Understanding the Mechanisms Affecting Gridded Ion Engine Operation in Ground Test Facilities

Abstract

Gridded ion engines are an option for high-power nuclear electric propulsion missions supporting science, cargo, and human missions to the moon, Mars, and beyond. Current state-of-the-art systems operate in the sub-10 kW range. High-power missions will require systems to operate at 100s of kW, with the power processed by multiple thrusters. Being able to test such high-power engines in a spacelike environment requires pumping speeds in the 10s of ML/s which is well beyond the capabilities of ground-based test facilities presently. Chamber size is important to minimize plasma wall interactions both from sputtering and electromagnetic coupling. In lieu of a facility with ideal attributes that circumvent these problems, a ground testing approach that utilizes current facilities must be used. To develop such high-power engines that will operate predictably in space, a combination of extensive measurements of engines operating at high-powers relative to chamber volume densities is needed in addition to computational models correcting for performance differences such that spacelike operation can be recovered.

In response to the need to understand electric propulsion ground testing and the impact of these facility effects on engine performance, the Joint AdvaNced PropUlsion InStitute (JANUS) was formed. As a part of the JANUS project, this work specifically focuses on understanding facility related mechanisms affecting gridded ion engine operation under high-power conditions, whether simulated or actual, in marginal and submarginal facilities, and using models to correct deviations from spacelike operation.

The approach here is to first measure the impact of facility effects in a chamber that is of modest size and pumping speed achieving the low-power densities akin to spacelike conditions. The next step would be to operate an engine at high-power densities to simulate 100 kW class engines to assess deviations, preferably in the same facility. Unfortunately, 100 kW class engines do not currently exist. To emulate the high-power density facility effects of a 100 kW system, a smaller, 8 cm NASA gridded ion engine was used in a small facility to simulate the effect of operating the larger engines. A comparison between the impacts of facility effects on thruster operation for low and high-power densities is then made with comments on recommended gridded ion engine ground testing best practices. Here these processes are identified that deviate from spacelike conditions, providing a test database for model validation such that these models can be later used to correct for facility effects and recover spacelike operation.

Overall, the goal of this thesis is to quantify these facility effects, provide a database for model validation, and to identify power density considerations. Along with this it was found that the ambient background plasma can have a large impact on gridded ion engine operations such as neutralization and facility coupling. Furthermore, it was identified that there is a problematic nature of graphite surfaces beyond simple sputter deposition that involves water embedded in the carbon lattices such that impacting high energy ions from the beam can cause plasma chemical effects that can lead to dirty probes which affects interpreted plasma properties or gives rise to changes in the operation or lifetime of the thruster. This effect suggests the need for chamber conditioning akin to what is done in the fusion community.