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Ion Thruster Accelerator Grids 223 These results suggest that carbon–carbon composite grids can be designed for reliable high-voltage standoff utilizing a field emission threshold of about 35 kV/cm, even for large gaps and voltages in excess of 10 kV, provided that the coulomb transfer is limited by the power supply to less than about 1 mC. This 35-kV/cm field limit is the highest voltage stress that should be allowed, and conservative design practices suggest that a 50% margin (to 23 kV/cm) should be considered in designing these types of grids. 5.5.4 Pyrolytic Graphite Pyrolytic graphite (PG) is also a candidate for accelerator grid electrodes in ion thrusters [47]. This material is configured with the carbon crystal planes parallel to the surface. Pyrolytic graphite is grown a layer at a time to near the desired shape on a mandrel and then finish machined to the final configuration. Flat test coupons were fabricated in this manner, but they featured small surface bumps and depressions that were residual from the growth process. Figure 5-18 shows the behavior of a PG grid sample that had apertures laser-machined into it and then the surface lightly grit blasted. The as-new PG material demonstrated threshold electric fields of 20 to 30 kV/cm for gaps of 1 mm or larger, which is lower than that found for the CC grid material. However, a series of ten 1-mC arcs tends to smooth and condition the surface and raise the threshold electric field to the order of 30 kV/cm. Higher coulomb arcs (up to about 10 mC) also improve the voltage standoff to about 40 kV/cm. The pyrolytic graphite is more susceptible to field emission and breakdown than the carbon–carbon material, but appears to tolerate higher coulomb-transfer arcs. 160 140 120 100 80 60 40 20 0 Fig. 5-18. Threshold electric field for pyrolytic graphite with grid apertures (from [36]). PG Grid Material New 1 mC 10 mC 012345 Electrode Gap (mm) Threshold E-Field (kV/cm)PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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