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Ion Thruster Accelerator Grids 207 body), the screen grid expands away from the accel grid and the perveance decreases as the gap gets larger. In addition, concave grids have a smaller discharge chamber volume for a given thruster size, which adversely affects the discharge loss. Ideally, the ion optics design would have sufficient margin to operate at full power over the range that the grid gap changes. This is possible for smaller thrusters and/or lower power levels where the grid deflection is a small fraction of cold gap. For thrusters with grid diameters greater than 15 to 20cm operating at power levels in excess of 1 kW, it is often necessary to design the optics for the highest power case with the small hot gap, and to start the thruster in the diode mode (discharge only) or at lower beam powers to pre-heat the grids to avoid breakdown during thermal motion. This establishes the grid gap dimension within the range the optics can tolerate for high-power operation with minimal grid interception. It should be noted that grids fabricated from the various forms of carbon (graphite, carbon–carbon composite, or pyrolytic) have smaller or negligible thermal expansion than refractory metal grids and will have smaller grid gap changes. Ion optics sets that utilize grids made of two different materials have to deal with this issue of different thermal expansion coefficients and potentially larger grid gap changes. Another significant grid issue is alignment of the grid apertures. The ion trajectories shown in Fig. 5-6 assumed perfect alignment of the screen and accel grid apertures, and the resultant trajectories are then axi-symmetric along the aperture centerline. Displacement of the accel grid aperture relative to the screen grid centerline causes an off-axis deflection of the ion trajectories, commonly called beam steering. The affect of aperture displacement on the beamlet steering has been investigated for many years in both ion sources and ion thrusters [22–25]. The beamlet is steered in the direction opposite to that of the aperture displacement due to the higher focusing electric field induced at the accel grid aperture edge. Studies of this effect in ion thruster grid geometries [24] show that small aperture displacements ( 10% of the screen aperture diameter) cause a deflection in the beamlet angle of up to about 5 degrees. This phenomenon can be used to compensate for the curvature of the grids to reduce the overall beam divergence, which is called compensation. However, the perveance of the aperture is reduced in this case, and interception of edge ions on the accel grid due to the non-uniform electric fields can be an issue. Mechanical misalignment of the grids due to manufacturing tolerances or thermal deformation can also produce aperture displacement and unintended beamlet steering. This problem has been identified as the cause of thrust vector variations observed as thrusters heat up [24]. For this reason, precise alignment of the grid apertures and grid support mechanisms that minimize non-uniformPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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