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.
A sensitivity analysis of a fully kinetic particle code was conducted to investigate the importance of uncertainties associated to physical parameters. A 500 W-class laboratory model magnetic-layer Hall thruster was used as the testbed. The sensitivities of the physical parameters, including thermal accommodation coefficient, anode/wall temperature, Bohm diffusion coefficient, electron injection current, cathode coupling voltage, and background pressure, were quantified one-byone on a conservative possible range. The results suggest the wall erosion prediction is more sensitive to the physical parameters than the thrust or the discharge current. Among the physical parameters, sensitivity to the Bohm diffusion coefficient and parameters related to the neutral flow (i.e., thermal accommodation coefficient and anode/wall temperatures) were dominant. It was hence found that uncertainties in the physical parameters related to the neutral flow had comparable influence on the Bohm diffusion coefficient despite the low attention they attracted.
Summary (Abstract) of doctoral thesis contentsAn external discharge plasma thruster (XPT), a prototype of alternative low power Hall thruster, which produces and sustains plasma discharge completely outside a non-anodic cavity, was proposed in order to address efficient scaling down and lifetime problems of low power Hall thrusters. XPT is the first Hall thruster that operates without any annular or cylindrical discharge channel walls. In particular, it is expected to operate in very low power regime where conventional Hall thrusters cannot prevent high ion/electron losses to the channel walls. Design details and key physics of this novel Hall thruster were described, and action items for improving thrust performance are presented.To validate a fully kinetic particle code, and to investigate the importance of uncertainties associated to physical parameters a sensitivity analysis was conducted. A 500W-class laboratory model magnetic-layer Hall thruster was used as the testbed.The sensitivities of the physical parameters, including thermal accommodation coefficient, anode/wall temperature, Bohm diffusion coefficient, electron injection current, cathode coupling voltage, and background pressure, were quantified one-by-one on a conservative possible range. The results suggest the wall erosion prediction is more sensitive to the physical parameters than the thrust or the discharge current. Among the physical parameters, sensitivity to the Bohm diffusion coefficient and parameters related to the neutral flow (i.e., thermal accommodation coefficient and anode/wall temperatures) were dominant. It was hence found that uncertainties in the physical parameters related to the neutral flow had comparable influence on the Bohm diffusion coefficient despite the low attention they attracted.XPT was simulated by the fully kinetic particle code, and the sensitivity analysis methodology was applied to determine upper and lower bounds for estimated thrust performance. Numerical results suggested that XPT can be as much efficient as conventional Hall thrusters of the same power range, which prompted us to manufacture the thruster and perform experimental investigation to verify numerical simulations.Experiment results showed that XPT has similar discharge characteristics with SPT.The thrust and the anode specific impulse ranged from 1.6 to 17.9 mN, and from 352 to 1276 sec respectively at anode potentials of 150-200-250 V with anode mass flow rates of 0.48-0.95-1.43 mg/s. The anode efficiency ranged from 6.7 to 26.8 % at discharge powers from 43 to 418 W. Performance of XPT is comparable to SPT at the same power level. Significantly, stable operation (Δ<0.2) was possible over wide range of operational conditions. These results supported the conclusion made from the numerical study.XPT does not possess an annular discharge channel or a cylindrical one and working (別紙様式 2) (Separate Form 2) gas is allowed to expand into vacuum right after leaving anode. It might be considered that such a "cavity-less plasma thruster" would have very l...
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