Model validation, dynamic edge localized mode discrimination, and high confidence resistive wall mode control in DIII-D

A linear model for feedback stabilization of n=1 resistive wall modes (RWMs) in the DIII-D [T.C. Simonen, J. Fusion Energy 11, 79 (1992)] tokamak is presented and validated with recent experimental data. The model uses a toroidal current sheet to represent the plasma surface and “picture frame” currents to represent the conducting structure. Since the model does not account for plasma rotation, recent low rotation DIII-D discharges are vital for validation. It is shown that edge localized modes (ELMs) cause the system to become unstable in DIII-D by affecting the magnetic field sensor measurements, and thus, exciting the active coils even though the RWM is already stabilized. Two procedures for discriminating ELMs from the sensor signals are suggested and by combining the two approaches, the ELM contributions in the closed loop can be removed almost completely. Filtered sensor signals and a validated closed loop model facilitates high confidence RWM feedback stabilization. Controllers with high stability robustness and low sensitivity to noise are designed, and achievable closed loop performance for different sensor configurations in DIII-D are compared.