Vacuum DC breakdown characteristics between grid electrodes with ion sputtering

In ion thrusters, grid electrodes are biased at high voltage to extract and accelerate ions. However, they are susceptible to vacuum breakdown which considerably undermines the ion thruster performance. To optimize the grid electrode design and improve the ion thruster longevity, the impact of ion sputtering on the vacuum direct-current breakdown characteristics of grid electrodes is systematically examined. The adopted seven-aperture grid electrodes are made of three materials including molybdenum, graphite, and stainless steel, which are exposed to Ar+ and Xe+ sputtering at energies of 400 eV and 1000 eV for durations ranging from 0.5 to 8 h. It is found that the initial breakdown voltage, which means the first breakdown voltage after sputtering, generally decreased regardless of the electrode material as the sputtering time increased. However, the breakdown voltage rapidly increases with more repeated discharges. Each kind of electrode material demonstrates unique responses to ion sputtering and breakdown. In comparison with metal electrodes, the graphite surface is relatively easy to appear defects but exhibits the least insulation degradation. The metal surface remains relatively smooth, but breakdown voltage is significantly affected, with electrode materials such as stainless steel failing to recover to their initial insulation levels. Xe+ sputtering leads to greater breakdown voltage fluctuations during repeated discharges due to higher sputtering yield. Dark current is measured, and the field enhancement factor β FE is calculated to reveal correlations between local electric field and the breakdown voltage. Furthermore, micro particles are identified as major contributors to the first few times breakdown. Based on above observations, two kinds of distinct discharge mechanisms, including the cascade discharge induced by micro particles and the discharge induced by field emission, are proposed to interpret experimental data.