Active Control of Kink Modes Using a Non-magnetic, Extreme Ultraviolet Sensor Array

Mode control in tokamaks often utilizes nonaxisymmetric magnetic sensors and actuators near the plasma surface. Placing magnetic coils behind walls would improve their longevity in a reactor at the expense of reducing frequency response, while a light-based detector could still respond quickly and with greater spatial sensitivity. We present the first demonstration of kink mode feedback control using only non-magnetic sensors consisting of extreme ultraviolet (EUV) detector arrays. Sixty-four poloidal views measure internal and external mode dynamics via plasma emissivity. Singular Value Decomposition (SVD) of EUV measurements is used to establish a basis set for calculating amplitude and phase of rotating perturbations. The poloidal spectrum of applied fields can adapt to changing structure of emissivity in real time. Feedback is completed using a graphics processing unit (GPU)-based control system with a total latency of 22$\mu$s [1]. We observe mode suppression and amplification as a function of the applied feedback phase angle relative to measured emissivity fluctuations. The system can directly extend to real-time tomographic reconstructions for mode or equilibrium control using an appropriate series-expansion method.\\ {[1]}N. Rath et al., Rev. Sci. Instr. 85, 045114 (2014)