Control of rotating helical magnetic field penetration into tokamak plasmas using electrode biasing in HYBTOK-II

Detailed measurements of rotating helical field (RHF) have been carried out in a small tokamak device HYBTOK-II in order to know the interaction between the tokamak plasma and RHF. The magnetic pick-up coils were inserted deeply inside the plasma to detect the response profile of tokamak plasma to the RHF. It was found that a large relative velocity between the plasma and RHF poloidal rotation velocities gives rise to a strong attenuation of RHF in the plasma because the large screening current flows around the resonance surface against the externally applied RHF. The electrode biasing was applied so as to increase the relative velocity so that the RHF was strongly attenuated in comparison with unbiased case.