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Ion Thruster Accelerator Grids 217 The ability of the accelerator grids to hold off high voltage reliably and to withstand occasional breakdowns without significant damage or loss of voltage standoff capability is therefore of critical importance for ion thrusters. The high-voltage behavior of vacuum-compatible materials has been summarized in recent books on high-voltage engineering [34,35]. In plasma devices [36], electric fields of up to 40 kV/cm were found useful for refractory metal electrodes and of the order of 25 kV/cm for carbon materials. Degradation of the voltage hold-off due to surface damage incurred during breakdowns has been investigated for molybdenum and carbon electrodes [36] commonly used in ion thruster applications. The surfaces of these materials can be carefully prepared to withstand high electric fields required to produce the highest thrust density. However, sputter erosion over time and electrical breakdowns between grids cause some fraction of the stored energy in the power supply to be deposited on the grid surface. The formation of an arc at the cathode electrode (the accel grid) and the deposition of a significant amount of electron power from discharge into the anode electrode (the screen grid) can cause both the screen and accel grid surfaces to be modified and/or damaged. The breakdown events usually impact the subsequent voltage hold-off capability of the grid surfaces, which affects the long-term performance of the thruster. 5.5.1 Electrode Breakdown The grids in ion thrusters have high voltages applied across small grid gaps, which can lead to high-voltage breakdown and unreliable thruster operation. High-voltage breakdown is usually described in terms of the electric field applied to the surface that causes an arc or discharge to start. Arc initiation is well correlated to the onset of field emission [37,38]. If sufficient field emission occurs due to excessive voltage or a modification to the surface that enhances field emission, the gap breaks down. Physical damage to arced surfaces during the breakdown is attributed to localized energy deposition on the electrode that causes melting or evaporation of the material. On the cathode surface (the accel grid), the energy is deposited primarily by ion bombardment from the arc plasma. On the anode surface (the screen grid), the energy is deposited from the plasma or electron stream that crosses the gap and results in localized surface heating and vaporization. The energy provided to the arc from the power supply is distributed between any series resistance in the electrical circuit, the voltage drop at the cathode surface, and the voltage drop in the plasma discharge and anode sheath. These voltage drops can be modeled using discrete series resistances in the energy balance of the system. Engineers often rate the possibility of a power supply damaging the electrodes by the amount of stored energy in the power supply. However, the amount of material removed from the surfaces and the lifetime of high-voltage electrodes is usually characterized [36] by the amount of current that passes through the arc. This “coulomb-transferPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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