Thrust performance, propellant ionization, and thruster erosion of an external discharge plasma thruster

Abstract

It is quite a challenge to design low power Hall thrusters with a long lifetime and high efficiency because of the large surface area to volume ratio and physical limits to the magnetic circuit miniaturization. As a potential solution to this problem, we experimentally investigated the external discharge plasma thruster (XPT). The XPT produces and sustains a plasma discharge completely in the open space outside of the thruster structure through a magnetic mirror configuration. It eliminates the very fundamental component of Hall thrusters, discharge channel side walls, and its magnetic circuit consists solely of a pair of hollow cylindrical permanent magnets. Thrust, low frequency discharge current oscillation, ion beam current, and plasma property measurements were conducted to characterize the manufactured prototype thruster for the proof of concept. The thrust performance, propellant ionization, and thruster erosion were discussed. Thrust generated by the XPT was on par with conventional Hall thrusters [stationary plasma thruster (SPT) or thruster with anode layer] at the same power level (∼11 mN at 250 W with 25% anode efficiency without any optimization), and discharge current had SPT-level stability (Δ < 0.2). Faraday probe measurements revealed that ion beams are finely collimated, and plumes have Gaussian distributions. Mass utilization efficiencies, beam utilization efficiencies, and plume divergence efficiencies ranged from 28 to 62%, 78 to 99%, and 40 to 48%, respectively. Electron densities and electron temperatures were found to reach 4 × 1018 m−3 (∂ne/ne = ±52%) and 15 eV (∂Te/Te = ±10%–30%), respectively, at 10 mm axial distance from the anode centerline. An ionization mean free path analysis revealed that electron density in the ionization region is substantially higher than the conventional Hall thrusters, which explain why the XPT is as efficient as conventional ones even without a physical ionization chamber. Our findings propose an alternative approach for low power Hall thruster design and provide a successful proof of concept experiment of the XPT.

Rotating spoke instabilities in a wall-less Hall thruster: experiments

“…The WL approach is a more drastic alternative to reducing the channel erosion. The principle is to entirely shift the ionization and acceleration regions outside the cavity by moving the anode towards the channel exit plane [36][37][38][39]. Figure 1 illustrates the anode architecture and the magnetic field topology in the ST and WL configurations.…”

Experimental Investigation of an External Discharge Very Low Power (<20W) Hall Thruster

“…Using the alternative de nition of the divergence angle -the angle at which 95% of the total ion current is contained, yields a value of 57.7 ∘ . Overall, the measured divergence angle is low compared to other wall-less devices [17], [19], [20]. Moreover, a thrust estimation of 29μN was found from Eq.…”

Evidence of a free-space ion acceleration layer in the plume of a quad confinement plasma source

“…This latter observation reveals that the acceleration front is connected to the system physical geometry and magnetic topology. The decoupling of ionization and acceleration regions and the generation of cross-field wall-less plasmas have been the subject of much research within the space propulsion community, aimed at mitigating wall-losses and reducing the erosion of the ceramic channel [34][35][36][37] thus extending the operational lifetime. These studies have demonstrated the feasibility of such discharge with ionization and acceleration regions located in free space outside of a physical channel.…”