The mechanism of grid current dynamic characteristics for microwave discharge ion thruster under the regulation of feedback control system applied in high precision space missions

High thrust noise limits the application of microwave discharge ion thruster in high precision space missions, which needs to be suppressed by feedback control system. A feedback control experimental platform is established for microwave discharge ion thruster choosing microwave power and ion beam current respectively as controlled variable and feedback parameter. Experiments show the feedback control system adjusts screen grid (SG) current by regulating microwave power to compensate for fluctuations of acceleration grid (AG) current, which is mainly because the ability of microwave power to adjust SG currentis dramatically stronger than that to adjust AG current. Besides, experiments show the AG current shows diametrically opposed characteristics for different operations under the fluctuations of decreasing flow rate and feedback control system. When the microwave power is improved to suppress the fluctuationsof decreasing flow rate, the neutral gas flux into the grid system decreases, which further reduces CEX reaction intensity. The backflow ions contributingto AG current are mainly CEX ions generated downstream of AG, which result in the decline of AG current with the fall of CEX reaction intensity. Besides, the primary ions from SG and CEX ions generated in the grid system both may be intercepted by AG. The fall of CEX reaction intensity increases the proportion of primary ions and reduce that of CEX ions. Compared with primary ions, the velocities of CEX ions are much slower, which makes their trajectories more affected by the electric field in the grid system. The divergent electric field under different operations results in the diametrically opposed characteristics of AG current with the variation of ion component.