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206 Chapter 5 12 11 10 9 8 7 6 Perveance Limit = 688.8.V 600 700 800 Beam Power Supply Voltage (V) 1100 1200 900 1000 Fig. 5-8. Accel grid current versus the screen supply voltage for the NSTAR thruster at TH15 parameters, showing the perveance limit. Technology Applications Readiness (NSTAR) engine operating at the full power parameters of TH15 but with the screen voltage decreasing. In this case, the perveance limit is found to be at 688.8 V, compared to the nominal 1100 V of the screen voltage at this throttle level. The perveance limit can also be defined by a given percentage increase in the accel current. However, the screen grid transparency usually decreases as the screen power supply voltage is decreased, which reduces the beam current and accel current during this measurement. The magnitude of the percentage increase in the accel current due to direct ion impingement then needs to be defined for the ion optics assembly. 5.3.3 Grid Expansion and Alignment A significant issue in ion thrusters that utilize refractory metal grids is thermal expansion of the grids during thruster operation changing the acceleration gap dimension between the screen and accel grids. This will directly affect the ion trajectories and the perveance of the ion optics. Since the screen grid is heated by direct contact with the discharge plasma and is usually dished outwards and designed with a minimum thickness to increase the effective transparency, the screen grid expansion is usually larger than the accel grid and the gap tends to decrease as the thruster heats up. This shift from the cold gap to the hot gap causes the perveance of the optics to increase for convex grid curvature (grids domed outward from the thruster body) and changes the beamlet trajectories at the given operating point. In addition, for grids designed to hold the applied voltage across the cold gap, the hot gap may be so small that field emission and high voltage breakdown become problems. For ion thrusters with refractory metal grids designed with concave grid curvature (grids domed into the thruster Accel Grid Current (mA)PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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