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224 Chapter 5 5.5.5 Hold-off and Conditioning in Ion Thrusters Tests have shown that the arc initiation voltage is directly related to the threshold voltage and electric field for field emission in Figs. 5-14 through 5-18 [36]. Arc initiation voltages tend to be less than 10% higher than the threshold values for field emission shown here. This is consistent with experimental observations that low levels of field emission and/or corona can be tolerated before full arc breakdown occurs, but arcing and recycling tend to increase once significant field emission starts. Molybdenum has been found to have a good tolerance for high coulomb-transfer arcs, and grids can be designed to reliably hold electric fields well in excess of 40 kV/cm. Carbon-based materials have more structure than the refractory metals and tend to form field emitters if excessive charge transfers are allowed. Nevertheless, grids utilizing carbon- based materials can be designed with electric fields in excess of 20 kV/cm if the coulomb transfer during breakdowns is limited to about 1 mC or less. Detailed investigations of the voltage hold-off and conditioning of carbon–carbon thruster grids were performed by Martinez [8], who documented the effect for larger area grid sets. Figure 5-19 shows their reduction in field emission from carbon–carbon grids plotted on a Fowler–Nordheim plot [43] for increasing numbers of 1-mC arcs. This work shows that even if the surface of carbon– carbon grids evolve field emitters over time due to erosion from ion bombardment, proper design of the power supply to limit the coulomb-transfer rate will result in reconditioning of the grid surfaces with every recycle event. −21 −23 −25 −27 −29 −31 1.5 10−4 3.5 10−4 5.5 10−4 7.5 10−4 9.5 10−4 1.2 10−3 1/V (V −1) T Fig. 5-19. Fowler–Nordheim plots of field emission, showing conditioning of carbon–carbon grids by increasing numbers of 1-mC arcs (from [8]). No Conditioning 1500 700 300 100 1-mC Arcs ln [J/V2] (A/V2) TPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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