High-gradient insulators HGI are periodic assemblies of conducting and insulating layers that have been shown to withstand higher pulsed voltages in vacuum than homogeneous insulators of the same length. We carried out calculations and experimental studies to understand the effect of geometry on the performance of wellconditioned, flat-surface HGI assemblies. We tested stacks with several different ( values of Ir r r r rM where I is the axial length of an insulating layer and M is the length ) of a metal layer . The experiments showed that HGI performance was substantially better than conventional insulators for Ir r r r rM -3 and somewhat worse for Ir r r r rM ) ) 3. Numerical calculations of electron orbits showed: 1 that the electric fields in HGI assemblies may have the favorable property of sweeping charged particles away ) from the surface and 2 that electron multiplication on the surface is suppressed when Ir r r r rM -3.Index Terms -High voltage insulators, electron avalanche, vacuum surface breakdown.
Results of the non-linear interaction of an extremely short (0.6 ns) high power (∼500 MW) X-band focused microwave beam with the plasma generated by gas ionization are presented. Within certain gas pressure ranges, specific to the gas type, the plasma density is considerably lower around the microwave beam axis than at its periphery, thus forming guiding channel through which the beam self-focuses. Outside these pressure ranges, either diffuse or streamer-like plasma is observed. We also observe high energy electrons (∼15 keV), accelerated by the very high-power microwaves. A simplified analytical model of this complicated dynamical system and particle-in-cell numerical simulations confirm the experimental results.
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