Investigation of transport-related fluctuations in the boundary layer of ASDEX
In a tokamak, the length of time for which particles and energy can be confined is determined by their transport perpendicular to the magnetic field. In experiments, a transport increased by several orders of magnitude is sometimes observed, which remains unexplained and cannot be predicted (anomalous transport). In addition to the macroscopic sawtooth instability of density and temperature in the centre of the plasma, there are also microscopic fluctuations in density, potential, temperature and magnetic field, which are associated with anomalous transport. An attempt was made to determine the relationship between fluctuations of density and potential in the plasma boundary layer in ASDEX and the transport of particles. The measurement techniques employed (H(alpha)-diagnosis, Langmuir probes) provide high temporal and spatial resolution. In the statistical analysis, correlation lengths for the fluctuations are determined - radial, poloidal and parallel with respect to the magnetic field. By means of power density spectra and coherence spectra, the fluctuation amplitude is shown for the first time to be asymmetrical between the inner boundary layer and the outer boundary layer. There is an extremely strong correlation in the fluctuation along the lines of magnetic field. In consequence, a relatively simple, two-dimensional theoretical model can be provided. It is shown how auxiliary heating by neutral gas injection affects the fluctuations and how their characteristics change when high-confinement mode is achieved. Finally, characteristics observed in ASDEX are compared with results from other machines, including Q-machines, and the experimental results are discussed.
Bibliographic Reference: Report: IPP III/160 DE (1990) 120 pp.
Availability: Available from Max-Planck-Institut für Plasmaphysik, 8046 Garching bei München (DE)
Record Number: 199011470 / Last updated on: 1994-12-02
Original language: de
Available languages: de