Breakdown of Dielectrics Due to Pulsed Electrons

Spontaneous dielectric breakdown was studied in methyl methacrylate polymer, Plexiglas, with a Field Emission Corporation Model 705 Febetron. The breakdown plane was influenced by the presence of embedded metal foils. Charge deposition was measured in a stack of thin sheets of Plexiglas and compared favorably with the calculated charge deposition under the condition of no charge buildup. The energy deposition behind samples of Plexiglas showed a reduction with increased sample thickness. An incident charge density of 4.l × 10-2 coulombs/M2 was found necessary to cause spontaneous breakdown in samples thicker than the electron range. The time between the deposition of maximum charge and spontaneous dielectric breakdown was 30 to 40 nanoseconds for an 1/8-inch thick sample with 7.4x10-2 coulombs M2 of incident electrons on an 1/4-inch diameter area. When Plexiglas was irradiated to 7.4x10-2 coulombs/M2, 1/16-inch thick samples did not show breakdown whereas 1/10-inch thick samples did show breakdown.