Measurement of the distribution of charge exchange ions in a Hall-effect thruster plume

Hall-effect thrusters (HET) are a stationary form of electric propulsion that have been widely used in space missions. However, the wide range of the divergence angle of the HET plasma plume and the high ion velocities can result in a large area where collisions and plume interactions occur. Of these, low-energy charge-exchange (CEX) ions typically originate from collisions, and can result in severe deposition and sputtering issues on spacecraft components. In this work, we propose a novel magnetic Faraday probe in coordination with a particle-in-cell (PIC) simulation code. This system is able to deflect CEX ions with a changing solenoid current, and can be used to obtain the ion density and energy levels of CEX ions through experimental measurements. In addition, a Faraday probe, a retarding potential analyzer (RPA), and a Langmuir probe were used to obtain the beam characteristics. The CEX ion density at the beam edge area was determined to be in the level of 2.54 1012 m-3 (the local ion density of the plasma plume is in the level of 1013 m-3), and the ion energy level at the beam edge area was found to be below 30.1 eV. Due to the difference in the vacuum levels between ground tests and in-space operation, CEX collisions will be more severe during ground tests, and can affect plume measurements and erosion of the thruster. Two different vacuum levels were tested experimentally here to study the relationship between CEX ions and the background neutrals. As a result, it was found that a high background neutral density can exacerbate CEX collisions at the beam edge area, which results in an increase in the CEX ion density and a decrease in the CEX ion energy. This study brings us new information on the CEX ion density and energy distribution at the beam edge area from an experimental point of view. Furthermore, data on CEX ions energy can be used as input data for future plume simulations.