Active mode control using current-injecting electrodes in a tokamak

Feedback control of instabilities is important for maintaining high performance tokamak plasmas near beta limits. A goal of this work is to expand the type of actuators, beyond 3D magnetic coils, that can be used to control instabilities. We demonstrate feedback control using an array of four biased electrodes that inject currents into the plasma along 3D paths. Electrode voltages are phased in response to magnetic sensors near the plasma surface. Resulting control current runs coherently along the magnetic field for some distance before conducting radially from electrodes to the wall, producing a geometry that naturally couples to modes. Injected currents suppress or amplify kink modes, depending on programmed phase between the detected fields and applied electrode voltages. Separate experiments with quiescent plasmas are used to map the driven current paths through the plasma by fitting magnetic sensors and wall currents. The VALEN code is then used to model the control system’s interaction with kink modes, including the fitted control filaments and passive wall segments. Biasing the electrodes 4cm into the plasma (r/a~0.85) leads to mode amplitude reduction of ~40%, while having the electrodes just inside the plasma edge (r/a~1) yields a suppression of ~30%.