Overcoming the Scaling Limitations of Ring-Cusp DC Ion Thruster Discharges

Ion thruster discharges exhibit impressive performance at conventional scales (~30 cm in diameter); however, scaling to smaller sizes (~3 cm) presents considerable challenges. This research effort addresses the miniaturization of direct current (DC) discharges via a series of careful investigations (i.e., near-field cusp-confinement, discharge experiments, computational efforts, and theoretical analyses). These investigations provided insight into the plasma behavior and loss mechanisms within small-scale DC discharges. This insight was then used to develop and demonstrate a highly efficient miniature-scale DC ion source called the Axial Ring-Cusp Hybrid (ARCH) discharge. Implemented as a 3 cm ion thruster, it can potentially achieve a discharge loss of 160 eV/ion and mass utilization efficiencies of 0.89 — performance values that have previously only been attainable by conventional-scale discharges (>10 cm). The improve performance will greatly expand the mission capabilities of many future spacecraft missions by providing the high efficiency of ion thrusters at small powers and scales. This thesis describes 3 efforts that were undertaken to better understand cusp-confinement physics and miniature DC discharge behavior. Since small-scale DC discharges are limited by their ability to effectively utilize the high-energy primary electrons, the first effort began with experiments utilizing an electron flood gun to investigate the loss behavior of magnetically-confined primary electrons. It was found that increasing the complexity of the upstream field affected the spatial complexity of the loss area. These efforts showed that, in contrast to existing theory, the primary electron loss can be strongly influenced by the upstream magnetic fields. This behavior is particularly important and noticeable for miniature-scale discharges where small perturbations of the primary electron’s gyro-center are preserved from one cusp to the next. The second effort was then used to investigate the overall confinement behavior of a cusp discharge, to baseline the probing techniques for magnetized plasma discharges, and to validate computational models. It was found that the plasma electron temperature and primary electron density must be analyzed via the electron energy distribution function (EEDF) data. The interpreted primary electron density showed good quantitative agreement with computational simulations.The final effort was to investigate and improve the discharge efficiency of miniature-scale DC ion thruster discharges. The plasma parameter maps for several magnetic field and discharge conditions were analyzed in conjunction with estimated performance measurements. For the 3 cm Miniature Xenon Ion (MiXI) thruster, the detail discharge maps revealed the mechanisms responsible for the MiXI’s flat beam profile and high discharge loss. Analyses of the ionization rates confirm previous computational findings that the plasma within the original MiXI design is created almost entirely by direct primary electron ionization of the xenon gas. Multiple ring cusp configurations (i.e., 3-ring, 4-ring, and 5-ring) were also examined. These efforts revealed that a major redesign is necessary to avoid some of the inherent limitations of ring-cusp discharges at the miniature scale. Ultimately, a new discharge design called the ARCH discharge was developed. Performance data and discharge maps show that this approach provides high primary electron confinement, high plasma electron density, high plasma uniformity along the extraction plane, and discharge performance that may rival well-designed conventionally-sized ion thrusters

Miniature ion thruster ring-cusp discharge performance and behavior

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