TRANSPORT ANALYSIS OF THE EDGE ZONE OF H-MODE PLASMAS BY COMPUTER-SIMULATION

Local transport and ideal ballooning stability in the L-phase and ELM-free H-phase in ASDEX are analyzed by computer modelling. It is found that the diffusivities χe and D at the edge are reduced by a factor of six a few milliseconds after the H-transition. During the L-phase and the quiescent H-phase ideal ballooning modes are found to be stable. Computer experiments further show that a significant reduction in the particle flux at the separatrix takes place which is closely connected with the H-transition process. This explains the observed buildup of a density shoulder on a millisecond time-scale and the drop of the particle flow into the divertor. A strong decrease of the electron heat conduction flux at the separatrix is, however, ruled out in ELM-free periods. On the assumption of electrostatic turbulence induced transport, these results are consistent with measured density fluctuation levels near the separatrix.