Effect of magnetic perturbation fields on power decay length in EMC3-EIRENE simulations and comparison to experiment in ASDEX upgrade

Abstract

Previous simulation results for the tokamak ASDEX Upgrade have shown that Magnetic Perturbation (MP) fields could lead to a decrease of the peak heat flux onto the divertor targets. However, experimentally, such a decrease has not been observed. To understand this effect, a well diagnosed ASDEX Upgrade discharge was simulated with the three dimensional transport code EMC3-EIRENE, where an ad hoc model was applied to mimic the screening field created by the plasma in response to the external MP field. The simulations show that only for sufficiently weak screening a broadening of the plasma wetted area occurs. The heat flux broadening effect was then studied more systematically by simulations where heat and particle diffusion coefficients were scanned for three different MP field strengths. The scans show that only for sufficiently low diffusion coefficients and high MP field strengths, a significant decrease of the peak heat flux can be achieved. Furthermore, the effect of a kink response, which is commonly observed during ELM suppression discharges in ASDEX Upgrade, was investigated with EMC3-EIRENE simulations. The simulation results indicate, that while a kink response can deform the midplane profiles strongly, the effect on the power decay length may still be small. Keywords: ASDEX upgrade, Magnetic perturbations, Divertor heat flux, MSC: 00-01, 99-00