Active Feedback Control of the Wall Stabilized External Kink Mode*

Active feedback control has been used in the HBT-EP tokamak to control the growth of the n=1 resistive wall mode. These experiments were carried out using a set of thin stainless-steel wall segments with magnetic diffusion time of ~0.4 ms positioned near the plasma boundary. In plasmas that would normally exhibit a strong ideal n = 1 external kink mode without a nearby conducting wall, the resistive wall slows the growth of the external kink to the ~1 ms time scale where it can be stabilized by active feedback control. The approach taken in these experiments is to use a network of active feedback coils mounted on the surface of the stainless-steel wall segments and driven by an active feedback control system that simulates the electrical response of a superconducting wall and minimizes the radial flux penetration of the perturbed mode field through the wall. This implementation of the so called ‘smart shell’ in HBT-EP has 30 independent sensor/driver feedback loops.