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Ion Thruster Accelerator Grids 193 screen supply if the backstreaming current becomes large. The ion beam is current neutralized and quasi-neutral (nearly equal ion and electron densities) by the electrons extracted from the neutralizer cathode. Fortunately, the thruster self-biases the neutralizer common potential sufficiently negative relative to the beam potential to produce the required number of electrons to current neutralize the beam. Figure 5-3 showed a generic thruster that includes a three-grid accelerator system, where a final grid called the “decel grid” is placed downstream of the accel grid. This grid shields the accel grid from ion bombardment by charge- exchanged ions produced in the beam backflowing toward the thruster, and eliminates the downstream “pits-and-grooves erosion” that will be discussed in Section 5.6. Three-grid systems then potentially have longer accel grid life than two grid systems and generate less sputtered material into the plume that can deposit on the spacecraft. These benefits are offset by the increased complexity of including the third grid. In actual design, the diameter of each accel grid aperture is minimized to retain unionized neutral gas in the plasma generator, and the screen grid transparency is maximized so that that the grids extract the maximum possible number of ions from the plasma. The electrode diameters and spacing are then optimized to eliminate direct interception of the beam ions on the accel grid, which would cause rapid erosion due to the high ion energy. A schematic example of a three- grid system showing the ion trajectories calculated by a two-dimensional (2-D) ion optics code [4] is shown in Fig. 5-4. The ions are focused sufficiently by this electrode design to pass through the accel grid without direct interception. On the downstream side of the accel grid, the negative accel-grid bias applied to avoid electron backstreaming results in a relatively small deceleration of the ions before they enter the quasi-neutral beam potential region. This high transparency, strong “accel–decel” geometry typical of ion thrusters results in some beamlet divergence, as suggested by the figure. However, this small beamlet angular divergence of typically a few degrees causes negligible thrust lost because the loss scales as cos , and because most of the beam divergence discussed in Chapter 2 related to the thrust correction factor is due to the dishing of the grids. The amount of current that an ion accelerator can extract and focus into a beam for a given applied voltage is related to the space-charge effects characterized by the Child–Langmuir equation and is called the perveance: P Ib . (5.1-1) V3/2PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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