Tail-Waving System for Active Feedback Stabilization of Flute Modes in Open Traps

Abstract

Feedback control is routinely used in modern plasma traps for adjusting plasma equilibrium on the transport time scale. Some intrinsic properties of magnetic mirrors make it possible to employ feedback control for stabilization of flute modes as well. Purely electromagnetic plasma-control system that is independent of line-tying or plasma conductivity to the end-plates is proposed. The system adds transverse flexibility to the plasma column, so that any growing perturbation can be deformed to become anti-ballooning. Anti-ballooning form means reduced flute amplitude in bad-curvature regions and enhanced amplitude in expanders or other traditional stabilizers, so that energy of the perturbation becomes positive and the mode is suppressed. Detailed analysis shows that transverse flexibility (or tail-waving) of the discharge can be employed for feedback stabilization even without good-curvature regions. The only requirement is that the discharge inertia (field-weighted plasma density) and the pressure-weighted field curvature are differently distributed along the discharge. If based on inertia, the stabilization mechanism resembles the rope-walker act. Estimates show that the power cost of such stabilization is reasonable and scales inversely with the trap length.