Interaction of externally applied rotating helical field with tokamak plasma

The penetration process of a rotating magnetic perturbation into a rotating tokamak plasma has been investigated, taking account of the mode structure of the perturbations, and using a small magnetic probe in the small research tokamak CSTN-IV. It was observed that the radial component of the perturbation was amplified in the plasma when magnetic islands are formed, while the poloidal component was attenuated but, deep inside the plasma, amplified. Using a resistive MHD code, such modifications of external perturbation were found to be caused by a redistribution of plasma current due to the formation of magnetic islands. Considering the phase velocity of both the perturbation and the intrinsic plasma rotation, it was also found that the sideband component substantially affects the penetration process with magnetic islands forming. Therefore, in dynamic ergodic divertor operation, it is necessary to put the emphasis not only on the frequency of the perturbation fields but also on their direction, mode structure and the radial profile of the intrinsic plasma rotation at the same time.