Effect of cathode electron emission on unexpected unstable discharges in a miniature microwave ion thruster

Under the demand of space gravitational wave detection, the miniature microwave ion thruster with propellant-free cathode discharge experiment was carried out. The propellant-free cathode cannot be adaptively coupled with the thruster. Improper coupling will induce unexpected unstable discharges (UUD), which will not only affect the thruster structure, but also interfere with the control system and the stability of the thrust output. Therefore, in this paper, in order to suppress the UUDs, cathode operating parameters are optimized. It is found that the difference between the screen grid current and the cathode current determines the frequency and transient characteristics of the UUDs. An excess of electrons or ions will result in a significantly higher UUD frequency. Optimizing cathode parameters can reduce the frequency of UUDs. A long-term test confirms that the frequency can be reduced to 0.045 mHz. It is shown that proper matching between the cathode and the thruster can suppress UUDs and improve the thrust stability in the low-frequency band.