A system of coils, sensors and amplifiers has been installed on the DIII-Dtokamak to study the physics of feedback stabilization of low-frequency MHDmodes such as the resistive wall mode (RWM). Experiments are being performedto assess the effectiveness of this minimal system and benchmark thepredictions of theoretical models and codes. In the last campaign theexperiments had been extended to a regime where the RWM threshold is loweredby a fast ramp of the plasma current. In these experiments the onset time ofthe RWM is very reproducible. With this system, the onset of the RWM had beendelayed by up to 100 ms without degrading the plasma performance. The growth rateof the mode increases proportional to the length of the delay, suggesting that theplasma is evolving towards a more unstable configuration. The present resultshave suggested directions for improving the feedback system, including bettersensors and improved feedback algorithms.
Stability and control of resistive wall modes in high beta, low rotation DIII-D plasmas