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Ion Thruster Accelerator Grids 221 edges associated with real grids cause the voltage hold-off to decrease. For molybdenum material with apertures, the resulting surface is susceptible to breakdown at electric fields of 40 to 50 kV/cm, which should be considered the maximum electric field for designing molybdenum grids. 5.5.3 Carbon–Carbon Composite Materials Carbon is a desirable material for ion thruster grid electrodes because of its low sputtering yield under xenon ion bombardment [44] as compared with most refractory grid materials. However, the structural properties of graphite are usually insufficient for thin graphite grids of any reasonable size (greater than a 5- to 10-cm diameter) to survive launch vibrations. This problem can be solved by using carbon material with better structural properties, such as carbon– carbon composites and pyrolytic graphite. Grids made of these materials have demonstrated low erosion in life tests and flown successfully [45]. However, the more complex structures of these materials leads to lower thresholds for field emission and less voltage standoff for grids made of these materials. Carbon–carbon composite material used for grid electrodes [46] is based on carbon fibers woven into a matrix with the fibers oriented in one or two dimensions. This material has enhanced strength and flexural modulus compared to pure graphite due to the carbon-fiber properties. The carbon-fiber weave is impregnated with a resin and built up to the desired shape by progressive laminate layers on a mold. The resulting material is usually densified and graphitized at high temperature, and may be further impregnated or over-coated with a thin chemical-vapor-deposition (CVD) carbon layer after this process to fill any voids or smooth the final surface. High-voltage breakdown tests were conducted with and without this final surface graphite coating. The threshold voltage of the carbon–carbon composite samples is shown in Fig. 5-16, where the threshold for field emission is plotted as a function of the electrode gap for various levels of coulomb-transfer arcing. New material (without arcing) with a fresh CVD layer has a high threshold for field emission, and therefore holds voltage well. High coulomb-transfer arcs (>1 mC) tend to damage that surface and return it to the state of the material without the CVD over-layer. Higher coulomb-transfer arcs also tend to damage the surface. In fact, in this example, the 10-mC arcs resulted in damage to the opposite anode electrode, which evaporated and redeposited material back on the cathode- potential surface, improving its voltage hold-off capability. For this reason, the coulomb-transfer limit for carbon–carbon (CC) grids should be set to about 1 mC such that conditioning and no damage to either the screen or accel grid occurs during any breakdowns.PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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