Key Results from the DIII-D/TEXTOR Collaboration on the Physics of Stochastic Boundaries projected to ELM Control at ITER

Full text: In this paper key results of a direct comparison of TEXTOR and DIII-D experiments with external resonant magnetic perturbation (RMP) fields are presented. These comparisons of resistive, low rotation L-mode plasmas at TEXTOR to highly conductive, high rotation H-mode plasmas at DIII-D identify generic physics mechanisms of the application of RMP fields with a strong field line pitch angle alignment in the plasma edge. We show evidence that this pitch alignment induces a stochastic edge layer in both very different plasma regimes with at least a thin layer of open field lines in the plasma edge breaking the axis-symmetry of the tokamak introducing a new three-dimensional (3D) plasma boundary. In both plasma regimes a reduction of the electron pressure pe with increasing extension of the modeled stochastic layer and pe recovery with decreasing stochastic layer width is found. This pe(q95) resonant modulation is caused consistently by a strongly q95 resonant reduction of the electron temperature correlated to the stochastic layer width. This compares well to the generic physics picture of an open stochastic layer and the pe reduction induced is identified as a pre-requisite for stabilizing type-I ELMs by RMP. The connection between the magnetic topology and plasma transport was studied with the EMC3-Eirene plasma and neutral transport code and revealed a comparable impact on both devices of the perturbed layer on the plasma profiles and the plasma wall interaction. The potential to reduce the RMP induced particle pump out by fine tuning the RMP spectral properties was demonstrated experimentally. At low resonant field amplitudes enhanced particle confinement is shown in high field side limited L-mode discharges on both devices while higher resonant field amplitude yield particle pump out. This shows the potential for optimizing ELM control by RMP towards maintaining high plasma density and proves the importance of taking into account the 3D plasma boundary induced during ELM suppression for design of first wall components at ITER. This work was supported by the US Department of Energy under DE-FG03-97ER54415, DE-AC52-07NA27344, DE-FC02-04ER57698, DE-FG02-07ER54917, DE-AC05-DOOR22725 and DE-FG02-89ER53297. (author)