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Ion and Hall Thruster Plumes 395 arrays, exposed components, and scientific instruments. Thruster plumes and their interactions with the spacecraft must be understood and accommodated in order for the spacecraft to perform to specification for the required mission life. 8.2 Plume Physics The thruster plume is composed of ions and electrons of various energies and some neutral gas. The energetic beam ions accelerated by the thruster fields are the dominant ion species and the major source of thrust. The velocity and angular distributions of these ions can be measured in the laboratory and calculated by the thruster computer models discussed in previous chapters. For ion thrusters, where the accelerating voltages are a thousand volts or more, the weak plume electric fields have little influence on energetic ion trajectories. In this case, the challenge is usually determining the ion trajectories from the shaped-grid accelerator structure. However, for Hall thrusters, where the accelerating voltages are a few hundred volts, the plume electric fields can significantly broaden the energetic ion plume. The second source of ions is due to charge-exchange reactions between beam ions and neutral xenon gas. The neutral gas is due to un-ionized particles leaving both the thruster and the neutralizer (hollow cathode), and, in the case of laboratory measurements, background neutrals present in the vacuum chamber. Charge-exchange reactions have usually been associated with inelastic collisions processes yielding low-energy ions at large angles with respect to the main-beam direction. However, as thruster voltages increase to provide higher specific impulse (Isp), the energy of these scattered ions can become significant. The total plume plasma density, including all three ion components, is shown schematically in Fig. 8-2 for a 4-kW Hall thruster. 8.2.1 Plume Measurements Thruster plume characteristics have been measured extensively in the laboratory and in space on a few spacecraft. In the laboratory, most measurements have been of the ion velocities and densities, and some thruster erosion products, but not of the un-ionized neutral gas, which is in most cases dominated by background gas in the test chambers. The balance of the thruster gas flow and the speed of the test facility’s vacuum pumps determine the background gas pressure. The maximum facility pressure during high power testing is usually limited to less than 10–4 torr. Therefore, the density of un- ionized propellant from Hall and ion thrusters is greater than the background only within a few centimeters of the thruster.PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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