Dynamics and control of resistive wall modes with magnetic feedback control coils: experiment and theory

Fundamental theory, experimental observations and modelling of resistive wall mode (RWM) dynamics and active feedback control are reported. In the RWM, the plasma responds to and interacts with external current-carrying conductors. Although this response is complex, it is still possible to construct simple but accurate models for kink dynamics by combining separate determinations for the external currents, using the VALEN code, and for the plasma's inductance matrix, using an magnetohydrodynamics code such as DCON. These computations have been performed for wall-stabilized kink modes in the HBT-EP device, and they illustrate a remarkable feature of the theory: when the plasma's inductance matrix is dominated by a single eigenmode and when the surrounding current-carrying structures are properly characterized, then the resonant kink response is represented by a small number of parameters. In HBT-EP, RWM dynamics are studied by programming quasi-static and rapid ‘phase-flip’ changes of the external magnetic perturbation and directly measuring the plasma response as a function of kink stability and plasma rotation. The response evolves in time, is easily measured, and involves excitation of both the wall-stabilized kink and the RWM. High speed, active feedback control of the RWM using VALEN-optimized mode-control techniques and high-throughput digital processors is also reported. Using newly installed control coils that directly couple to the plasma surface, experiments demonstrate feedback suppression of the kink instability in rapidly rotating plasmas near the ideal wall stability limit.