High-power Hall thrusters are enabling for a variety of space missions. Developments in the field have led to an increase in the achievable power levels and the lifetime of these devices. Magnetic ...shielding increases the lifetime of Hall thrusters by three orders of magnitude, while nested Hall thrusters allow these devices to be scaled to the high powers necessary for Mars missions. Despite these advancements, there remain open questions about the exact implications on Hall thruster operation that pose large risks for transitioning the thrusters to flight. One of the most critical is how neutral gas particles emanating from various sources influence the overall performance and lifetime of these thrusters. There exist several sources of neutrals such as the background facility pressure, the cathode, and adjacent channels on nested Hall thrusters. This work examines the role neutrals can play on the performance and lifetime of high-power Hall thrusters. We start with a theoretical framework for how neutrals should impact these parameters. In particular, we detail how neutrals will shift the main acceleration region of the Hall thrusters resulting in a reduced divergence angle and increased thrust. We discuss how this shift could lead to changes in the erosion profile. Previous results on nested Hall thrusters indicated that the performance in multi-channel operation was higher than the sum of each individual channel. We postulate that these difference are due to the increased neutral density when moving from single-channel to multi-channel operation. Again, the neutrals drive the acceleration region inward which improves the thrust. Additionally, neutrals from an adjacent channel can be ingested resulting in higher performance. We provide a framework to calculate the increased thrust due to both of these parameters. In order to experimentally characterize these impacts, we use three Hall thrusters: the H9, a 9-kW Hall thruster, the X2, a two-channel 10-kW nested Hall thruster, and the N30, a two-channel 33-kW nested Hall thruster. We employ a variety of diagnostics including a thrust stand, a neutral pressure probe, a Faraday probe, and laser-induced fluorescence to measure the thruster properties in addition to the standard telemetry. Our results show that the acceleration region of Hall thrusters does shift with a change in neutral density. We show this shift is independent of the neutral source and the critical parameter is the neutral density at the exit plane. Crucially, we also show that this shift can lead to changes in performance. For nested Hall thrusters, we show that the thrust improves 5% when moving from single channel to dual channel operation on the X2. This 5% increase is explainable by our theoretical framework which shows that this improvement is due to neutral effects. Finally, we show that the near-wall ion trajectories (for both single-channel and nested-channel magnetically shielded thrusters) are not substantially changed with varying local neutral density - indicating that the erosion profile of the thruster does not depend on the outside pressure environment substantially. Ultimately, we have improved our understanding of the mechanisms behind neutral impacts on the performance and lifetime of magnetically shielded and nested Hall thrusters. This improves of fundamental understand of Hall thruster operation and decreases the risk for transitioning to flight. For nested Hall thrusters, we have expanded our understanding into the differences between them and traditional Hall thruster and increased the technology readiness level.
Centaurs, minor planets with a semi-major axis between the orbits of Jupiter and Neptune (5–30 AU), are thought to be among the most diverse small bodies in the solar system. These important targets ...for future missions may have recently been Kuiper Belt Objects (KBOs), which are thought to be chemically and physically primitive remnants of the early solar system. While the Kuiper Belt spans distances of 30–50 AU, making direct observations difficult, Centaurs' proximity to the Earth and Sun make them more accessible targets for robotic missions. Thus, we outline a mission concept designed to reconnoiter 10199 Chariklo, the largest Centaur and smallest ringed body yet discovered. Named for a legendary Centaur tamer, the conceptual Camilla mission is designed to fit under the cost cap of the National Aeronautics and Space Administration (NASA) New Frontiers program, leveraging a conservative payload to support a foundational scientific investigation to these primitive bodies. Specifically, the single flyby encounter utilizes a combined high-resolution camera/VIS-IR mapping spectrometer, a sub-mm point spectrometer, and a UV mapping spectrometer. In addition, the mission concept utilizes a kinetic impactor, which would provide the first opportunity to sample the composition of potentially primitive subsurface material beyond Saturn, thus providing key insights into solar system origins. Such a flyby of the Chariklo system would provide a linchpin in the understanding of small body composition, evolution, and transport of materials in the solar system.
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•Flyby concept for 10199 Chariklo, the largest Centaur and smallest ring system.•Opportunity to learn about Kuiper Belt Objects much closer to Earth.•Impactor would provide deepest yet subsurface sampling in the outer Solar System.•Mission concept fits well within NASA New Frontiers Program cost cap.•Mission concept may fit within NASA Discovery Program cost cap.
The University of Michigan, in collaboration with NASA and the Air Force Office of Scientific Research, has developed two nested-channel Hall thrusters: the X2 and the X3. The X2 is a 6-kW ...two-channel thruster that was designed as a proof of concept for nesting channels concentrically. The thruster runs off a single centrally-mounted cathode. The successful results of the X2 project lead to the development of the X3, a 200-kW three-channel thruster.
Fas and the tumor necrosis factor receptor (TNFR)1 regulate the programmed cell death of lymphocytes. The death domain kinase, receptor interacting protein (rip), is recruited to the TNFR1 upon ...receptor activation. In vitro, rip-/- fibroblasts are sensitive to TNF-induced cell death due to an impaired nuclear factor kappaB response. Because rip-/- mice die at birth, we were unable to examine the effects of a targeted rip mutation on lymphocyte survival. To address the contribution of RIP to immune homeostasis, we examined lethally irradiated mice reconstituted with rip-/- hematopoietic precursors. We observed a decrease in rip-/- thymocytes and T cells in both wild-type C57BL/6 and recombination activating gene 1-/- irradiated hosts. In contrast, the B cell and myeloid lineages are unaffected by the absence of rip. Thus, the death domain kinase rip is required for T cell development. Unlike Fas-associated death domain, rip does not regulate T cell proliferation, as rip-/- T cells respond to polyclonal activators. However, rip-deficient mice contain few viable CD4+ and CD8+ thymocytes, and rip-/- thymocytes are sensitive to TNF-induced cell death. Surprisingly, the rip-associated thymocyte apoptosis was not rescued by the absence of TNFR1, but appears to be rescued by an absence of TNFR2. Taken together, this study implicates RIP and TNFR2 in thymocyte survival.
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