Identification and suppression of compressional Alfvén wave instability in magnetic-nozzle plasma thrusters

Plasma instabilities limit the efficiency of magnetic nozzle-based plasma thrusters, particularly in the low-power regime, which is critical for space missions with constrained power. We present a linear dispersion model for plasma thrusters with isothermal electrons and β-distributed ions, identifying an unstable axisymmetric (m = 0) compressional Alfvén wave. This mode is experimentally observed in a high-temperature superconducting applied-field MPD thruster, appearing in the 3–6 MHz range and driven by high-energy ions in the plume, which induce oscillations in the ion energy distribution and trigger the instability, degrading performance. Suppression of these ion fluctuations at 300 sccm effectively stabilizes the instability mode. As a result, the HTS-based MPD thruster achieves 35% efficiency at 14.6 kW, producing 390 mN of thrust with a specific impulse of 4471 s. These results demonstrate record performance for a sub-15 kW MPD thruster and provide a framework for identifying and controlling compressional Alfvénic instabilities in magnetically confined plasma systems.