Miniature ion thrusters are an attractive option for a wide range of space missions due to their low power levels and high specific impulse. Thrusters using ring-cusp plasma discharges promise the highest performance, but are still limited by the challenges of efficiently maintaining a plasma discharge at such small scales (typically 1-3 cm diameter). This effort significantly advances the understanding of miniature-scale plasma discharges by comparing the performance and xenon plasma confinement behavior for 3-ring, 4-ring, and 5-ring cusp by using the 3 cm Miniature Xenon Ion thruster as a modifiable platform. By measuring and comparing the plasma and electron energy distribution maps throughout the discharge, we find that miniature ring-cusp plasma behavior is dominated by the high magnetic fields from the cusps; this can lead to high loss rates of high-energy primary electrons to the anode walls. However, the primary electron confinement was shown to considerably improve by imposing an axial magnetic field or by using cathode terminating cusps, which led to increases in the discharge efficiency of up to 50%. Even though these design modifications still present some challenges, they show promise to bypassing what were previously seen as inherent limitations to ring-cusp discharge efficiency at miniature scales. Published by AIP Publishing. https://doi

The miniaturization of conventional direct-current ion sources is predominantly restricted by efficiency limitations associated with the increased surface area-to-volume ratio of smaller-scale discharge chambers-reducing the effective confinement length of the high-energy 'primary' electrons that is necessary for efficient plasma generation. The Axial Ring-Cusp Hybrid (ARCH) plasma discharge addresses this scaling limitation by using a new approach that combines magnetic and electrostatic confinement to decouple the primary and plasma electrons loss mechanisms. Simulated ion thruster performance measurements show that the ARCH discharge may be capable of achieving a discharge loss and a propellant mass utilization of 175 eV/ion and 0.87, respectively. These estimates are supported by full internal maps of the plasma properties, including the electron energy distribution function, inside the discharge chamber. The measurements show highly effective confinement of the primary electrons, high average plasma electron temperatures of ∼5eV, and low plasma sheath potential relative to the anodeattributes generally found only in efficient conventional-scale discharges with good overall plasma confinement. As such, the new ARCH discharge design approach may allow miniature ion thrusters to achieve the performance and efficiency levels similar to those of highly efficient conventional ion thrusters.

For this study, we use a combined experimental and computational approach to investigate the near-surface dynamics and structure of plasma confined by a permanent magnet cusp. Improved understanding in this region allows electric propulsion designers to take advantage of cusp confinement for micro-scale discharges (≤ 1 cm), enabling high performance microthrusters that are attractive for both microsatellite and formation flying missions. An electron gun experiment designed specifically for this effort provides detailed data on the plasma for a single permanent magnet cusp for electron-only conditions and multispecies (electron, neutral, and ion) weakly ionized conditions. For these experimental conditions, a multispecies particle-in-cell model is developed that uses an adaptive mesh and analytical solutions for permanent magnets to provide high resolution for particle motion and interactions in the cusp region. Results from the experiment and model agree well and are consistent with existing theory for the "leak radius" at the cusp. Nomenclature a = coefficients for 2D quadratic equation A = amplitude of oscillation dS = differential area along a surface, m 2 E = electric field, V/m E = particle energy, eV E 0 = atomic unit of energy (= 27.21eV) m = particle mass, kg q = charge, C r = radial position, m r l = leak radius, mm R = error of the 2D quadratic equation, V t = time, s U = random number between 0 to 1 v = particle velocity, m/s w l = leak width, mm z = axial position, m Subscripts i = initial k = cell number n = number of neighboring cells f = final Symbols β = magnetic pressure Δt = time step, s ρ h , ρ e , ρ i = hybrid, electron, and ion gyroradii ρ p , ρ s = primary and secondary electron gyroradii φ = electric potential, V χ = deflection angle, rad

An experimental effort was under taken to understand and improve the discharge efficiency of the Miniature Xenon Ion (MiXI) thruster. Analyses were performed on the 3 cm discharge with a 3-ring and 5-ring cusp configurations, which lead to a new axial ring cusp design that shows considerable promise. For each configuration, a Langmuir probe was used to make 2D maps of the electron energy distribution function (EEDF) and other important plasma parameters throughout the discharge chamber. For the standard ring-cusp configurations, the results show that the plasma structure is dominated by the cusp fields at the miniature scale and suggest that primary electron loss to the walls likely dominates discharge losses at this scale. The insight derived from the testing of the ring-cusp configurations led to the development of a new design approach: the Miniature Axial Ring Cusp Ion (MARCI) prototype thruster. Discharge mapping and performance testing of the MARCI discharge demonstrate favorable plasma conditions within the discharge and near the extraction plane, and that the device can potentially achieve discharge losses and mass utilization efficiencies near 245 W/A and 0.89, respectively. As such, the new MARCI design approach may allow miniature ion thrusters to achieve the performance and efficiency levels of highly efficient conventional ion thrusters.

An experimental effort was used to examine the primary electron loss behavior for micro-scale (≲3 cm diameter) discharges. The experiment uses an electron flood gun source and an axially aligned arrangement of ring-cusps to guide the electrons to a downstream point cusp. Measurements of the electron current collected at the point cusp show an unexpectedly complex loss pattern with azimuthally periodic structures. Additionally, in contrast to conventional theory for cusp losses, the overall radii of the measured collection areas are over an order of magnitude larger than the electron gyroradius. Comparing these results to Monte Carlo particle tracking simulations and a simplified analytical analysis shows that azimuthal asymmetries of the magnetic field far upstream of the collection surface can substantially affect the electron loss structure and overall loss area.