Studies of Global Stability of Field-Reversed Configuration Plasmas using a Rigid Body Model
Authors: H. Ji, M. Yamada, R. Kulsrud, N. Pomphrey, and H. Himura
Global stability of field-reversed configuration (FRC) plasmas has been studied using a simple rigid body model in the parameter space of s (ratio of separatrix radius to average ion gyroradius) and plasma elongation E (ratio of separatrix length to separatrix diameter). Tilt stability is predicted, independent of s, for FRC's with low E (oblate), while the tilt stability of FRC's with large E (prolate) depends on s/E. It is found that plasma rotation due to ion diamagnetic drift can stabilize the tilt mode when s/E is less than or equal to 1.7. The so-called collisionless ion gyroviscosity also is identified to stabilize tilt when s/E is less than or equal to 2.2. Combining these two effects, the stability regime broadens to s/E is less than or equal to 2.8, consistent with previously developed theories. A small additional rotation (e.g., a Mach number of 0.2) can improve tilt stability significantly at large E. A similar approach is taken to study the physics of the shift stability. It is found that radial shift is unstable when E < 1 while axial shift is unstable when E >1. However, unlike tilt stability, gyroviscosity has little effect on shift stability.