Closed-loop feedback of MHD instabilities on DIII-D

A system of coils, sensors and amplifiers has been installed on the DIIID t okamak to study the physics of feedback stabilization of low-frequency MHD modes such as the resistive wall mode (RWM). Experiments are being performe d to assess the effectiveness of this minimal system and benchmark the pred ictions of theoretical models and codes, In the last campaign the experimen ts had been extended to a regime where the RWM threshold is lowered by a fa st ramp of the plasma current. In these experiments the onset time of the R WM is very reproducible. With this system, the onset of the RWM had been de layed by up to 100 ms without degrading the plasma performance. The growth rate of the mode increases proportional to the length of the delay, suggest ing that the plasma is evolving towards a more unstable configuration. The present results have suggested directions for improving the feedback system , including better sensors and improved feedback algorithms.