Net charge balance in an alternating-polarity porous-emitter ionic liquid electrospray thruster

Ionic liquid electrospray thrusters (ILETs) are promising for micro-/nanosatellite propulsion due to their compact size, low mass, low power consumption, and high specific impulse. During long-term single-polarity operation, however, sustained emission may lead to net charging of the thruster and potential drift risks. This work investigates net charge balance of a porous-emitter ILET under alternating-polarity operation, using the net exported charge per cycle and its accumulation rate as practical metrics, and systematically examining the effects of switching frequency, positive/negative voltage amplitudes and duty ratio. The results show that polarity-dependent emission asymmetry prevents a symmetric bipolar actuation from automatically yielding zero net charge. Transient responses indicate that, after each polarity reversal, the plume current requires a finite time to build up and reach a quasi-steady plateau; as switching frequency increases, the quasi-steady duration within each half-cycle decreases and the emission becomes increasingly transient, eventually limiting stable alternating-polarity operation. Under stable operating conditions, near-zero net exported charge can be achieved by tuning the positive/negative voltage amplitudes and duty ratio, and the net charge accumulation rate varies approximately linearly with both voltage amplitude and duty ratio, enabling predictable adjustment of charge-balance performance. These results provide experimental evidence and practical guidance for alternating-polarity charge-balanced operation of porous-emitter ILETs.