Hot Gap Measurement of a Three-Grid System for a 30 cm Diameter Ion Thruster With Beam Extraction

The grid gap affects the extraction perveance and focusing of the ion beam. In order to obtain the hot gap variation of a three-grid system for a 30 cm diameter ion thruster, a digital imaging technique with measuring accuracy of 5 $\mu $ m is developed to obtain the real-time position of the probes on the grids. In the measurement, 15 ceramic probes are designed and installed in groups of three probes at different locations along radial direction of the grid. After calibration of the magnification factor and coordinate rotation, the absolute error of probe position obtained by laser rangefinder and image processing system is within 5 $\mu $ m. The measurement results show that due to different temperature rise of the three grids, the grid gap decreases rapidly first and then increases gradually, and reaching a minimum after 300-400 s of beam extraction, and this variation characteristic occurs every time the operating power is switched. In addition, the variation of grid gap presents a symmetrical distribution, and the gap variation at the same position from the grid center is basically the same, and the change of the gap in the center of the grids is greater than that in other regions. When the ion thruster is operating at 3000 W condition, the gap between the screen grid and the accelerator grid at the center is reduced to a maximum of 0.372 mm after 312 s of beam extraction, which is 0.528 mm less than the initial gap, and the gap between the accelerator grid and the decelerator grid is reduced by up to 0.541 mm after 388 s. Therefore, more attention should be paid to the grid gap variation during thruster startup, especially within 400 s of operation.