Time-Dependent Modeling of Feedback Control of Resistive Wall Modes in Low-Rotation DIII-D Plasmas

Resistive wall modes (RWMs) are plasma instabilities that occur at high beta in magnetically confined plasmas. In DIII-D, suppressing these instabilities via plasma rotation has allowed up to a factor of 2 increase in beta [1], which is a significant step forward toward realizing an economical fusion reactor. In the absence of plasma rotation, active feedback using magnetic coils is a promising approach to RWM stabilization. The finite bandwidth of realistic electronics poses two limitations on a linear feedback system: 1) limitation on the largest stabilizable growth rate, and 2) limitation on the largest stable feedback gain. Here, time-dependent simulations with a simple, ideal magnetohydrodynamics model show that the largest, linearly stable feedback gain can be exceeded with hardware limitations providing pseudo-stability. The largest stabilizable growth rate depends on the type of feedback sensor and algorithm. Modeling of the DIII-D feedback system is compared to experimental results.\par \vskip6pt \noindent [1] A.M. Garofalo, et al., Phys. Plasmas 9, 1997 (2002).