Exploration on the Possible Bump-on-Tail Instability in VASIMR

The bump-on-tail (BOT) instability is generally caused by a beam of energetic particles existing in relatively cold background plasma. The employment of second-stage wave-driven module in the variable specific impulse magnetoplasma rocket (VASIMR) yields the production of energetic ions, which could drive the BOT instability. The present work explores this possibility for the first time via numerical simulations based on the experimental data on the VASIMR, i.e., referring to VX-50. It is found that the BOT instability does exist even in the plume region away from antenna. The results indicate that velocity space diffusion provides a stabilizing effect on the nonlinear evolution of waves, while dissipation in the bulk plasma essentially impedes it. To show the practical values implied by these computations, the influences of this BOT instability on the power coupling and thrust are investigated particularly. These findings are valuable for VASIMR, as well as other plasma thrusters that yield energetic particles inside relatively cold background plasma, to suppress BOT instability and thus increase the power coupling efficiency and thrust performance.