Influence of rotating resonant magnetic perturbation on the plasma radial electric field on TEXTOR

The plasma radial electric field (E-r ) has been changed by applying an n = 1 counter-rotating resonant magnetic perturbation (RMP) field with a frequency of 5 kHz in ohmic plasmas on TEXTOR. The change in the E-r (Delta E-r) is negative, different from the observations in previous experiments where Delta E-r was always positive when a static or low frequency (similar to 1 kHz) rotating RMP field was applied in the plasma on TEXTOR. The Er profile in the present experiment shows two distinct evolution stages. In the first stage, Delta E-r from the q = 2 to q = 3 surfaces have a similar decrease as the amplitude of the 5 kHz counter-rotating field increases. In the second stage, the decrease rate of Delta E-r is faster for the positions closer to the q = 2 surface. As a result, the E-r around the q = 2 surface has a significant change in this second stage while no change of E-r is observed near the q = 3 surface even after the excitation of an m/n = 2/1 tearing mode. A reduced MHD code, 4FC, has been used to model the experiment. Two simulations have been performed. The first one is by applying a single 2/1 perturbation while both, 2/1 and 3/1 perturbations, have been applied in the second simulation. The result from the second simulation is qualitatively consistent with the experimental observations while the first simulation including only a single 2/1 perturbation cannot explain the evolution of the E-r profile in the second stage as observed in the experiment.