Effects of Energetic Beam Ions on Stability Properties of Field-reversed Configurations
Authors: E.V. Belova, R.C. Davidson, H. Ji, M. Yamada, and S. Gerhardt
Date of PPPL Report: December 2006
Presented at: the Twenty-First IAEA Fusion Energy Conference, 16–22 October 2006, Chengdu, China.
Stability properties of Field-Reversed Configurations (FRCs) formed by counter-helicity spheromak merging method have been studied numerically using the nonlinear hybrid and MHD simulation code HYM, including the effects of neutral beam injection. It is shown that the beam ions can have a stabilizing or destabilizing effect on the global modes in FRCs, depending on the toroidal mode number n, the mode polarization, and the beam parameters. Linear simulation results agree well with a qualitative analysis based on a generalized energy principle. Nonlinear simulations show that the beamdriven instabilities saturate nonlinearly due to changes in the distribution function of the beam ions. A new stability regime has been found for FRCs with elongation E~1, which requires a close-fitting conducting shell and energetic beam ion stabilization. It is shown that the n = 1and n = 2MHD modes can be effectively stabilized by a combination of conducting shell and beam ion effects, and that the residual weakly unstable n > 2 modes saturate nonlinearly at low amplitudes. The resulting configuration remains stable with respect to all global MHD modes, as long as the FRC current is sustained.