This thesis demonstrates the capability to control the particle heating mechanisms in radio-frequency (rf) hollow cathode electrothermal micropropulsion sources via the application of multi-harmonic `tailored' voltage waveforms. Modulation of the electrical asymmetry of such waveforms, through varying the phase offset between successive harmonics, results in a direct variation of the phase-resolved sheath dynamics. This enables the preferential deposition of power into either electrons or ion species, selectively enhancing the ionisation rate or neutral gas heating efficiency, respectively. Further, by considering the neutral depletion arising from neutral gas heating, this work predicts the formation of collisionless ion populations within otherwise collisional plasmas. These outcomes are achieved in tandem through the use of 2D fluid/Monte-Carlo numerical simulations with comparison to experimental measurements of the lab-based prototype Pocket Rocket hollow cathode microthruster. Application of these electrical control schemes to plasma sources for spacecraft propulsion enables the prospect of variable thrust and variable specific impulse operation, significantly increasing the in-mission versatility of electric thrusters.