The sputtering-deposition balance in the beam and backflow regions of electric propulsion plume

This paper investigates the sputtering and deposition effects in the backflow and beam regions under the influence of plasma flow in a Krypton propellant Hall thruster. Quartz crystal microbalance measurements were used to study deposition and sputtering in both the beam and backflow regions. The experimental results show that the plasma density in the backflow region is approximately on the order of 10¹⁴m⁻³, with a spatial potential ranging from 9.7 V to 13.8 V. The backflow region is predominantly characterized by low-energy ions generated by the charge-exchange effect. Additionally, the ion energy in the backflow region is more sensitive to the anode voltage. The results indicate that the backflow region is mainly dominated by the deposition of contaminants, while the beam region exhibits a combined effect of high-energy ion sputtering and the deposition of erosion products. The deposition rate in the backflow region increases with the propellant flow rate, showing consistency with the plasma density. The deposited products include B, C, N, and some metals, with deposition rates and spatial distributions of the primary products derived from frequency calibration. The sputtering mass fraction within the divergence half-angle accounts for 87.7 % of the total sputtering, and the sputtering effect is influenced by both the anode voltage and propellant flow rate. Higher anode voltage helps reduce the sputtering range of high-energy beams. It was found that the actual deposition amount is at least 4.7 times greater than the measured deposition. The new model considers the deposition of products from the thruster and secondary sputtering, allowing for a qualitative analysis of the actual deposition values based on the ion energy and the results from the energy dispersive spectrometer.