Study of gas breakdown phenomena in high-frequency electric field driven ExB discharges for air-breathing electric propulsion in VLEO

Abstract We perform a computational study of very low-pressure gas breakdown phenomena in a perpendicular electric field and applied magnetic field configuration (i.e., ExB discharge) with high-frequency oscillations of the electric field. This work is particularly relevant to an ionization stage of proposed two-stage air-breathing electric propulsion (ABEP) devices, operating in the rarefied atmosphere of very low Earth orbit (VLEO) satellites. In the ionizer, radio-frequency (RF) power is delivered between parallel plates, and the setup is classified as a capacitively coupled RF discharge. A one-dimensional three-velocity (1D-3V) particle-in-cell Monte-Carlo collision (PIC-MCC) technique is employed for the study with nitrogen gas as a surrogate to represent the rarefied atmosphere of VLEO. For ambient pressures less than 1 mTorr, a static magnetic confinement combined with high RF electric field excitation is necessary to efficiently trap electrons and achieve ignition. Observations indicate that for low initial seed electron density of ∼10 13 m −3 and frequencies of ∼100 MHz, gas breakdown requires RF voltages > 2 kV. At higher operating frequencies of about 300 MHz, gas breakdown can be achieved for lower RF voltage of about 500 V. The results indicate that a stochastic, i.e. collisionless, electron heating is a dominant heating mechanism at examined low pressure regimes of 0.1 mTorr. Under these conditions, interactions between electrons and the sheath during its expansion phase produce a high-energy electron beam which is essential for sustaining an enhanced ionization rate. The generated high-energy electrons lead to bounce resonance heating (BRH) due to repeated reflections by the oscillating sheath. Furthermore, with a driving voltage of 500 V and at a high excitation frequency of 300 MHz, an observed resonance with the plasma frequency appears to improve electron confinement and ionization, particularly under the lowest gas pressure and plasma density investigated in this study.