Semiquantitative analysis of feedback systems for resistive wall modes

Resistive wall modes (RWMs) are long wavelength plasma instabilities that would be stabilized if the surrounding walls were perfectly conducting. In DIII–D [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159], RWMs limit lifetime and β achieved in advanced tokamak regimes, thus the feasibility of RWM stabilization through application of external magnetic fields from a feedback system is being investigated. To understand the behavior of different feedback algorithms and as a guide in the optimization of parameter settings, a “simple” feedback model is desirable. Here, a formulation of the Intelligent Shell problem which includes a realistic modeling of the electronics system involved is described. Analysis using this formulation is reduced to finding roots of a polynomial. Experimental parameter scans with present DIII–D RWM feedback system show quantitative agreement with the model.