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Ion Thruster Accelerator Grids 199 Tmax 4 o Ts 2e VT3/2 2eVb 8 2 A=9e M 2MM=9 o TsRE. (5.2-10) g e The maximum thrust density from an ion thruster increases with the screen grid transparency and the square of the electric field [8]. Ion thrusters with thin, high transparency grids operating near the perveance limit and at the maximum possible electric field in the acceleration gap will produce the most thrust for a given grid area. A key feature of ion thrusters illustrated by Eq. (5.2-10) is that the thrust density is independent of propellant mass. The net-to-total voltage ratio from Eq. (5.2-9) is given by R=Vb = Vb . (5.2-11) VT Vs + Va This equation describes the relative magnitude of the accel grid bias relative to the screen potential. Operating with small values of R increases the total voltage between the screen and accel grids, which, from Eq. (5.2-2), results in a higher current density of ions accelerated from the thruster. While it appears desirable to operate with very small values of R (large accel grid negative bias) to increase the current capability of a grid set, this results in higher energy ion bombardment of the accel grid and shortens grid life. Operating with small values of R will also change the beam divergence, but this is a relatively small effect in ion thrusters for most grid designs. For applications where thruster life is important, the magnitude of accel grid bias voltage is usually minimized to the value required to just avoid electron backstreaming, and the value of R typically ranges from 0.8 to 0.9. Finally, Eq. (5.2-10) suggests that the thrust density depends on the square root of R and would increase slowly with higher beam-to-total voltage ratios. This is misleading because the total voltage also appears in the electric field term (E = VT / e ) , and so higher thrust densities actually occur with more negative accel grid bias because of the higher voltage applied across the screen-to-accel gap for a given net (beam) voltage. Aside from mechanical tolerances, the minimum “hot-gap” grid separation, g , is limited by the vacuum breakdown field of the grid material: E = V < Ebreakdown. (5.2-12) g In practice, grid breakdowns initiated by arcing or small micro-discharges between the grids cause “recycles” in which the voltages are temporarilyPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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