Plasma Characteristics Measured in the Plume of a Next Multi-Thruster Array

Abstract Plasma properties in the plume produced by a “3+1” NEXT thruster array operating at full power were mapped using a series of planar Langmuir probes. The Langmuir probes were swept across the diameters of each thruster as well as the centerline of the array at multiple downstream axial locations to produce a plasma “map” of the plume produced by the array. Such maps yielded the spatial distribution of the plasma density, the electron temperature and the plasma potential in the near field of the array. This spatial information provides insight into local plasma particle flow. Flow direction is particularly important from both an array lifetime and spacecraft-plume plasma interaction standpoint. The variation in the plume plasma parameters tended to vary in a manner consistent with both plume shape and overlap of adjacent plumes. I. Introduction To address lifetime and performance requirements for ion thruster missions requiring large propellant throughputs and high total impulse, a multi-thruster array can be utilized. In this case, a high performance ion thruster is integrated into an array of like thrusters such that lifetime and performance requirements of the system match the mission needs. A given engine’s throughput requirement could be less than that which would be required if the thruster was operated alone-which translates into longer life. As an additional benefit, each engine could also be operated at full power for periods during the mission to reduce overall trip time. If the time at the full power operation is sufficiently small such that the wear accrued is less than that which would occur during reduced power operation for longer periods of time, then lifetime is also optimized. Such high power operation is possible particularly early on in the mission when the solar flux at the spacecraft is high (ref. 1). Many multi-thruster array studies, both theoretical and experimental, have been carried out in the past to investigate these aforementioned positive attributes. Such investigations tested up to three thrusters featuring a host of electrostatic probes to carry out the investigation (refs. 2 to 9). These studies did not reveal any unexpected phenomena or potential barriers to the actual implementation of this approach. A multi-thruster array consisting of four NASA’s Evolutionary Xenon Ion Thrusters (NEXT) was tested at the NASA Glenn Vacuum Facility 6. As illustrated in figure 1, NEXT Engineering Model (EM) thrusters were arranged in a “3+1” configuration where three thrusters were active and while a dormant thruster served as the spare. Here NEXT EM1, EM2, EM4, and EM5 were utilized in this test. This configuration is identical to the configuration designed for the Titan Orbiter mission (ref. 10). A detailed description of the thruster array may be found elsewhere (ref. 11). Of particular interest is the plasma particle and field distribution in the plume of an operating array of thrusters. The multi-thruster plume plasma determines the flow fields for charge exchange (CEX) ions as well as for charged sputter products. Both species effect thruster lifetime and performance. The backflow of charged species also affects the spacecraft itself particularly onboard scientific instruments. Backflowing CEX ions that find their way to the accelerator grid can give rise to erosion there. Sputtered components created local to the engine can also flow back to the thrusters, thereby giving rise to the formation of coatings or flakes. The charged, sputtered-material as well as the low energy CEX ions from the plume itself can backflow unto the spacecraft, potentially contaminating instruments or affecting the performance of electronics associated with the particular science mission. Depending on the potential of the spacecraft structures, the backflowing ions could also pose a sputter erosion threat as well. Finally, it should be pointed out that even if the incident energies of the charged specifies are sufficiently low, their collection on spacecraft surfaces will invariably affect the charge balance. It is conceivable that such charging could lead to arcing events and also affect the overall spacecraft potential. In order to assess the plume induced plasma environment in the near field of the array, a series of electrostatic probe measurements were made in the plume of the multi-thruster array. The objective of the investigation was to map the plasma properties in the plume. These properties include plasma potential, electron temperature, and plasma