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Ion Thruster Accelerator Grids 195 corresponding to higher voltages or lower beamlet currents than the optimal combination, increases the Child–Langmuir (CL) length and pushes the sheath to the left farther into the plasma. In the extreme case, this situation can launch ions at a very large angle from the edge region near the screen aperture and cause “cross-over” trajectories, which can then produce excessive erosion of the accel grid by direct ion impingement. Likewise, operating at higher than the optimal perveance, corresponding to higher beamlet currents or lower voltages than optimal, reduces the Child–Langmuir sheath thickness, and the plasma boundary pushes toward the screen aperture. This “over-perveance” condition flattens the sheath edge and accelerates ions directly into the accel grid, again causing excessive erosion. The optical performance and life of any grid design, therefore, is acceptable only over a limited range in voltage and current density, which will be discussed in Section 5.3. For this reason, the uniformity of the plasma over the grid area is important to avoid either cross-over or direct interception in different regions of the ion optics that strongly degrade the life of the grids. In the two- or three-grid configurations, the geometry of the grid apertures and gaps is intended to eliminate or at least minimize direct impingement by beam ions on the most negative potential electrode in the system, namely, the accel grid. This is required to minimize sputtering of the grid by the high-energy beam ions. The screen grid does receive ion bombardment from the discharge plasma due to its finite transparency, but the ions arrive with only an energy of the order of the discharge voltage in DC discharge thrusters or the floating potential in rf or microwave thrusters. Sputter erosion of the screen grid then becomes an issue only at high discharge voltages or due to the production of high-energy ions in the hollow cathode region [5,6] that can bombard the screen grid. Likewise, the decel grid is biased near the beam plasma potential and backflowing ions produced in the beam by charge exchange impact with very low energy, which causes little or no sputtering. For two grid systems, the backflowing ions bombard the accel grid with essentially the grid bias voltage. This can cause significant sputtering of the downstream face of the accel grid and may determine the grid life. The decelerating field produced downstream of the accelerator grid by the accel grid bias acts as a weak defocusing lens for the ions, but keeps electrons emitted by the neutralizer from entering the high field region and backstreaming at high energy into the discharge chamber. This decelerating field is set up either by applying a potential between the accelerator grid and the decel grid or by applying the bias between the accelerator grid and the hollow cathode neutralizer and allowing the low energy plasma downstream of the accelerator grid to act as a virtual anode. Unfortunately, ions generated between the grids by either charge exchange with unionized neutral gas escaping thePDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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