Suppression of rotating external kink instabilities using optimized mode control feedback

Rotating external kink instabilities have been suppressed as well as excited in a tokamak using active magnetic coils that directly couple to the plasma through gaps in passive stabilizing conducting shells that surround the plasma. The kink instability has a complex growth rate, approximately (3+i2π5)×103s−1, and is near the ideal wall stability limit when discharges are prepared with a rapid plasma current ramp and adjusted to have an edge safety factor near 3. The active control coils are driven by a digital mode control feedback system that uses multiple field-programmable gate arrays to analyze signals from 20 poloidal field sensors and achieve high-speed feedback control. The feedback coil geometry used was designed to optimize feedback effectiveness. Signal processing is of critical importance to optimize phase transfer functions for control of rotating modes.