Accelerator Grid Tests on an Electron-Bombardment Ion Rocket

A systemic investigation of screen-accelerator grids was made with an ion rocket having a 10-centimert-diamter electron-bombardment ion source. Mercury ion beams of 0.03 to 0.44 ampere were operated at net accelerating voltages of 900 to 6100 volts. A multihole screen-accelerator grid set passed the greatest percent of an ideal plane diode current with a ration of between-grid spacing to individual hole diameter of approximately 1. Breakdown between the grids occurred at field strengths of 2000 to 3000 volts per millimeter for grid spacings of 1 to 3 millimeters. The accelerator impingement was about 1 percent and caused little erosion of the molybdonum grids, which indicated a grid life of the order of 10000 hours or better. The experimental work was conducted in a bell jar connected to a 5-foot-diameter by 16-foot-long vacuum tank at Lewis Research Center.