of local and global energy confinement with elongation kappa and current in L- and H-mode plasmas is explored. It is shown that the elongation enters global confinement only through the : geometrical quantities of plasma volume and surface area. while local transport does not contribute. The global scaling tau(E) proportional to kappa(0.8) is caused by W proportional to n T-0 V with the volume V proportional to kappa and the surface area leading to T-0 proportional to kappa(-0.2). Empirical scalings of the effective heat diffusivity chi with kappa, the current inside a flux surface I (x). B-p and q are presented. Applying I (x) makes chi independent Of kappa, while a formulation with q yields strong implicit and explicit kappa dependences.
2002
Article
http://edoc.mpg.de/15593
urn:ISSN:0029-5515
Nuclear Fusion, v.42, L8-L9 (2002)
en
oai:edoc.mpg.de:155942006-11-2333:1309
Contribution of the different erosion processes to material release from the vessel walls of fusion devices during plasma operation
Behrisch, R.
In high temperature plasma experiments several processes contribute to erosion and loss of material from the, vessel walls. This material may enter the plasma edge and the central plasma where it acts as impurities. It will finally be re- deposited at other wall areas. These erosion processes are: evaporation due to heating of wall areas. At very high power deposition evaporation may become very large, which has been named "blooming". Large evaporation and melting at some areas of the vessel wall surface may occur during heat pulses, as observed in plasma devices during plasma disruptions. At tips on the vessel walls and/or hot spots on the plasma exposed solid surfaces electrical arcs between the plasma and the vessel wall may ignite. They cause the release of ions, atoms and small metal droplets, or of carbon dust particles. Finally, atoms from the vessel walls are removed by physical and chemical sputtering caused by the bombardment of the vessel walls with ions as well as energetic neutral hydrogen atoms from the boundary plasma. All these processes have been, and are, observed in today's plasma experiments. Evaporation can in principle be controlled by very effective cooling of the wall tiles, arcing is reduced by very stable plasma operation, and sputtering by ions can be reduced by operating with a cold plasma in front of the vessel walls. However, sputtering by energetic neutrals, which impinge on all areas of the vessel walls, is likely to be the most critical process because ions lost from the plasma recycle as neutrals or have to be refuelled by neutrals leading to the charge exchange processes in the plasma. In order to quantify the wall erosion, "Materials Factors" (MF) have been introduced in the following for the different erosion processes.
2002
Article
http://edoc.mpg.de/15594
urn:ISSN:0863-1042
Contributions to Plasma Physics, v.42, 431-444 (2002)
en
oai:edoc.mpg.de:155952008-01-3133:1309
The Helias reactor HSR4/18
Beidler, C. D.
Harmeyer, E.
Herrnegger, F.
Igitkhanov, Y.
Kendl, A.
Kisslinger, J.
Kolesnichenko, Y. I.
Lutsenko, V. V.
Nührenberg, C.
Sidorenko, I.
Strumberger, E.
Wobig, H.
Yakovenko, Y. V.
helias reactor
The Helias reactor is an upgraded version of the Wendelstein 7- X experiment. A straightforward extrapolation of Wendelstein 7- X leads to HSR5/22, which has 5 field periods and a major radius of 22 m. HSR4/18 is a more compact Helias reactor with 4 field periods and an 18 m major radius. Stability limit and energy confinement times are nearly the same as in HSR5/22, thus the same fusion power (3000 MW) is expected in both configurations. Neoclassical transport in HSR4/18 is very low. and the effective helical ripple is below 1%. The article describes the power balance of the Helias reactor, and the blanket and maintenance concepts. The coil system of HSR4/18 comprises 40 modular coils with NbTi superconducting cables. The reduction from 5 to 4 field periods and the concomitant reduction in size will also reduce the cost of the Helias reactor.
2001
Article
http://edoc.mpg.de/15595
urn:ISSN:0029-5515
Nuclear Fusion, v.41, 1759-1766 (2001)
en
oai:edoc.mpg.de:156052006-11-1433:1309
Quantitative layer analysis of single crystal surfaces by LEIS
Beikler, R.
Taglauer, E.
ion scattering; LEIS; computer simulation; alloys; surface composition; surface structure
The quantitative interpretation of energy and angle resolved low-energy ion scattering intensity distributions is achieved by comparing the experimental results with those from numerical simulations using the MARLOWE code with extensions for layer selective analysis. The example used is the CuAu(1 0 0) alloy crystal surface. For adequate analysis the potential parameter (i.e. the screening length) has to be calibrated. This is done by using elemental single crystal standards. Anisotropic thermal vibrations are taken into account in correspondence with published Debye temperature values. Neutralization effects are found to be of minor importance for Na+ and He+ scattering from CuAu. A quantitative analysis of the first and second layer composition of CuAu(1 0 0) as a function of temperature could thus be obtained, the results being in good agreement with theoretical predictions. (C) 2002 Elsevier Science B.V. All rights reserved.
2002
Article
http://edoc.mpg.de/15605
urn:ISSN:0168-583X
Nuclear Instruments and Methods in Physics Research B: Beam Interactions with Materials and Atoms, v.193, 455-459 (2002)
en
oai:edoc.mpg.de:156062006-11-1433:1309
Transport of edge-localized mode energy in a scrape-off layer in the presence of collisionless fast electrons
Bergmann, A.
The transport of the energy delivered by an edge-localized mode (ELM) into the scrape-off layer (SOL) of a tokamak plasma is studied with particle-in-cell simulations. A simple one- dimensional model with a Monte-Carlo procedure for Coulomb collisions is used. The SOL is collisional before the ELM, but the mean-free-path length in the hot ELM plasma is longer than the connection length, such that fast electrons can travel directly from the midplane to the target plate. However, the sheath potential increases rapidly, and most of the energy is transported with a velocity of order of the ion sound speed if secondary electron emission (SEE) is absent or weak. In the case of strong SEE, though, a large fraction of the energy can be carried by fast electrons during the ELM pulse.
2002
Article
http://edoc.mpg.de/15606
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 1162-1167 (2002)
en
oai:edoc.mpg.de:156082006-11-1433:1309
Two-dimensional particle simulation of the current flow to a flush-mounted Langmuir probe in a strong oblique magnetic field
Bergmann, A.
The current flow to flat Langmuir probes mounted flush into divertor target plates or limiter in a tokamak is studied with particle-in-cell simulations. The simulation model is two- dimensional in real space and three-dimensional in velocity space resolving the gyration of electrons and ions. The following two effects are studied: (i) The lack of saturation of the ion current at grazing incidence of the magnetic field. Simulations are performed with realistic values of the dimensionless parameters (ratios between probe size, gyroradius and Debye length) and the grazing angle of the magnetic field. (ii) The influence of the self-consistent ExB drift. A contribution to the ion current nonsaturation (at negative bias) is found as well as a reduction of the electron current (at positive bias). (C) 2002 American Institute of Physics.
2002
Article
http://edoc.mpg.de/15608
urn:ISSN:1070-664X
Physics of Plasmas, v.9, 3413-3420 (2002)
en
oai:edoc.mpg.de:156102006-11-1433:1309
Spectroscopy of heliumlike argon resonance and satellite lines for plasma temperature diagnostics
Biedermann, C.
Radtke, R.
Fournier, K. B.
The n = 2 - 1 spectral emission pattern of heliumlike argon, together with the associated satellite emission originating from lithiumlike argon have been measured with high-resolution x-ray spectroscopy at the Berlin electron-beam ion trap. The observed line intensity across a wide range of excitation energies was weighted by an electron-energy distribution to analyze as a function of plasma temperature the line ratios between KLL dielectronic recombination satellites, in particular the j+z, j, and k satellites, and the w-resonance line. A good agreement between various theoretical models is found, supporting the method of line-ratio measurement as a temperature diagnostic for plasmas. A value for the so-called R-line ratio is determined and calculations with the HULLAC suite of codes predict it to be electron density independent over a wide range.
2002
Article
http://edoc.mpg.de/15610
urn:ISSN:1063-651X
Physical Review E, v.66 (2002)
en
oai:edoc.mpg.de:156112006-11-1433:1309
Reactions of gas-phase H atoms with atomically and molecularly adsorbed oxygen on Pt(111)
Biener, J.
Lang, E.
Lutterloh, C.
Küppers, J.
The interaction of gas-phase H atoms with ordered and disordered adlayers of atomic oxygen, hydroxyl, and molecular oxygen on Pt(111) surfaces was investigated by in situ mass spectrometry and post-reaction TPD (temperature programed desorption). Exposure of oxygen adlayers to gas-phase H atoms at 85 K leads to formation of H2O via two consecutive hydrogenation reactions: H(g)+O(a)-->OH(a) followed by H(g)+OH(a)-->H2O(g,a). Both reaction steps are highly exothermic, and nascent H2O molecules partially escape into the gas phase before being thermally accommodated on the surface. Empty surface sites and hydrogen bonding promote thermalization of H2O. Separate experiments performed with OH-covered Pt(111) surfaces reveal that the hydrogenation of hydroxyl is a slow reaction compared to the hydrogenation of atomic oxygen; additionally, the abstraction of H from OH by gas-phase D atoms, OH(a)+D(g)-->O(a)+HD(g), was detected. Abstraction of H from adsorbed H2O was not observed. Admission of gas-phase H atoms to O-2-covered Pt(111) surfaces at 85 K leads to the desorption of O-2 and H2O. The thermodynamic stability of the HO2 radical suggests that the reaction is initiated by hydrogenation of molecular oxygen, O-2(a)+H(g)-->HO2. The intermediate HO2 either decomposes via dissociation of the HO-O bond, HO2-->OH(a)+O(a), finally leading to the formation of H2O (similar to85%), or via dissociation of the H-O-2 bond thus leading to desorption of O-2 (similar to15%). The whole reaction sequence of formation and decomposition of HO2 is fast compared to the formation of H2O via hydrogenation of atomic oxygen and hydroxyl. The observed coverage dependence of the reaction kinetics indicates the dominance of hot-atom mediated reactions. (C) 2002 American Institute of Physics.
2002
Article
http://edoc.mpg.de/15611
urn:ISSN:0021-9606
Journal of Chemical Physics, v.116, 3063-3074 (2002)
en
oai:edoc.mpg.de:156152012-06-2033:1309
Simulation of fast wave current drive in tokamaks in the ion cyclotron frequency range
Bilato, R.
Brambilla, M.
Pavlenko, I.
Meo, F.
To perform realistic simulations of radio-frequency current drive in tokamaks in the ion cyclotron range of frequencies, the wave fields evaluated by the toroidal full-wave code TORIC have been used to build the quasilinear electron diffusion coefficient, which, in turn, is used by a recently developed solver of the bounce-averaged quasilinear Fokker-Planck kinetic equation, QLEFOPS. Applications to possible fast wave current drive experiments in ASDEX Upgrade are presented. The effects of varying the target electron temperature, the background Ohmic current, and, within the relatively limited possibilities, the directivity of the antenna have been investigated. The results are in reasonable agreement with the predictions of the parametrization of the current drive (CD) efficiency proposed by Ehst and Karney based on the adjoint method for the solution of the kinetic equation. The detailed analysis which was necessary to guarantee consistency between the two codes, on the other hand, has put into evidence characteristics of this CD scheme, which, to some extent, are likely to limit the efficiency achievable in practice. In particular, the normalized quasilinear diffusion coefficient for realistic power densities turns out to be far too small for the development of a quasilinear plateau. In the absence of suprathermal electrons, sufficient absorption can be achieved only by positioning the spectrum of parallel phase velocities excited by the antenna close to the thermal region, where the theoretical efficiency is poor.
2002
Article
http://edoc.mpg.de/15615
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 1085-1093 (2002)
en
oai:edoc.mpg.de:156232006-11-1433:1309
Fast wave current drive in the ion cyclotron frequency range from the single-particle point of view
Bilato, R.
Brambilla, M.
The quasilinear model is commonly used to determine the heating rate and the current drive efficiency of the fast wave in the ion cyclotron range of frequency. The validity of this model at these frequencies is a controversial issue [Becoulet et al.. Phys. Plasmas 1, 2908 (1994)]. Here this problem is addressed from the single-particle point of view by deriving an approximated Hamiltonian which accounts also for the inhomogeneity of the confining magnetic field. Realistic values of the wave field are calculated by means of the toroidal full- wave code TORIC for typical scenarios of an ASDEX Upgrade [Gruber et al., Nucl. Fusion 41, 1369 (2001)] experiment, and apply the Chirikov criterion to determine the threshold for the development of the intrinsic stochasticity. These results are verified by numerically integrating the Hamiltonian equations. It is found that, due to the discreteness of the parallel phase velocity spectrum of a fast wave at these frequencies, for the parameters of ASDEX Upgrade the intrinsic stochasticity is marginal, especially close to integer and low-rational values of the safety factor. The failure of intrinsic stochasticity to guarantee the validity of the conventional quasilinear model, however, does not imply the onset of nonlinear saturation in the power absorption and in the driven current, since in all realistic situations Coulomb collisions suffice, by a large margin, to ensure the applicability of the linearized Vlasov equation. Under this condition, power and momentum transfer rates to resonant electrons are identical to those predicted by the quasilinear theory. (C) 2003 American Institute of Physics.
2003
Article
http://edoc.mpg.de/15623
urn:ISSN:1070-664X
Physics of Plasmas, v.10, 179-186 (2003)
en
oai:edoc.mpg.de:156262006-11-1433:1309
The spatial structure of edge fluctuations in the Wendelstein 7-AS stellarator
Bleuel, J.
Endler, M.
Niedermeyer, H.
Schubert, M.
Thomsen, H.
In the scrape-off layer and outermost edge of the Wendelstein 7-AS (W7-AS) stellarator, fluctuations of, mostly, ion saturation current I-sat and floating potential Phi(fl) have been measured with high poloidal, radial (both 2-3 mm) and temporal (0.5 mus) resolution. Parallel to the magnetic field, measurements with two probe tips on the same magnetic field line were performed. The basic spatio-temporal structure of the fluctuations is presented in terms of correlation functions and of wavenumber-frequency (kf) spectra. They are characterized by the parameters of a fit function to the correlation function, which describe the spatio-temporal structure. The behaviour of these fluctuation parameters under variations of the discharge parameters is investigated. A high correlation of the fluctuations parallel to the magnetic field is documented, and we analyse, which frequency components contribute to this high correlation. In our radially and poloidally resolving measurements, we find that the fluctuation structures are inclined in the poloidal-radial plane at our position of measurement. The consequences of such inclined structures for the interpretation of measurements with one-dimensional arrays are discussed, with an emphasis on the risk of an erroneous interpretation of purely radially resolving measurements. There is strong evidence that this inclination is due to the local magnetic shear in W7-AS in conjunction with the high correlation of the fluctuations parallel to the magnetic field. A further contribution to the inclination may originate from the radial shear of the poloidal velocity of the fluctuations in the plasma edge. The different consequences of magnetic shear and velocity shear for an inclination of the fluctuations in the poloidal-radial plane are discussed.
2002
Article
http://edoc.mpg.de/15626
urn:ISSN:1367-2630
New Journal of Physics, v.4, 38 (2002)
en
oai:edoc.mpg.de:156272006-11-1433:1309
Investigation of modification of the surface layer of graphite at high fluence irradiation by nitrogen ions
Bogomolova, L. D.
Borisov, A. M.
Krasilnikova, N. A.
Kulikauskas, V. S.
Mashkova, E. S.
Eckstein, W.
2002
Article
http://edoc.mpg.de/15627
urn:ISSN:0367-6765
Izvestiya Akademii Nauk Seriya Fizicheskaya, v.66, 551-557 (2002)
ru
oai:edoc.mpg.de:156312006-11-1433:1309
Al2O3-films on Ni3Al(111): a template for nanostructured cluster growth
Becker, C.
Rosenhahn, A.
Wiltner, A.
von Bergmann, K.
Schneider, J.
Pervan, P.
Milun, M.
Kralj, M.
Wandelt, K.
In scanning tunnelling microscope images of thin Al2O3-films grown on Ni3Al(111) at 1000 K two super-lattices with periodicities of 2.6 and 4.5 nm, respectively, can be identified. These well-ordered nanostructures can be used as nucleation centres for metal particle growth. It can be shown that both nanostructures act as a template for the fabrication of ordered assemblies of metal clusters by mere physical vapour deposition. The degree of ordering of these nanostructures is largely dependent on the metal deposited. Here we report on the growth of Cu, Ag, Au, Mn, and V clusters on the Al2O3-films. The best results as far as ordering of the clusters is concerned was reached for V deposition at 550 K, which resulted in a nearly perfect hexagonal array of clusters with a spacing of 2.6 nm.
IoP Publishing Ltd and Deutsche Physikalische Gesellschaft
2002
Article
http://edoc.mpg.de/15631
urn:ISSN:1367-2630
New Journal of Physics, v.4 (2002)
en
oai:edoc.mpg.de:156342006-11-1433:1309
Benefits to European industry from involvement in fusion
Bogusch, E.
Bolt, H.
Chevalier, A.
Forty, C.
Gnesotto, F.
Heller, R.
Laurenti, A.
Link, G.
Lister, J.
Munther, R.
Rey, G.
Schedler, B.
Thumm, M.
Vallee, A.
Waterman, N.
European industry; fusion; Euratom Framework Programme
Industry benefits in a number of different ways from the research and technology activities of the European Fusion Programme, overseen by the European Commission. These benefits range from: winning contracts to supply equipment and services to fusion laboratories; access to fundamental (or basic) science data; opportunities for technology and knowledge transfer; and the use of facilities and availability of expert personnel from the many fusion institutions. Close fusion laboratory relationships with industry are fully supported by the Commission, which actively encourages the various 'industry liaison' activities of the fusion institutions. Industry involvement in the European fusion scene is highly valued, considered to be essential to the construction and operation of International Thermonuclear Experimental Reactor (ITER) and also to the long-term future of fusion. (C) 2002 Elsevier Science B.V. All rights reserved.
2002
Article
http://edoc.mpg.de/15634
urn:ISSN:0920-3796
Fusion Engineering and Design, 679-687 (2002)
en
oai:edoc.mpg.de:156352006-11-1433:1309
Plasma facing and high heat flux materials - needs for ITER and beyond
Bolt, H.
Barabash, V.
Federici, G.
Linke, J.
Loarte, A.
Roth, J.
Sato, K.
Plasma-facing materials (PFMs) have to withstand particle and heat loads from the plasma and neutron loads during reactor operation. For ITER knowledge has been accumulated by operation experience and dedicated tests in present tokamaks as well as by laboratory experiments and modelling. The rationale for the selection of PFMs in ITER (Be, W, carbon fibre reinforced carbon) is described with regard to the critical issues concerning PFMs, esp. erosion during transient heat loads and the T-inventory in connection with the redeposition of carbon. In the fusion reactor generation after ITER the very stringent conditions of increased surface power to be removed from the plasma, a lifetime requirement of several operational years, high neutron fluences and increased operation temperature are exerting even more severe constraints on the selection of possible materials. Comparing these boundary conditions with materials under development and their further potential, only a narrow path is left regarding heat sink and PFMs. In this context the investigations on W as first wall material carried out e.g. in ASDEX Upgrade are being discussed as well as laboratory results on W-based material systems. The implications of these results are the starting point of what should form a consistent programme towards plasma-facing and heat sink materials for a fusion reactor. (C) 2002 Elsevier Science B.V. All rights reserved.
2002
Article
http://edoc.mpg.de/15635
urn:ISSN:0022-3115
Journal of Nuclear Materials, v.307, 43-52 (2002)
en
oai:edoc.mpg.de:156362006-11-2233:1309
The two-mesh grid refinement method for the B2 code
Bonnin, X.
Bürbaumer, H.
Schneider, R.
Coster, D.
Aumayr, F.
Winter, H. P.
We introduce version 6.0 of the 2D multispecies plasma fluid code 132, to which a flexible dynamic grid adaptation module has been added. The method followed in this paper involves the creation of two grid structures. A fine 2D mesh produced from the magnetic flux surface structure serves as a scaffolding on which a rougher, adaptable mesh is constructed depending on the physical situation. The conditions for grid refinement/coarsening involve normalized gradients of the plasma parameters. The algorithm and its numerical implementation are described in detail. As a sample application, the detachment behaviour of an ASDEX-Upgrade L- Mode discharge has been studied. A comparison of the results from the old and new B2 code versions is presented. Similar results in the overall plasma behaviour are seen but differences appear in the detachment zone, due to the grid adapting to the strong local gradients. Increased (decreased) gas puffing leads to an upward (downward) shift of the detachment front as well as to a shift of the zone of maximum grid density. The now code allows one to increase the grid density in the zone of interest without a strong increase CPU runtime, thus enabling a more realistic modelling of the detachment front.
2002
Article
http://edoc.mpg.de/15636
urn:ISSN:0863-1042
Contributions to Plasma Physics, v.42, 175-180 (2002)
en
oai:edoc.mpg.de:174792006-11-1433:1309
Study of acceleration and confinement of high-energy protons during ICRF and NBI heating in LHD using a natural diamond detector
Krasilnikov, A. V.
Sasao, M.
Isobe, M.
Kumazawa, R.
Mutoh, T.
Takeiri, Y.
Watari, T.
Hartman, D. A.
Murakami, S.
Alekseev, A. G.
Amosov, V. N.
Kaschuck, Y. A.
Portnov, D. V.
Saito, K.
Seki, T.
Kaneko, O.
Torii, Y.
Iizuka, S.
Osakabe, M.
Goto, M.
Yamada, H.
Narihara, K.
Ohyabu, N.
Motojima, O.
Energetic tail formation by ion-cyclotron-range-of-frequencies (ICRF) heating was studied in the large helical device (LHD), by measuring high-energy charge-exchange atom spectra with a specially developed natural diamond detector, which views the LHD plasma perpendicularly. Comparison of measured effective perpendicular temperature of ICRF-driven H+ minority ions with the classical Stix model indicates that perpendicular ions with energy at least up to 150 keV are well confined in the centre region of the LHD. However, deviation from the classical prediction was observed for high-energy perpendicular protons in the outer region of the plasma, indicating ripple-induced transport and charge-exchange losses. With perpendicular fast atom spectrum and flux measurements, small yet notable differences were detected co-injected and counter-injected ICRF-driven beam ion confinement during combined neutral beam injection (NBI) and ICRF heating. These results can be explained by the differences in the orbit topology and ICRF- induced drift.
2002
Article
http://edoc.mpg.de/17479
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 759-767 (2002)
en
oai:edoc.mpg.de:199572006-11-1433:1309
Modelling of Alfven waves in JET plasmas with the CASTOR-K code
Borba, D.
Berk, H. L.
Breizman, B. N.
Fasoli, A.
Nabais, F.
Pinches, S. D.
Sharapov, S. E.
Testa, D.
A hybrid magnetohydrodynamic (M H D)-gyro-kinetic model CASTOR- K developed for the study of Alfven eigenmode (AE) stability in the presence of energetic ions has been applied to the interpretation of recent measurements of Alfven waves in JET. These include the detailed AE damping measurements performed using the AE antenna excitation system and also the observations of Alfven cascades in strongly reversed shear scenarios at JET. The mode conversion between the AEs and kinetic Alfven waves and the relation to the Alfven continuum is studied and the calculated damping is compared with the experimental data. The contribution of ion cyclotron resonant heating driven minority ions to the growth rate of the novel- type mode localized around the point of zero magnetic shear is calculated. This mode is shown to be clearly linked to the ideal MHD 'Alfven continuum', computed with the CSCAS code and consistent with the observation of a quasi-periodic pattern of upward frequency sweeping Alfven cascades in JET.
2002
Article
http://edoc.mpg.de/19957
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 1029-1038 (2002)
en
oai:edoc.mpg.de:200302006-11-1533:1310
Design of VUV/XUV spectrometers for impurity studies on W7-X
Biel, B.
Bertschinger, G.
Burhenn, R.
König, R.
European Physical Socciety
2002
Conference-Paper
http://edoc.mpg.de/20030
Behn, R.; Varandas, C.: 29th European Physical Society Conference on Plasma Physics and Controlled Fusion. Contributed Papers, European Physical Socciety (2002)
en
oai:edoc.mpg.de:200892006-11-1433:1309
Tungsten as plasma-facing material in ASDEX Upgrade
ASDEX Upgrade Team
Neu, R.
Dux, R.
Geier, A.
Gruber, O.
Kallenbach, A.
Krieger, K.
Maier, H.
Pugno, R.
Rohde, V.
Schweizer, S.
2003
Article
http://edoc.mpg.de/20089
Fusion Engineering and Design, v.Vol. 65, 367-374 (2003)
en
oai:edoc.mpg.de:201412012-07-0533:1309
Possibilities for multifrequency operation of a gyrotron at FZK
Borie, E.
Drumm, O.
Illy, S.
Koppenburg, K.
Kartikeyan, M. V.
Piosczyk, B.
Wagner, D.
Yang, X.
Dammertz, G.
Thumm, M. K.
gyrotrons; step tunability
We investigate the possibility of multifrequency operation of a 140-GHz gyrotron, which is designed to operate in the TE22,8 mode at 140 GHz and the TE19,6 mode at 111 GHz or the TE17,6 mode at 105 GHz, for which existing equipment can be used. The present calculations compute beam properties for a given set of coil currents, accelerating voltage and current, and then use these beam properties to compute the output power and efficiency. These calculations are performed separately for each mode.
2002
Article
http://edoc.mpg.de/20141
urn:ISSN:0093-3813
IEEE Transactions on Plasma Science, v.30, 828-835 (2002)
en
oai:edoc.mpg.de:201422006-11-1433:1309
Sputtering and ion-induced electron emission of graphite under high-dose nitrogen bombardment
Borisov, A. M.
Eckstein, W.
Mashkova, E. S.
The dependence of the sputtering yield Y and the electron emission coefficient gamma of isotropic graphites (POCO-AXF-5Q and Russian MPG-LT) on ion fluence and ion incidence angle 0 at near room temperatures and the dependence of gamma on target temperature under high dose 30 keV N-2(+) ion irradiation were measured. It was found that Y and gamma are stabilized at fluences F greater than or equal to 1 x 10(19) N/cm(2). A specific target surface topography develops. At steady-state conditions, the N concentration in MPG-LT is 19 at.% and in POCO16 at.%. In the angular range 0 = 0-80degrees, Y and gamma increase and the angular dependence of Y is slightly stronger than that of gamma. Sputtering yields of POCO are 1.5 times higher than those of MPG-LT. The reasons of the difference between the experimental and calculated sputtering yields using the TRIM.SP code are discussed. The dependence of gamma on the target temperature manifests a step-like increase at similar or equal to250 degreesC which may be due to radiation induced structure transformation in the modified surface layer. (C) 2002 Published by Elsevier Science B.V.
2002
Article
http://edoc.mpg.de/20142
urn:ISSN:0022-3115
Journal of Nuclear Materials, v.304, 15-20 (2002)
en
oai:edoc.mpg.de:201662006-11-1433:1309
Natural density formation as an H-mode operational limit
Borrass, K.
ASDEX Upgrade Team
EFDA-JET Work Programme Collaborators
The natural H-mode density, i.e. the plasma density evolving in an H-mode discharge without active fuelling, reaches Greenwald fractions in JET typically higher than in ASDEX Upgrade. According to general thinking this reflects device-specific differences as regards recycling-induced fuelling and beam fuelling. This paper presents evidence for a different view, namely that at sufficiently low plasma fuelling rates any fuelling rate dependence of the plasma density vanishes and the plasma particle content is completely determined by the plasma itself. It is shown that this limit, which would impose a lower boundary on the H-mode density, is reached in JET and ASDEX Upgrade natural density discharges, and its scaling is determined. Possible overlapping with existing density limit scalings in next-generation tokamaks is discussed with a view to a narrowing or even vanishing H-mode operation window.
2002
Article
http://edoc.mpg.de/20166
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 1251-1256 (2002)
en
oai:edoc.mpg.de:201752006-11-1433:1309
Alles Blasen, oder was?
Bosch, H.-S.
2002
Article
http://edoc.mpg.de/20175
Physik Journal, v.1, 21-22 (2002)
de
oai:edoc.mpg.de:201762006-11-1433:1309
Erste Erfolge bei "sphärischen Tokamaks"
Bosch, H.-S.
2002
Article
http://edoc.mpg.de/20176
Physik Journal, v.1, 19-20 (2002)
de
oai:edoc.mpg.de:201792006-11-1433:1309
Our energy future
Bradshaw, A. M.
Hamacher, T.
2002
Article
http://edoc.mpg.de/20179
Physicalia Magazine, v.24, 127-137 (2002)
en
oai:edoc.mpg.de:201802006-11-1433:1309
Fusionsforschung im Max-Planck-Institut für Plasmaphysik
Bradshaw, A. M.
Milch, I.
The Max Planck Institute for Plasma Physics (IPP) works on the basic principles of a future fusion power plant. Some 1000 persons at the locations of Garching, Greifswald, and Berlin are engaged in research work within the framework of the European Nuclear Fusion Program coordinated by EURATOM. IPP is financed by the European Union, the Federal Republic of Germany, and the federal states of Bavaria, Mecklenburg-Western Pomerania, and Berlin. IPP is the only institute in the world operating the two most favored plant lines, tokamak and stellarator, in parallel. Tokamak facilities generate the ring- shaped magnetic field cage by external magnet coils and by a pulsed electric current induced in the plasma. The ASDEX Upgrade plant has been operated at IPP since 1991, with several extensions and modifications, and serves to study one of the main problems of fusion research: the interactions between the hot plasma and the walls surrounding it. Important findings for future work have been made. They will also be used for the planned international ITER fusion plant, in the planning of which IPP is involved. Fusion plants operating by the stellarator principle work without a plasma current, having a field generated only by external coils, and thus are able to be run in the continuous mode. WENDELSTEIN 7-AS has been in operation at Garching IPP since 1988 and constitutes the first new-generation plant in the world. Its underlying conceptual design principles have been confirmed in plant operation so far. WENDELSTEIN 7-X will be the world's largest fusion experiment of the stellarator type. The plant is to be commissioned in 2006. After 2020, following the successful operation of ITER or WENDELSTEIN 7-X, more extensive planning could be started of fusion plants for future energy supply.
2002
Article
http://edoc.mpg.de/20180
urn:ISSN:1431-5254
Atw. Internationale Zeitschrift für Kernenergie, v.47, 37-42 (2002)
de
oai:edoc.mpg.de:201822006-11-1433:1309
Electron energy transport in the presence of rational surfaces in the Wendelstein 7-AS stellarator
Brakel, R.
W7-AS Team
Electron energy transport in the Wendelstein 7-AS stellarator exhibits a strong sensitivity to the boundary value of the rotational transform when the magnetic shear is low. Pronounced confinement maxima close to the low order rational t values correlate with a low density of other rational numbers in these favourable t ranges. The t dependence is lost if magnetic shear is increased by large plasma currents. The hypothesis is proposed that, at rational flux surfaces, electron energy transport is increased above the already anomalous level and that magnetic shear reduces the additional enhancement. An empirical model that incorporates these features into a simple expression for the electron heat conductivity can reproduce the experimental dependences over a wide range of external control parameters, e.g. the rotational transform and the net plasma current. Except for the explicit shear dependence, this model bears a remarkable similarity to the RTP model, which successfully describes the electron transport barriers near low order rational q surfaces in RTP and other tokamaks.
2002
Article
http://edoc.mpg.de/20182
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 903-912 (2002)
en
oai:edoc.mpg.de:202002007-01-0233:1309
ICRF system enhancements at ASDEX Upgrade
Braun, F.
Hofmeister, F.
Wesner, F.
Becker, W.
Faugel, H.
Hartmann, D.
Noterdaeme, J. M.
ion cyclotron range of frequency (ICRF); ASDEX Upgrade
2001
Article
http://edoc.mpg.de/20200
urn:ISSN:0920-3796
Fusion Engineering and Design, v.55-56, 551-555 (2001)
en
oai:edoc.mpg.de:202022006-11-1433:1309
Surface resonances versus surface states on Fe(110)
Braun, J.
Math, C.
Postnikov, A.
Donath, M.
The spin-dependent surface electronic structure of ferromagnetic Fe(110) is investigated experimentally and theoretically. Spin-resolved, inverse-photoemission results show a complex multipeak structure close to the Fermi level. Part of it is surface derived with an unexpected spin dependence and light-polarization dependence. The puzzling experimental findings are interpreted on the basis of a comprehensive theoretical analysis. Electronic slab calculations find surface-related features, caused by the crystal-vacuum interface, only well below the Fermi level. Calculations of the (inverse) photoemission intensities within the relativistic one-step model based on a bulk band structure, but with a realistic surface barrier, reveal an additional surface resonance around the Fermi level. Its nearly vanishing exchange splitting at Gamma and abnormal energy dispersion behavior as a function of the wave vector parallel to the surface are in accordance with the experimental findings.
2002
Article
http://edoc.mpg.de/20202
urn:ISSN:0163-1829
Physical Review B, v.65 (2002)
en
oai:edoc.mpg.de:202052006-11-1433:1309
Local physics basis of confinement degradation in JET ELMy H mode plasmas and implications for tokamak reactors
Budny, R. V.
Alper, B.
Borba, D. N.
Cordey, J. G.
Ernst, D. R.
Giraud, C.
Gowers, C. W.
Gunther, K.
Hahm, T. S.
Hammett, G. W.
Hawkes, N. C.
Horton, L. D.
Saibene, G.
Sartori, R.
Synakowski, E. J.
von Hellermann, M. G.
Zastrow, K. D.
First results of gyrokinetic analysis of JET ELMy H mode plasmas are presented. ELMy H mode plasmas form the basis of conservative performance predictions for tokamak reactors of the size of ITER. Relatively high performance for long durations has been achieved and the scaling appears to be favourable. It will be necessary to sustain low Z(eff) and high density for high fusion yield. The article studies the degradation in confinement and the increase in the anomalous heat transport observed in two JET plasmas: one with an intense gas puff and the other with a spontaneous transition from type I to III ELMS at the heating power threshold. Linear gyrokinetic analysis gives the growth rate gamma(lin) of the fastest growing modes. The flow shearing rate gamma(ExB) and gamma(lin) are large near the top of the pedestal. Their ratio decreases approximately at the time when the confinement degrades and the transport increases. This suggests that tokamak reactors may require intense toroidal or poloidal torque input to maintain sufficiently high gamma(ExB)/gamma(lin) near the top of the pedestal for high confinement.
2002
Article
http://edoc.mpg.de/20205
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 66-75 (2002)
en
oai:edoc.mpg.de:202072006-11-1433:1309
Microturbulence and flow shear in high-performance JET ITB plasma
Budny, R. V.
Andre, R.
Becoulet, A.
Challis, C. D.
Conway, G. D.
Dorland, W.
Ernst, D. R.
Hahm, T. S.
Hender, T. C.
McCune, D.
Rewoldt, G.
EFDA-JET Work Programme Collaborators
The transport, flow shear, and linear growth rates of microturbulence are studied for a JET plasma with high central q in which an internal transport barrier (ITB) forms and crows to a large radius. The linear microturbulence growth rates of the fastest growing (most unstable) toroidal modes with high toroidal mode number are calculated using the GS2 and FULL gyrokinetic codes. These linear growth rates, gamma(lin) are large, but the flow-shearing rates, Y-ExB (dominated by the toroidal rotation contribution), are also comparably large when and where the ITB exists.
2002
Article
http://edoc.mpg.de/20207
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 1215-1228 (2002)
en
oai:edoc.mpg.de:202132006-11-1433:1309
Basic analytical investigation of plasma-chemically modified carbon fibers
Bubert, H.
Ai, X.
Haiber, S.
Heintze, M.
Brüser, V.
Pasch, E.
Brandl, W.
Marginean, G.
2002
Article
http://edoc.mpg.de/20213
Spectrochimica Acta. B, v.57, 1601-1610 (2002)
en
oai:edoc.mpg.de:202152006-11-1433:1309
Resonant Faraday shield ICRH antenna
Cattanei, G.
ICRH has proved to be an efficient method of heating the plasma in toroidal devices. The high voltages needed at the coupling structure are, however, a severe handicap of this method. The possibility is investigated of having the highest voltages between the bars of the Faraday shield (FS), where they are both necessary and easier to maintain. For this purpose a resonant Faraday shield (RFS) antenna where the first and last bars of the FS are connected by an inductive strip is proposed. In front of this strip there is a second strip, fed, as in a conventional antenna, by an RF generator. It is shown that if the toroidal length of the FS is larger lambda/2 the strip connecting the bars of the FS acts as the secondary coil of a tuned transformer, the strip fed by the generator being the primary. It is therefore possible, by varying the frequency and the distance between the two strips, i.e. the coupling coefficient, to match the impedance of the primary to that of the generator.
2002
Article
http://edoc.mpg.de/20215
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 541-546 (2002)
en
oai:edoc.mpg.de:202162006-11-1433:1309
Resonant Faraday shield ICRH antenna
Cattanei, G.
W7-AS Team
ICRH has proved to be an efficient method of heating the plasma in toroidal devices. The high voltages needed at the coupling structure are, however, a severe handicap of this method. The possibility is investigated of having the highest voltages between the bars of the Faraday shield (FS), where they are both necessary and easier to maintain. For this purpose a resonant Faraday shield (RFS) antenna where the first and last bars of the FS are connected by an inductive strip is proposed. In front of this strip there is a second strip, fed, as in a conventional antenna, by an RF generator. It is shown that if the toroidal length of the FS is larger lambda/2 the strip connecting the bars of the FS acts as the secondary coil of a tuned transformer, the strip fed by the generator being the primary. It is therefore possible, by varying the frequency and the distance between the two strips, i.e. the coupling coefficient, to match the impedance of the primary to that of the generator.
2002
Article
http://edoc.mpg.de/20216
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 541-546 (2002)
en
oai:edoc.mpg.de:202212006-11-1433:1309
Effect of growth rate on the spatial distributions of dome-shaped Ge islands on Si(001)
Cho, B.
Schwarz-Selinger, T.
Ohmori, K.
Cahill, D. G.
Greene, J. E.
2002
Article
http://edoc.mpg.de/20221
Physical Review B, v.66 (2002)
en
oai:edoc.mpg.de:202232006-11-1433:1309
Self-Organisation and intermittent coherent oscillations in the EXTRAP-T2 reversed field pinch
Cecconello, M.
Malmberg, J. A.
Sallander, E.
Drake, J. R.
2002
Article
http://edoc.mpg.de/20223
Physica Scripta, v.65, 69-72 (2002)
en
oai:edoc.mpg.de:202242006-11-1433:1309
Turbulence behaviour during electron heated reversed shear discharges in JET
Conway, G. D.
Hogeweij, G. M. D.
de Baar, M. R.
Baranov, Y.
Barnsley, R.
Hawkes, N. C.
Litaudon, X.
Mailloux, J.
Righi, E.
Rimini, F. G.
Zastrow, K. D.
EFDA-JET Work Programme Collaborators
Low-frequency/long-wavelength density turbulence is found to be reduced within the volume enclosed by an electron thermal internal transport barrier generated with lower hybrid microwave heating in the JET tokamak. The turbulence reduction coincides with a region (typically r/a < 0.4) of negative, or reversed magnetic shear s and reduced electron thermal diffusion chi(e). In the tokamak edge region (r/a > 0.8), the turbulence amplitude is also reduced coinciding with large positive magnetic shear s > 1. Estimates of the core turbulence wavelengths (lambda(perpendicular to) > 0.1 m) are not in agreement with expectations from electron temperature gradient instabilities.
2002
Article
http://edoc.mpg.de/20224
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 1167-1180 (2002)
en
oai:edoc.mpg.de:203952006-11-1433:1309
Coupling of turbulence and reflectometer simulation codes and comparison with experiment
Conway, G. D.
Kurzan, B.
Scott, B.
Holzhauer, E.
Kaufmann, M.
First proof-of-principle results are presented from the coupling of a numerical (Landau-fluid) turbulence code and three two-dimensional reflectometer simulation codes (finite- difference time domain full-wave, time-invariant network full- wave, and physical optics). With a view to validating the numerical turbulence code, the results are favourably compared with real reflectometer signals from an L-mode edge discharge in the ASDEX Upgrade tokamak.
2002
Article
http://edoc.mpg.de/20395
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 451-463 (2002)
en
oai:edoc.mpg.de:203972006-11-1433:1309
Bootstrap current destabilization of ideal MHD modes in three- dimensional reactor configurations
Cooper, W. A.
Margalet, S. F.
Allfrey, S. J.
Isaev, M. Y.
Mikhailov, M. I.
Shafranov, V. D.
Subbotin, A. A.
Narushima, Y.
Okamura, S.
Suzuki, C.
Yamazaki, K.
Fu, G. Y.
Ku, L. P.
Monticello, D. A.
Redi, M. H.
Reiman, A. H.
Zarnstorff, M. C.
Nührenberg, J.
Todd, T. N.
In current-free stellarators, the parallel current density is normally too weak to drive global external kink modes. However, at finite values of beta, the bootstrap current (BC) can provide sufficient free energy to trigger this class of mode in some stellarator systems. The effect of the BC in the collisionless 1/nu regime has been investigated in several different types of stellarator reactor systems all with a volume V similar to 1000 m(3). In quasiaxisymmetric and quasihelically symmetric stellarators, the BC is large at, finite beta and this can cause low order resonances to move into and emerge out of the plasma which in turn can destabilize global internal and external kink modes. In a six-field period system with poloidally closed contours of the magnetic field strength B, the BC is small and decreases the rotational transform only slightly. As a result, only intermediate to high n modes can become weakly destabilized. Furthermore, it is demonstrated in this system that the contours of the second adiabatic invariant J(parallel to) close poloidally for all trapped particles at finite beta* similar to 6%. This condition leads to the loss of a very small fraction of the collisionless alpha-particle orbits. In Sphellamak configurations with peaked toroidal currents required to generate nearly isodynamic maximum-B confining field structures, the BC accounts only for a small fraction of the total current. The loss of a-particles born within the inner quarter of the plasma volume is negligible while about 1/3 of those born at half volume escape the device within a slowing down time.
2002
Article
http://edoc.mpg.de/20397
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, B357-B373 (2002)
en
oai:edoc.mpg.de:204002006-11-1433:1309
Energy confinement in steady-state ELMy H-modes in JET
Cordey, J. G.
McDonald, D. C.
Borrass, K.
Charlet, M.
Coffey, I.
Kallenbach, A.
Lawson, K.
Lomas, P.
Ongena, J.
Rapp, J.
Ryter, F.
Saibene, G.
Sartori, R.
Stamp, M.
Strachan, J.
Suttrop, W.
Valovic, M.
EFDA-JET Work Programme Collaborators
It is shown statistically that, divertor closure, plasma shaping and density peaking improve the energy confinement of ELMy H-modes, whilst the confinement degrades as the Greenwald density limit is approached. A prediction of the influence of these effects on the next step device ITER is given.
2002
Article
http://edoc.mpg.de/20400
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 1929-1935 (2002)
en
oai:edoc.mpg.de:204022006-11-1433:1309
Tokamak edge model validation and improvement
Coster, D. P.
Kim, J. W.
Nishimura, Y.
Schneider, R.
Bonnin, X.
Rozhansky, V.
Voskoboynikov, S.
Because of the complexity of the physics in the edge of divertor tokamaks, extrapolation from present-day machines to future reactor or reactor-type devices is done using two- dimensional-edge codes. Amongst the issues which are important for future machines, are those of inboard/outboard divertor power asymmetries and the compression of hydrogen and helium. The issues driving the in-out power asymmetry are explored, and a reasonable match is found to existing experiments, as well as an 'adjustable knob' that could be used to further improve the match. In this work, one further effect that is known to play a role-that of E x B and diamagnetic drifts-is identified but not analysed, because the codes have not yet been able to include the drift terms in combination with the impurities and kinetic neutrals. It is also shown that the codes reproduce the fact the only small differences were seen on the ASDEX Upgrade experiment in the compression of helium when the DivII divertor was replaced by the DivIIb configuration.
2002
Article
http://edoc.mpg.de/20402
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 979-984 (2002)
en
oai:edoc.mpg.de:204032006-11-1433:1309
JET quasistationary internal-transport-barrier operation with active control of the pressure profile
Crisanti, F.
Litaudon, X.
Mailloux, J.
Mazon, D.
Barbato, E.
Baranov, Y.
Becoulet, A.
Becoulet, M.
Challis, C. D.
Conway, G. D.
Dux, R.
Eriksson, L. G.
Esposito, B.
Frigione, D.
Hennequin, P.
Giroud, C.
Hawkes, N.
Huysmans, G.
Imbeaux, F.
Joffrin, E.
Lomas, P.
Lotte, P.
Maget, P.
Mantsinen, M.
Moreau, D.
Rimini, F.
Riva, M.
Sarazin, Y.
Tresset, G.
Tuccillo, A. A.
Zastrow, K. D.
Quasistationary operation has been achieved on the Joint European Torus tokamak in internal-transport-barrier (ITB) scenarios, with the discharge time limited only by plant constraints. Full current drive was obtained over all the high performance phase by using lower hybrid current drive. For the first time feedback control on the total pressure and on the electron temperature pro le was implemented by using, respectively, the neutral beams and the ion-cyclotron waves. Although impurity accumulation could be a problem in steady state ITBs, these experiments bring some elements to answer to it.
2002
Article
http://edoc.mpg.de/20403
urn:ISSN:0031-9007
Physical Review Letters, v.88 (2002)
en
oai:edoc.mpg.de:204062006-11-1433:1309
Modelling of mixed-phasing antenna-plasma interactions on JET A2 antennas
D'Ippolito, D. A.
Myra, J. R.
Ryan, P. M.
Righi, E.
Heikkinen, J.
Lamalle, P. U.
Noterdaeme, J. M.
The use of mixed(monopole-dipole)-phasing of a set of ion cyclotron range of frequency antennas is potentially useful to optimize tokamak performance. However, recent mixed-phasing experiments on JET, described here, showed undesirable antenna- plasma interactions under certain circumstances. A possible physical mechanism to explain the experimental results is discussed, namely, rf-driven dc parallel currents flowing between adjacent antennas with different phasings can lead to arcing on the antenna with the largest sheath voltage. Means of controlling the interaction are discussed.
2002
Article
http://edoc.mpg.de/20406
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 1357-1365 (2002)
en
oai:edoc.mpg.de:204092011-08-1033:1309
Development of a 140-GHz 1-MW continuous wave gyrotron for the W7-X stellarator
Dammertz, G.
Alberti, S.
Arnold, A.
Borie, E.
Erckmann, V.
Gantenbein, G.
Giguet, E.
Heidinger, R.
Hogge, J. P.
Illy, S.
Kasparek, W.
Koppenburg, K.
Kuntze, M.
Laqua, H. P.
LeCloarec, G.
LeGoff, Y. C.
Leonhardt, W.
Lievin, C.
Magne, R.
Michel, G.
Müller, G.
Neffe, G.
Piosczyk, B.
Schmid, M.
Schwörer, K.
Thumm, M. K.
Tran, M. Q.
beam tunnel; depressed collector; diamond window; electron gun; gyrotron; high-power microwaves
The development of high-power gyrotrons (118 GHz, 140 GHz) in continuous-wave (CW) operation for heating nuclear fusion plasmas has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. The 140-GHz gyrotron being under development for the installation at the W7-X stellarator now under construction at the IPP Greifswald, Germany, operates in the TE28,8 mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high mode- purity low-Ohmic loss cavity, an optimized nonlinear up-taper, a highly efficient internal quasi-optical mode converter, a single-stage depressed collector and An edge-cooled, single disk CVD-diamond window. RF measurements at pulse duration of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 W. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reduction of the output power. Long pulse operation (10 s at 1 MW) was possible without any signs of a limitation caused by the tube. For this output power the efficiency of the tube could be increased from about 30% without to about 50% with depression voltage. The best performance reached so far has produced an energy per pulse as high as 90 MJ (power 0.64 MW, pulse length 140 s) which is the highest value achieved in gyrotrons operating at this frequency and power level. The pulse-length limitations so far are mainly due to the external system.
2002
Article
http://edoc.mpg.de/20409
urn:ISSN:0093-3813
IEEE Transactions on Plasma Science, v.30, 808-818 (2002)
en
oai:edoc.mpg.de:204182006-11-1433:1309
Laser absorption spectroscopy with a blue diode laser in an aluminium hollow cathode discharge
Dinklage, A.
2002
Article
http://edoc.mpg.de/20418
Review of Scientific Instruments, v.73, 378-382 (2002)
en
oai:edoc.mpg.de:204282006-11-1433:1309
Confinement properties of high density impurity seeded ELMy H- mode discharges at low and high triangularity on JET
Dumortier, P.
Andrew, P.
Bonheure, G.
Budny, R. V.
Buttery, R.
Charlet, M.
Coffey, I.
de Baar, M.
de Vries, P. C.
Eich, T.
Hillis, D.
Ingesson, C.
Jachmich, S.
Jackson, G.
Kallenbach, A.
Koslowski, H. R.
Lawson, K. D.
Liu, C.
Maddison, G.
Messiaen, A. M.
Monier-Garbet, P.
Murakami, M.
Nave, M. F. F.
Ongena, J.
Parail, V.
Puiatti, M. E.
Rapp, J.
Sartori, F.
Stamp, M.
Strachan, J. D.
Suttrop, W.
Telesca, G.
Tokar, M.
Unterberg, B.
Valisa, M.
von Hellermann, M.
Weyssow, B.
EFDA-JET Work Programme Collaborators
The design value for ITER is based on operation at n/n(GW) = 0.85, beta(n) = 1.8 and H98(y, 2) = 1. These values have been routinely achieved in JET in argon seeded ELMy H-mode discharges in different divertor configurations and with different triangularities. Two main scenarios are emerging from the experiments. First, low triangularity (delta(u) = 0.19) in septum configuration. In this case large D-2 fuelling rates lead to confinement degradation towards L-mode. The seeding of Ar during the D-2 fuelling phase gives rise to a density close to the Greenwald value. After the switch-off of the D-2 gas fuelling ('afterpuff ' phase), the confinement recovers to H- mode quality whereas the density stays near the value reached at the end of the main fuelling phase and Z(eff) stays close to or below 2. Acting on the refuelling of Ar and D-2 in the 'afterpuff 'phase allows us to improve the stationarity of the high performance phase while maintaining up to the end of the heating phase the good confinement, density and radiation level. Second, high triangularity (delta(u) = 0.45) in vertical target configuration. In this case large fuelling rates do not lead to strong confinement degradation and the D-2 fuelling is applied continuously throughout the discharge. A radiated power fraction of up to 70%, H98(y, 2) = 0.9 at beta(n) = 2.1 and n = 1.15n(GW)-together with the formation of a radiating mantle and moderate Z(eff)-are achieved in this scenario. Furthermore, there are indications of significantly reduced heat load on the divertor target plates.
2002
Article
http://edoc.mpg.de/20428
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 1845-1861 (2002)
en
oai:edoc.mpg.de:204352011-07-1833:1309
8th International Workshop on Plasma Edge Theory in Fusion Devices: Proceedings of Invited and Contributed Papers
Düchs, D. F.
Heikkinen, J. A.
2002
Issue
http://edoc.mpg.de/20435
urn:ISSN:0863-1042
Contributions to Plasma Physics, v.42, 2-4 (2002)
en
oai:edoc.mpg.de:204402006-11-1433:1309
Carbon concentration in the Wendelstein 7-AS stellarator measured with the high-energy Li beam
Ehmler, H.
McCormick, K.
Wolfrum, E.
Burhenn, R.
Grigull, P.
Klinger, T.
W7-AS Team
2002
Article
http://edoc.mpg.de/20440
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 1411-1422 (2002)
en
oai:edoc.mpg.de:204432006-11-1433:1309
Limits of ion-beam depth-profiling as used in diffusion studies on oxidation-sensitive materials
Ehmler, H.
Rehmet, A.
Rätzke, K.
Faupel, F.
2002
Article
http://edoc.mpg.de/20443
Defect Diffusion Forum, v.203, 147-152 (2002)
en
oai:edoc.mpg.de:204452006-11-1433:1309
The impact of magnetic shear on the spatial structure of edge fluctuations in Wendelstein 7-AS
Endler, M.
Bruchhausen, M.
Bleuel, J.
Schubert, M.
W7-AS Team
2002
Article
http://edoc.mpg.de/20445
Stellarator News, v.83, 1-4 (2002)
en
oai:edoc.mpg.de:204532006-11-1433:1309
Electrostatic potential and plasma flow near the edge of a divertor tokamak
Feneberg, W.
Weitzner, H.
2002
Article
http://edoc.mpg.de/20453
Physics of Plasmas, v.9, 1036-1038 (2002)
en
oai:edoc.mpg.de:204552006-11-1433:1309
Transport in island divertors: physics, 3D modelling and comparison to first experiments on W7-AS
Feng, Y.
Sardei, F.
Grigull, P.
McCormick, K.
Kisslinger, J.
Reiter, D.
Igitkhanov, Y.
Basic plasma transport properties in island divertors are compared to those of standard tokamak divertors. A realistic plasma transport modelling of high-density discharges in island divertors has become possible by implementing a self-consistent treatment of impurity transport in the EMC3-EIRENE code. In contrast to standard tokamak divertors, the code predicts no high recycling prior to detachment, with the downstream density never exceeding the upstream density. This is mainly due to momentum losses arising from the cross-field transport associated with the specific island divertor geometry. This momentum loss is effective already at low densities, high temperatures and is responsible for the high upstream densities needed to achieve detachment. Numerical scans of carbon concentration for high-density plasma typically show first a smooth, then a sharp increase of the carbon radiation, the latter being accompanied by a sharp drop of the downstream temperature and density indicating detachment transition. The jumps of the radiation and temperature are due to a thermal instability associated with the form of the impurity cooling rate function and can be reproduced by a simple ID radial energy model based on cross-field transport and impurity losses. This model is used as a guideline to illustrate and discuss the detachment physics in details, including detachment condition and thermal instability. Major EMC3-EIRENE code predictions have been verified by the first W7-AS divertor experiments. A comparison of calculations and measurements is presented for high-density, high-power W7-AS divertor discharges and the physics related to rollover and detachment is discussed in detail. The code has been recently extended to general SOL configurations with open islands and arbitrary ergodicity by using a new highly accurate field-line mapping technique. The method correctly reproduces flux surfaces and islands over a high number of toroidal field periods, thus ensuring a clear distinction between parallel and radial transport. The technique has been tested successfully on W7-AS, W7-X, LHD and TEXTOR DED, and first applied to solve the coupled heat conduction equations for a typical ergodic W7-AS configuration.
2002
Article
http://edoc.mpg.de/20455
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 611-625 (2002)
en
oai:edoc.mpg.de:204582006-11-2233:1309
Island divertor transport modelling and comparison with experiment
Feng, Y.
Sardei, F.
Kisslinger, J.
Reiter, D.
Igitkhanov, Y.
A realistic modelling of island divertor transport, taking into account the complex 3D island structures and the discontinuous divertor plates and baffles, has become possible by the development, of the 3D Monte Carlo code EMC3-EIRENE [1,2], which has been already used to predict basic features of the W7-AS island divertor [3,4]. Recently the code has been extended to include a self-consistent treatment of impurity transport [5]. This paper presents first applications of the extended code to W7-AS for different edge densities up to detachment conditions and a discussion of the related physics. In addition, a new mapping technique is presented, which extends the application range of the EMC3-EIRENE code to open magnetic field structures like "open" islands and arbitrary stochastic fields.
2002
Article
http://edoc.mpg.de/20458
urn:ISSN:0863-1042
Contributions to Plasma Physics, v.42, 187-192 (2002)
en
oai:edoc.mpg.de:204602006-11-1433:1309
Ideal magnetohydrodynamic equations for low-frequency waves in toroidal plasmas
Fesenyuk, O. P.
Kolesnichenko, Y. I.
Wobig, H.
Yakovenko, Y. V.
Reduced linear equations of magnetohydrodynamics in high- aspect-ratio toroidal devices are derived, which are intended, first of all, for studying the Alfven eigenmodes in stellarators and tokamaks. The equations take into account the effects of the plasma pressure and compressibility, which are known to be of importance for toroidicity-induced Alfven eigenmodes, and are applicable to perturbations with arbitrary perpendicular wavelength. The reduction consists in eliminating high-frequency fast magnetoacoustic waves from the system and is shown not to affect the continuous spectrum of Alfven and slow magnetoacoustic waves, which, to a large extent, determines the behavior of the waves of interest. (C) 2002 American Institute of Physics.
2002
Article
http://edoc.mpg.de/20460
urn:ISSN:1070-664X
Physics of Plasmas, v.9, 1589-1595 (2002)
en
oai:edoc.mpg.de:204642006-11-1433:1309
Depth profiles and resolution limits in accelerator-based solid state analysis
Fischer, R.
depth profiles reconstruction; resolution enhancement; BPT
A ubiquitous problem in solid state analysis is the determination of the elemental composition of a sample as a function of the depth. The determination of the depth profiles from ion-beam experiments is an ill-posed inversion problem due to ion-beam and detector-induced energy spreads as well as energy-loss straggling and small-angle scattering effects. The inversion problem is solved in the framework of Bayesian probability theory, which provides a method for quantifying and combining uncertain data and uncertain additional information. By deconvolving the apparatus transfer function and modeling the scattering events in the sample we reconstructed depth profiles of C-13 in tetrahedral amorphous carbon (ta-C) and depth profiles in C-12/C-13 marker probes. An enhancement of the energy resolution by a factor of 6 was obtained.
2002
Article
http://edoc.mpg.de/20464
urn:ISSN:16182642
Analytical and Bioanalytical Chemistry, v.374, 619-625 (2002)
en
oai:edoc.mpg.de:204672006-11-1433:1309
Thomson scattering analysis with the Bayesian probability theory
Fischer, R.
Wendland, C.
Dinklage, A.
Gori, S.
Dose, V.
2002
Article
http://edoc.mpg.de/20467
urn:ISSN:07413335
Plasma Physics and Controlled Fusion, v.44, 1501-1519 (2002)
en
oai:edoc.mpg.de:204692006-11-1433:1309
Magnetic fluctuation probe design and capacitive pickup rejection
Franck, C. M.
Grulke, O.
Klinger, T.
In this article the capacitive pickup of magnetic fluctuation probes for plasma applications is studied. The nine most commonly used probe designs are compared with respect to their capacitive pickup rejection, magnetic sensitivity, and minimum plasma disturbance. For absolute calibration, well defined electric and magnetic field fluctuations are produced separately in a Faraday cup and in a Helmholtz magnetic field coil configuration, respectively. A sample measurement in a radio frequency helicon plasma demonstrates that the optimum probe design is well suited for measuring magnetic fluctuations in a plasma environment. (C) 2002 American Institute of Physics.
2002
Article
http://edoc.mpg.de/20469
urn:ISSN:0034-6748
Review of Scientific Instruments, v.73, 3768-3771 (2002)
en
oai:edoc.mpg.de:204752006-11-1433:1309
Transition from unbounded to bounded plasma whistler wave dispersion
Franck, C. M.
Grulke, O.
Klinger, T.
Whistler wave dispersion measurements are done in a linear magnetized helicon plasma experiment. The waves are excited by an induction loop and detected by movable magnetic probes for a frequency range of 100-800 MHz, corresponding to 0.05-0.9 omega(ce). The dispersion of whistler waves is measured for various plasma densities and magnetic field strengths. A key issue is to study the transition from an unbounded to bounded plasma wave dispersion. A comparison with theoretically derived dispersion relations is made. For small wavelengths, the dispersion can be described with whistler wave theory for unbounded plasmas whereas for larger wavelengths, the bounded geometry must be taken into consideration. The experimental results agree with theoretical dispersion relations derived for the bounded and the unbounded situation. (C) 2002 American Institute of Physics.
2002
Article
http://edoc.mpg.de/20475
urn:ISSN:1070-664X
Physics of Plasmas, v.9, 3254-3258 (2002)
en
oai:edoc.mpg.de:204792006-11-1433:1309
Interpretation of recent power width measurements in JET MkIIGB ELMy H-modes
Fundamenski, W.
Sipilä, S.
Matthews, G. F.
Riccardo, V.
Andrew, P.
Eich, T.
Ingesson, L. C.
Kiviniemi, T. P.
Kurki-Suonio, T.
Philipps, V.
EFDA-JET Work Programme Collaborators
2002
Article
http://edoc.mpg.de/20479
urn:ISSN:0029-5515
Plasma Physics and Controlled Fusion, v.44, 761-794 (2002)
en
oai:edoc.mpg.de:204802006-11-1433:1309
Improvement of the thermo-mechanical properties of fine grain graphite by doping with different carbides
Garcia-Rosales, C.
Ordas, N.
Oyarzabal, E.
Echeberria, J.
Balden, M.
Lindig, S.
Behrisch, R.
The possibilities for optimization of doped fine grain graphites with high thermal conductivity and high I thermal shock resistance are demonstrated at laboratory scale. A mixture of MCMB powder and different carbides (B4C, TiC, VC, ZrC and WC) was used as starting material. VC acts as catalyst of the graphitization at the lowest temperature, and ZrC is the most effective catalyst of all investigated carbides. A direct proportionality between the mean crystallite height, L-c, and the thermal conductivity at room temperature was found for all materials except for the B4C- and the ZrC-doped graphites. With increasing graphitization temperature the open porosity of all doped materials becomes gradually closed, suggesting the existence of a diffusion mechanism responsible for both the catalytic effect and the closing of the open porosity. The addition of carbides does not strongly influence the mechanical properties of pure graphite. A high ratio flexural strength to Young's modulus was achieved. (C) 2002 Elsevier Science B.V. All rights reserved.
2002
Article
http://edoc.mpg.de/20480
urn:ISSN:0022-3115
Journal of Nuclear Materials, v.307, 1282-1288 (2002)
en
oai:edoc.mpg.de:204812011-09-2833:1309
Comparative analysis of design options for current leads for Wendelstein 7-X magnet system
Gavrilin, A. V.
Miller, J. R.
Schauer, F.
van Sciver, S. W.
current lead optimizing; high temperature superconductors; over-current mode; Wendelstein 7-X
The Wendelstein 7-X Project at Max Plank Institute for Plasma Physics, Germany, has a need of analytical and engineering design study for forced flow cooled 18 kA current leads. The main criteria for the current leads' design should be reliability, serviceability, investment and total operating costs. Since the magnet duty cycle has very long idle current periods, two options look worth considering: gas-cooled conventional, current leads properly optimized for the duty cycle to operate in over-current mode (so-called. overloaded leads), and binary current leads with high temperature superconducting part. The options are analyzed in detail and optimized to minimize total operating cost with holding the serviceability in emergency case.
2002
Article
http://edoc.mpg.de/20481
urn:ISSN:1051-8223
IEEE Transactions on Applied Superconductivity, v.12, 1301-1304 (2002)
en
oai:edoc.mpg.de:204832006-11-1433:1309
Determination of the tungsten divertor retention at ASDEX Upgrade using a sublimation probe
Geier, A.
Maier, H.
Neu, R.
Krieger, K.
ASDEX Upgrade Team
For the measurement of the divertor retention at ASDEX Upgrade, a sublimation probe has been built with which it is possible to puff metallic impurities into the divertor plasma. This probe offers the opportunity to cross calibrate the influx of the metal with the influx of carbon or oxygen. For the measurement of the divertor retention R, a special deposition probe was used in the main chamber. The value found for R is in good agreement with earlier estimations from long-term deposition probes and computer modelling. The different measurements suggest a propagation of tungsten within the scrape-off layer from the divertor into the main chamber and from there across the separatrix into the confined plasma.
2002
Article
http://edoc.mpg.de/20483
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 2091-2100 (2002)
en
oai:edoc.mpg.de:204842011-09-1533:1309
Radiation power profiles and density limit with a divertor in the W7-AS stellarator
Giannone, L.
Burhenn, R.
McCormick, K.
Brakel, R.
Feng, Y.
Grigull, P.
Igitkhanov, Y.
NBI Team
ECRH Team
W7-AS Team
The addition of a divertor into the W7-AS stellarator has allowed access to a high density regime where the radiation profiles reach a steady state. In earlier limiter discharges, the plasma suffered a radiative collapse at high densities. In contrast to limiter experiments, where the impurity confinement time measured by Al laser blow-off increased with increasing line integrated density, in divertor discharges, above a density threshold, the impurity confinement time decreased with increasing line integrated density. The observation that the divertor plasma radiates mainly at the plasma edge rather than the plasma centre is a further indication that changes to the impurity transport coefficients at these high densities are the basis for the achievement of steady state discharges in the divertor configuration of W7-AS. The maximum line integrated density reached with a divertor is compared to that reached with a limiter. The previously derived scaling law for the density limit with a limiter shows that the achieved densities do not exceed those predicted when the higher deposited power is taken into account. In a divertor the radiated power is located at the plasma edge and increasing the density, cooling the plasma edge and radiating sufficient power to cause plasma detachment determines the density limit.
2002
Article
http://edoc.mpg.de/20484
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 2149-2165 (2002)
en
oai:edoc.mpg.de:204852011-09-1533:1309
Derivation of bolometer equations relevant to operation in fusion experiments
Giannone, L.
Mast, K.
Schubert, M.
NBI Team
ECRH Team
W7-AS Team
An overview of the bolometer diagnostic in the W7-AS stellarator is presented. Bolometers are used in plasma physics to measure radiated power losses that are mainly produced by impurity lines and neutral particles. With an array of collimated bolometers, the spatial and temporal evolution of these losses can be measured. The accuracy of the inversion of the line integrated measurements into local values necessitates a careful calibration and conversion of the measured signal into an incident power of each foil. Using transmission line theory to determine the equivalent circuit for the sensor and cable, the equations for bolometer calibration and measurement, taking into account the cable resistance and capacitance between amplifier and detector, are derived. The ability to switch the bolometer amplifiers from measurement to calibration mode between discharges allows continuous monitoring of the calibration constants of the bolometer foil. Measurements by bolometer cameras and their Abel inversion for the radial profile of radiated power in W7-AS are presented. (C) 2002 American Institute of Physics.
2002
Article
http://edoc.mpg.de/20485
urn:ISSN:0034-6748
Review of Scientific Instruments, v.73, 3205-3214 (2002)
en
oai:edoc.mpg.de:204862006-11-1433:1309
Helium and neon enrichment studies in the JET mark IIAP and mark IIGB divertors
Groth, M.
Andrew, P.
Fundamenski, W.
Guo, H. Y.
Hillis, D. L.
Hogan, J. T.
Horton, L. D.
Matthews, G. F.
Meigs, A. G.
Morgan, P. D.
Stamp, M. F.
Stork, D.
von Hellermann, M.
Adequate helium enrichment and exhaust have been achieved in reactor relevant ELMy H mode plasmas in JET performed in the Mark II divertor geometry. These quantities, describing the retention of impurities in divertors, have been experimentally inferred from CXRS measurements in the core plasma, and from a spectroscopic analysis of a Penning gauge discharge in the exhaust gas. The retention of helium was found to be sufficient with respect to the requirements in a next step device, with helium enrichment factors exceeding 0.1 in high density ELMy H mode discharges. With increasing core plasma density the helium partial pressure in the exhaust channel increases. While in L mode plasmas the helium enrichment decreases with increasing core plasma density, it remains almost constant in ELMy H mode plasma. The noble gas neon is better enriched in the divertor at high core plasma densities in both confinement regimes. These experimental results can be explained by the significant differences between the penetration depths of the impurity neutrals and by their subsequent different impurity transport mechanisms. Analytical and numerical analyses of these plasmas using the impurity code package DIVIMP/NIMBUS support the proposition that, owing to their much longer ionization mean free path, helium particles can escape from the divertor chamber as neutrals, while neon escapes by means of ion leakage. Consequently, the divertor plasma conditions strongly influence the noble gas compression and enrichment. Variations of the divertor plasma configuration and modifications to the divertor geometry have enhanced the pumping capabilities of the tokamak, but have been found not to affect the helium enrichment.
2002
Article
http://edoc.mpg.de/20486
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 591-600 (2002)
en
oai:edoc.mpg.de:204872006-11-1433:1309
Large-scale fluctuation structures in plasma turbulence
Grulke, O.
Klinger, T.
The role of large-scale fluctuation structures in electrostatic drift-wave-type plasma turbulence is highlighted. In particular, well-defined laboratory experiments allow one to study the dynamics of drift wave mode structures as well as 'eddies' in drift wave turbulence. In the present paper we discuss the mutual relationships between observations made in linear magnetic geometry, purely toroidal geometry and magnetic confinement. The simplest structure, a saturated, nonlinear drift mode, is the starting point for a Ruelle-Takens-Newhouse transition route to chaos and weakly developed turbulence. Both spectral and phase space analysis are applied to characterize in detail the transition scenario, which is enforced due to an increased drive by the plasma equilibrium state. In addition to direct multi-probe observation, statistical approaches are most revealing for the systematic study of the spatiotemporal dynamics in fully developed drift wave turbulence. In particular, the propagation of large-scale 'eddy' structures is traced by conditional statistics methods. Finally, the control of drift wave turbulence by spatiotemporal synchronization is discussed.
IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
2002
Article
http://edoc.mpg.de/20487
urn:ISSN:1367-2630
New Journal of Physics, v.4 (2002)
en
oai:edoc.mpg.de:204882006-11-1433:1309
Temporal evolution of neoclassical tearing modes and its effect on confinement reduction in ASDEX Upgrade
Gude, A.
Günter, S.
Maraschek, M.
Zohm, H.
It is shown that (3,2) neoclassical tearing modes (NTMs) in ASDEX Upgrade can, under similar external control parameters, occur at significantly different values of beta(N), with the NTMs occurring at higher beta(N) sometimes having a less detrimental impact on the energy confinement. This is found to be due to a coupling between the (3,2) NTM and a (4,3) mode, which can either occur continuously and clamp the (3,2) amplitude for the time of the existence of the (4,3) or occur in bursts on a timescale much shorter than the usual resistive timescale, where each burst leads to a rapid reduction of the (3,2) NTM called an amplitude drop. The amplitude drops may be due to stochastization in the presence of two islands of different helicity and sufficient amplitude, which can act as an anomalous increase in resistivity. These observations point towards the possible existence of a regime where the energy confinement deterioration due to NTMs is acceptable for reactor scale experiments.
2002
Article
http://edoc.mpg.de/20488
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 833-840 (2002)
en
oai:edoc.mpg.de:204892006-11-1433:1309
Abstraction of sulfur from Pt(111) surfaces with thermal H atoms toward adsorbed and gaseous H2S
Güttler, A.
Kolovos-Vellianitis, D.
Zecho, T.
Küppers, J.
atom-solid reactions; surface chemical reaction; platinum; sulphur; hydrogen sulphide
Sulphur layers on Pt(1 1 1) surfaces with coverages of 0.25 and 0.33 were prepared by H2S adsorption at 85 K and subsequent annealing. If,S adsorption on Pt, S/Pt and H/Pt surfaces and S adsorbate layers were characterized by Auger electron and thermal desorption spectroscopies. Admission of thermal H atoms to S covered Pt(I 1 1) at 85 K leads to formation of gaseous (80%) as well as adsorbed H2S (20%). The yield of adsorbed H2S decreases due to its isothermal desorption above 90 K. The interaction of H(g) with S(a) involves three reaction steps: 1. H(g) + S(a) --> SH(a), 2. H (9) + SH(a) --> H2S(g, a), and 3. H(g) + SH(a) --> H,(g) + S(a) with apparent cross-sections sigma = 0.3 Angstrom(2), sigma(2) = 0.6 Angstrom(2) and sigma(3) = 0.03 Angstrom(2). Above 140 K the hydrogenation of SH toward H2S(a,g) is blocked by thermal decomposition of H2S. Impact of D on coadsorbed S, SH, and H'S leads to desorption of H2S. (C) 2002 Elsevier Science B.V. All rights reserved.
2002
Article
http://edoc.mpg.de/20489
urn:ISSN:0039-6028
Surface Science, v.516, 219-229 (2002)
en
oai:edoc.mpg.de:204902006-11-1433:1309
Superconducting coil system for a Stellarator fusion reactor
Harmeyer, E.
Kisslinger, J.
Wieczorek, A.
Wobig, H.
Helias reactor; Stellarator fusion reactor; superconducting coil system
A superconducting coil system has been developed for the Stellarator fusion reactor. The distributions of the magnetic field and the forces are calculated. In order to support the magnetic forces a scheme of mutual support of the coils is applied. The pancakes of the coil winding pack of each individual coil are embedded in a strong coil housing of stainless steel. Linear and nonlinear finite element computations are done with half a field period, taking into account the stellarator symmetry. The mechanical stress and strain distributions inside the winding pack and the coil structure are analyzed. Furthermore, for the superconducting coil system the quench protection system is addressed.
2002
Article
http://edoc.mpg.de/20490
urn:ISSN:1051-8223
IEEE Transactions on Applied Superconductivity, v.12, 558-561 (2002)
en
oai:edoc.mpg.de:204912006-11-1433:1309
Stellarators
Hartmann, D. A.
Stellarators are toroidal devices where the required rotational transform of the magnetic field lines is generated by external field coils and not via an induced net toroidal plasma current. This confinement scheme has the advantages that, in principle, steady-state plasma operation is possible and that it does not have to brace itself against disruptions of a toroidal plasma current. At the cost of having to give up toroidal symmetry the properties of the stellarator field can be tailored to suit reactor needs. Research focuses on the plasma confinement properties of different stellarator fields and investigates the problems arising when one extrapolates to reactor parameters.
2002
Article
http://edoc.mpg.de/20491
urn:ISSN:1536-1055
Transactions on Fusion Science and Technology, v.41, 56-67 (2002)
en
oai:edoc.mpg.de:205072011-07-1833:1309
The formation and evolution of extreme shear reversal in JET and its influence on local thermal transport
Hawkes, N. C.
Andrew, Y.
Challis, C. D.
De Angelis, R.
Drozdov, V.
Hobirk, J.
Joffrin, E.
Lotte, P.
Mazon, D.
Rachlew, E.
Reyes-Cortes, S.
Sattin, F.
Solano, E.
Stratton, B. C.
Tala, T.
Valisa, M.
EFDA-JET Work Programme Collaborators
In JET discharges where lower hybrid heating and current drive (LHCD) is applied early during the current ramp, a region of the plasma with zero current density is formed near the axis. At the boundary of this region the current density is large and B-theta increases rapidly over a small distance. In the central region the safety factor, q, is effectively infinite, but this falls steeply in the boundary region. Outside the boundary region q reaches a minimum, where the magnetic shears equivalent to r/q (dq/dr) becomes zero. The formation of this region of zero current is dependent on both the heating and the current drive effects of the LHCD. When LHCD is switched off the current profile begins to relax towards the resistive peaked current distribution of fully inductive tokamak operation. If LHCD is not used in the current rise then these current profiles are not established. Although the physical mechanism exists to drive the central plasma current below zero, in most cases it appears to be prevented from going negative. At least one MHD mechanism has been identified which could be responsible for this. The presence of the zero central current is closely linked to the periodic relaxation events seen in these discharges. In these discharges, internal transport barriers have been observed with additional heating powers substantially below the values required to obtain barriers in monotonic q profile cases.
2002
Article
http://edoc.mpg.de/20507
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 1105-1125 (2002)
en
oai:edoc.mpg.de:205092006-11-1433:1309
Observation of excitonic satellites in the photoelectron spectra of Ne and Ar clusters
Hergenhahn, U.
Kolmakov, A.
Riedler, M.
de Castro, A. R. B.
Lofken, O.
Möller, T.
We have recorded synchrotron radiation excited photoelectron spectra of free Ne and Ar cluster beams in the valence and inner valence region. Varying the cluster sizes from a few up to some hundred atoms, the development of inelastic energy losses of the outgoing photoelectrons is clearly visible. The first few energy loss features can be related to creation of excitons and interband transitions within the cluster. (C) 2002 Elsevier Science B.V. All rights reserved.
2002
Article
http://edoc.mpg.de/20509
urn:ISSN:0009-2614
Chemical Physics Letters, v.351, 235-241 (2002)
en
oai:edoc.mpg.de:205112006-11-1433:1309
Vibrational excitation of C 1s-ionized ethane
Hergenhahn, U.
Kugeler, O.
Rennie, E. E.
Rüdel, A.
Marburger, S.
The effects of interaction between the vibrational and electronic structure, so-called vibronic coupling, are reviewed in the context of core-ionized states. High resolution C Is spectra of ethane and deuterated ethane are used as an example to compare our reasoning with measurements.
2002
Article
http://edoc.mpg.de/20511
urn:ISSN:0218-625X
Surface Review and Letters, v.9, 13-19 (2002)
en
oai:edoc.mpg.de:205152006-11-1433:1309
Overview on stationary and transient divertor heat loads
Herrmann, A.
The divertor concept and the divertor materials envisaged for ITER FEAT restrict the maximum values of the stationary and transient heat fluxes. The maximum stationary heat flux is limited by the active cooling structure. The limit for transient events is given by the maximum tolerable surface temperature. This paper will review different options for divertor heat flux reduction. Proper geometric orientation of the divertor targets reduces the heat flux to the target plates at most by a factor of 60 relative to the parallel heat flux determined by upstream transport. The radiation capability of closed divertor configurations is doubled compared to open configurations. A further reduction of maximum heat load can be achieved by increasing the wetted area in double-null plasma configurations. The energy transported by transient events (type I ELMS) is up to 30% of the total energy deposited in the divertor. The heat flux profile during ELMS is not significantly broadened and is comparable to the profile between ELMS. The duration of the energy deposition varies with edge plasma parameters between 0.1 and 1 ms. The expected transient heat flux for ITER FEAT conditions is above the tolerable value. This requires the further development of operation scenarios with tolerable ELMS (e.g. type 11) preserving the high energy confinement of type I ELM scenarios.
2002
Article
http://edoc.mpg.de/20515
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 883-903 (2002)
en
oai:edoc.mpg.de:205172006-11-1433:1309
Electron heated internal transport barriers in JET
Hogeweij, G. M. D.
Baranov, Y.
Conway, G. D.
Cortes, S. R.
De Baar, M. R.
Hawkes, N.
Imbeaux, F.
Litaudon, X.
Mailloux, J.
Rimini, F. G.
Sharapov, S. E.
Stratton, B. C.
Zastrow, K. D.
EFDA-JET Work Programme Collaborators
By applying lower hybrid current drive (LHCD) for strong electron heating and off-axis current drive starting very early in the discharge, an electron internal transport barrier (eITB) can be generated. The barrier is formed just inside the location of minimum q, and slowly moves inward with this location. During the current flat-top the barrier can be sustained during many seconds, either with continued LHCD, or by ion cyclotron resonance heating. In this paper both scenarios are analysed and compared.
2002
Article
http://edoc.mpg.de/20517
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 1155-1165 (2002)
en
oai:edoc.mpg.de:205192006-11-1433:1309
Chemical sputtering of hydrocarbon films by low-energy Ar+ ion and H atom impact
Hopf, C.
von Keudell, A.
Jacob, W.
2002
Article
http://edoc.mpg.de/20519
urn:ISSN:0029-5515
Nuclear Fusion, v.42, L27-L30 (2002)
en
oai:edoc.mpg.de:205202006-11-1433:1309
Dependence of H-mode pedestal parameters on plasma magnetic geometry
Horton, L. D.
Hatae, T.
Hubbard, A.
Janeschitz, G.
Kamada, Y.
Kurzan, B.
Lao, L.
McCarthy, P. J.
Mossessian, D.
Osborne, T. H.
Pinches, S. D.
Saarelma, S.
Sugihara, M.
Suttrop, W.
Thomsen, K.
Urano, H.
2002
Article
http://edoc.mpg.de/20520
Plasma Physics and Controlled Fusion, v.44, A273-A278 (2002)
en
oai:edoc.mpg.de:205212006-11-1433:1309
Observation of mode patterns for high purity mode operation in the submillimeter wave gyrotron
Idehara, T.
Ogawa, I.
Maeda, S.
Pavlichenko, R.
Mitsudo, S.
Wagner, D.
Thumm, M.
2002
Article
http://edoc.mpg.de/20521
International Journal of Infrared and Millimeter Waves, v.23, 973-980 (2002)
en
oai:edoc.mpg.de:205222006-11-2333:1309
A physics picture of type I ELMs
Igitkhanov, Y.
Pogutse, O. P.
Wilson, H. R.
Connor, J. W.
Wobig, H.
Grigull, P.
Hirsch, M.
Janeschitz, G.
Sugihara, M.
Loarte, A.
Saibene, G.
Sartori, R.
Pacher, G.
Pacher, H.
2002
Article
http://edoc.mpg.de/20522
urn:ISSN:0863-1042
Contributions to Plasma Physics, v.42, 272-276 (2002)
en
oai:edoc.mpg.de:205232006-11-1433:1309
Seeding of impurities in JET H-mode discharges to mitigate the impact of ELMs
Jachmich, S.
Maddison, G.
Beurskens, M. N. A.
Dumortier, P.
Eich, T.
Messiaen, A.
Nave, M. F. F.
Ongena, J.
Rapp, J.
Strachan, J.
Stamp, M.
Telesca, G.
Unterberg, B.
EFDA-JET Work Programme Collaborators
ELMy H-mode is one of the foreseen operational scenarios for ITER-FEAT. Good confinement and density is usually associated with Type I ELMs, which, however, will have a severe impact on the divertor target plates in ITER. At the JET-tokamak, impurities have been seeded into a variety of discharge configurations, by which the radiation level can be risen by a factor of 2. This delays the appearance of ELMs and reduces the power load onto the divertor target plates. In this paper, the influence of the impurities on the ELM behaviour and the plasma edge properties will be discussed. Several divertor diagnostics, such as electrical probes and infrared camera indicate a reduction of the heat power load at the inner divertor target tiles and a shrinking of the impact zone at the outer divertor.
2002
Article
http://edoc.mpg.de/20523
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 1879-1891 (2002)
en
oai:edoc.mpg.de:205252006-11-1433:1309
A new quasi-stationary, very high density plasma regime on the W7-AS stellarator
Jaenicke, R.
Bäumel, S.
Baldzuhn, J.
Brakel, R.
Burhenn, R.
Ehmler, H.
Endler, M.
Erckmann, V.
Feng, Y.
Gadelmeier, F.
Geiger, J.
Giannone, L.
Grigull, P.
Hartfuß, H.-J.
Hartmann, D.
Hildebrandt, D.
Hirsch, M.
Holzhauer, E.
Kick, M.
Kisslinger, J.
Klinger, T.
Klose, S.
Knauer, J.
König, R.
Kühner, G.
Laqua, H.
Maaßberg, H.
McCormick, K.
Narayanan, R.
Niedermeyer, H.
Pasch, E.
Ruhs, N.
Rust, N.
Saffert, J.
Sardei, F.
Schneider, F.
Schubert, M.
Speth, E.
Wagner, F.
Weller, A.
Wenzel, U.
Werner, A.
Würsching, E.
Stellarators have the intrinsic property of steady state operation. However, on present-day stellarators the pulse length is usually not only limited due to technical reasons, but also by physical problems. Lack of density control and a subsequent radiation collapse terminate the discharges quite often at high densities. To improve the control of the plasma- wall interaction, the island divertor concept was developed for optimized stellarators. To test this divertor concept on W7-AS, all limiters were removed and replaced by ten divertor modules. In subsequent divertor experiments a promising new plasma operational regime has been discovered which is termed 'high density H-mode' (HDH-mode). During the transition into that regime a clear reduction of ELM-like events and turbulent fluctuations is observed. The HDH-mode combines good energy confinement with very low impurity confinement resulting in low core radiation, but high edge-localized radiation. Consequently, stationary discharges at densities of typically 2 x 10(20) m(-3) can be performed within the accessible pulse length of about 1 s. At densities above 3 x 10(20 m-3) a controlled transition from attached to partially detached plasmas is observed. The still edge-localized radiation reaches 90% of the heating power so that the power load onto the divertor target plates is further reduced. At a lower toroidal field of 0.9 T average beta-values could be raised from earlier 2% to more than 3% in magnetic field configurations with rather smooth flux surfaces at the plasma boundary. The recently obtained results render excellent prospects for W7-X, the larger superconducting successor experiment of W7-AS.
2002
Article
http://edoc.mpg.de/20525
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, B193-B205 (2002)
en
oai:edoc.mpg.de:205272006-11-1433:1309
Prediction of significant tokamak turbulence at electron gyroradius scales
Jenko, F.
Dorland, W.
The experimental conditions under which tokamak turbulence at hyperfine (electron gyroradius) scales is predicted to be significant and observable are described. The first quantitative predictions of fluctuation amplitudes, spectral features, and the associated electron energy transport are presented. A novel theoretical model which quantitatively describes the boundaries of the high-amplitude streamer transport regime is presented and shown to explain the gyrokinetic simulation results. This model uniquely includes consideration of two distinct secondary instabilities.
2002
Article
http://edoc.mpg.de/20527
urn:ISSN:0031-9007
Physical Review Letters, v.89 (2002)
en
oai:edoc.mpg.de:205292006-11-1433:1309
Radial and zonal modes in hyperfine-scale stellarator turbulence
Jenko, F.
Kendl, A.
Electromagnetic plasma turbulence at hyperfine (i.e., electron gyroradius) scales is studied in the geometry of an advanced stellarator fusion experiment, Wendelstein 7-AS [H. Renner, Plasma Phys. Controlled Fusion 31, 1579 (1989)], by means of nonlinear gyrokinetic simulations. It is demonstrated that high-amplitude radial streamers may also exist in non-tokamak devices, raising the electron heat flux to experimentally relevant values. Moreover, some statistical characteristics of the fully developed turbulence are computed, highlighting the (co-)existence, nature, and role of self-generated zonal flows and fields. (C) 2002 American Institute of Physics.
2002
Article
http://edoc.mpg.de/20529
urn:ISSN:1070-664X
Physics of Plasmas, v.9, 4103-4106 (2002)
en
oai:edoc.mpg.de:205302006-11-1433:1309
Stellarator turbulence at electron gyroradius scales
Jenko, F.
Kendl, A.
Electromagnetic gyrokinetic simulations of electron- temperature-gradient-driven modes on electron gyroradius scales are performed in the geometry of an advanced stellarator fusion experiment, Wendelstein 7-AS. Based on linear simulations, a critical electron- temperature-gradient formula is established which happens to agree quite well with a previously derived formula for tokamaks in the appropriate limit. Nonlinear simulations are used to study the turbulence and transport characteristics which are dominated by the presence of high- amplitude radially elongated vortices or 'streamers'. The role of Debye shielding effects is also examined.
IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
2002
Article
http://edoc.mpg.de/20530
urn:ISSN:1367-2630
New Journal of Physics, v.4 (2002)
en
oai:edoc.mpg.de:205762006-11-1433:1309
q=1 advanced tokamak experiments in JET and comparison with ASDEX Upgrade
Joffrin, E.
Wolf, R.
Alper, B.
Baranov, Y.
Challis, C. D.
de Baar, M.
Giroud, C.
Gowers, C. W.
Hawkes, N. C.
Hender, T. C.
Maraschek, M.
Mazon, D.
Parail, V.
Peeters, A.
Zastrow, K. D.
EFDA-JET Work Programme Collaborators
The ASDEX Upgrade advanced tokamak scenario with central q close to 1 has been reproduced on JET. For almost identical q profiles, the comparative analysis does show similar features like the fishbone activity and the current profile evolution. In JET, transport analyses indicates that an internal transport barrier (ITB) has been produced. Gradient length criterions based on the ion temperature gradient turbulence stabilization are used to characterize the ITBs in both devices. The trigger of ITBs is associated with rational surfaces in both devices although the underlying physics for this triggering seems different. This experiment has the prospect to get closer to identity experiments between the two tokamaks.
2002
Article
http://edoc.mpg.de/20576
urn:ISSN:0741-3335
Plasma Physics and Controlled Fusion, v.44, 1203-1214 (2002)
en
oai:edoc.mpg.de:205782011-06-1533:1309
Scaling of the pedestal density in type-I ELMy H-mode discharges and the impact of upper and lower triangularity in JET and ASDEX Upgrade
Kallenbach, A.
Beurskens, M. N. A.
Korotkov, A.
Lomas, P.
Suttrop, W.
Charlet, M.
McDonald, D. C.
Milani, F.
Rapp, J.
Stamp, M.
EFDA-JET Work Programme Collaborators
ASDEX Upgrade Team
Experiments have been performed to disentangle the individual role of tipper and lower triangularity on density buildup of lower single null, type-I ELMy H-mode discharges in JET. Comparison with corresponding data from ASDEX Upgrade allowed us to determine a dimensionless representation of the relation between the main chamber recycling and core density and to widen the triangularity variation in the data base. To incorporate the recycling flux density F in a dimensionless form, an effective scrape-off layer density n(e,SOL) proportional to Gamma(0.5) is introduced allowing us to,,SOL parametrize and scale the core density by the density rise factor (n) over bar (e)/n(e,SOL). The scaling uses edge- specific definitions of normalized Larmor radius, collisionality and beta. Rewritten in dimensional form, a behaviour is found which is very similar to energy confinement scalings for the ELM-averaged conditions considered here: the density rise factor exhibits an almost linear dependence on plasma current, a weak negative toroidal field dependence as well as power degradation. While a pronounced positive dependence of density buildup on the upper triangularity is observed, no significant correlation with the lower triangularity is found. In particular the dependences on plasma current and upper triangularity emphasize the importance of transport physics for the density buildup.
2002
Article
http://edoc.mpg.de/20578
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 1184-1192 (2002)
en
oai:edoc.mpg.de:205872006-11-1433:1309
Decomposition of multicomponent mass spectra using Bayesian probability theory
Kang, H. D.
Preuss, R.
Schwarz-Selinger, T.
Dose, V.
mass spectra decomposition; mixed gases; Bayesian probability theory; cracking coefficients; concentrations
2002
Article
http://edoc.mpg.de/20587
urn:ISSN:10765174
Journal of Mass Spectrometry, v.37, 748-754 (2002)
en
oai:edoc.mpg.de:205892006-11-1433:1309
On estimating the epistemic probability of realizing Q = Pfus / Paux larger than a specified lower bound in ITER
Kardaun, O. J. W. F.
A simplified analysis is given of the problem of estimating the 'epistemic probability' for ITER to attain a power amplification factor Q larger than a certain lower bound. Attention is restricted to the parameters of ITER-FEAT and the 1998 ITER design. The probabilistic framework is briefly discussed and the distributional interval estimates for Q following from the interval estimates of the confinement time are derived (a) for ITER operation at constant fusion output power P-fus, and (b) for operation at a temperature that maximizes Q = P-fus/P-aux. The second situation requires a radial integration of the flux surface averaged energy balance. Instead of a local transport model, a simple class of temperature profiles is used. The results are represented graphically. A generalization of the widely used fusion triple product plot against temperature is suggested. The analysis presupposes that at the reference operating point in situation (a), and in the range of operating temperatures projected to be achievable in situation (b), the conditions for reaching standard ELMy H mode in ITER are met. The practical conclusion of the article is that under this premise ITER-FEAT has a fairly large epistemic probability of obtaining plasma conditions with prevalent alpha particle heating.
2002
Article
http://edoc.mpg.de/20589
urn:ISSN:0029-5515
Nuclear Fusion, v.42, 841-852 (2002)
en
oai:edoc.mpg.de:205952006-11-1433:1309
On the 'Magnetic' Nature of electron transport barriers in tokamaks
Kasilov, S. V.
Reiter, D.
Runov, A. M.
Kernbichler, W.
Heyn, M. F.
2002
Article
http://edoc.mpg.de/20595
Plasma Physics and Controlled Fusion, v.44, 985-1004 (2002)
en
oai:edoc.mpg.de:205972006-11-1433:1309
Linear aspects of streamer formation in electron temperature gradient turbulence
Kendl, A.
The dependence of radially elongated streamer formation in toroidal electron temperature gradient driven turbulence on the magnetic field geometry (specified by shear s and shift alpha) is well described by means of a linear gyrokinetic analysis of the radial mode number spectrum. (C) 2003 American Institute of Physics.
2003
Article
http://edoc.mpg.de/20597
urn:ISSN:1070-664X
Physics of Plasmas, v.10, 317-318 (2003)
en
ResultSet_1n4m1zr2eay_range_100-199