MAGNETIC-FIELD EFFECTS ON VACUUM INSULATOR FLASHOVER

The influence of magnetic fields (both dc and pulsed) on dielectric surface breakdown in vacuum and simulated low-earth-orbit conditions has been investigated using pulsed test voltages. Predictions from the saturated secondary electron emission avalanche breakdown model and the experimental results both show magnetic insulation effects (i.e., an increase in flashover voltage) at magnetic-field amplitudes as low as 0.1 T. The most favorable configuration for magnetic insulation is with the magnetic field oriented parallel to the insulator surface and perpendicular to the electric field. An increase in flashover voltage with increasing magnetic field is seen when the vector E x B points away from the surface, while a decrease followed by an increase in flashover voltage is seen for E x B into the surface. The magnitude of the insulation effect depends on the dielectric material, ambient pressure, surface roughness, and the presence of background plasma. Predictions from single-particle computer simulations of the secondary electron avalanche process, using nonuniform fields, point to the importance of conditions at the cathode in producing magnetic insulation effects. It was found that it is sufficient to apply the magnetic field in the cathode region only, and that significant magnetic insulation effects can be observed using small, lightweight permanent magnets. An applied magnetic field will also increase the flashover voltage in a low-density (n(e) almost-equal-to 10(4) cm-3) plasma environment. The dependence of the flashover voltage on electrode separation (gap distance) is observed to remain sublinear with the application of an insulating magnetic field. Prebreakdown luminance measurements are presented which further support the saturated secondary electron emission avalanche model.