Investigation of the pump-out effect by resonant magnetic perturbations in ASDEX Upgrade

Future fusion reactors are foreseen to run in the high confinement mode at low collisionality due to the confinement requirements for self-sustained fusion power generation. However, this regime is accompanied by edge localized modes, which expell particles and energy in a pulsed manner. Since this instability has the potential to damage the first wall material, it has to be mitigated or suppressed. While resonant magnetic perturbations (RMPs) can achieve this, they have the drawback of causing a strong reduction of density. Theories trying to explain the transport mechanism underlying this ''pump-out'' effect are tested experimentally in ASDEX Upgrade. The results indicate that the resonance between the location of penetrating RMP fields and resonant surfaces is not crucial for the pump-out. Reflectometry measurements reveal the presence of toroidally asymmetric turbulence in the steepest gradient region. Those fluctuations correlate well with D-alpha light measurements in the scrape-off layer. Experiments with a modulation of the RMP field strength show, that the change in density happens first also in this region of asymmetric turbulence.