SECONDARY ELECTRON TRAJECTORIES IN HIGH-GRADIENT VACUUM INSULATORS WITH FAST HIGH-VOLTAGE PULSES

Vacuum insulators composed of alternating layers of metal and dielectric, known as high-gradient insulators (HGIs), have been shown to withstand higher electric fields than conventional insulators. Primary or secondary electrons (emitted from the insulator surface) can be deflected by magnetic fields from external sources, the high-current electron beam, the conduction current in the transmission line, or the displacement current in the insulator. These electrons are deflected either toward or away from the insulator surface and this affects the performance of the vacuum insulator. This paper shows the effects of displacement current from short voltage pulses on the performance of high gradient insulators. Generally, vacuum insulator failure is due to surface flashover, initiated by electrons emitted from a triple junction. These electrons strike the insulator surface thus producing secondary electrons, and can lead to a subsequent electron cascade along the surface. The displacement current in the insulator can deflect electrons either toward or away from the insulator surface, and affects the performance of the vacuum insulator when the insulator is subjected to a fast high-voltage pulse. Vacuum insulators composed of alternating layers of metal and dielectric, known as high-gradient insulators (HGIs), have been shown to withstand higher electric fields than conventional insulators. HGIs, being tolerant of the direct view of high-current electron and ion beams, and having desirable RF properties for accelerators, are a key enabling technology for the dielectric-wall accelerators (DWA) being developed at Lawrence Livermore National Laboratory (LLNL). Characteristically, insulator surface breakdown thresholds go up as the applied voltage pulse width decreases. To attain the highest accelerating gradient in the DWA, short accelerating voltage pulses are only applied locally, along the HGI accelerator tube, in sync with the charged particle bunch, and the effects of displacement current on trajectories of electrons emitted from HGI surface are particularly interesting. This paper presents simulated electron trajectories experiencing either constant or short-duration applied voltage pulses. Comparisons of these trajectories clearly indicate the importance of the voltage pulse shape, especially the rise time, in the flashover initiation process for HGIs