This is a preprint of a paper intended forpubli_tion in • jourual orproceedin8_. Since : " _:__: changesmay be madebefore publication,thispreprint is madeavailablewith the ", i_i: ii understanding that it will not be cited or reproduced without the permission of the :.

We have recently initiated an investigation of elecii .. llii i This work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48.

This h a preprint of apaper intended for publication in a journal orproceerIing9. Since changes may be nude before publidion, this preprint is made available with the andexstanding that it will not be cited or reproduced without the permission of the author. Fast, low jitter command triggered switching is key to the successful implementation of the dielectric wall accelerator (DWA). We are studying a W induced vacuum surface flashover switch for this purpose. We present our initial data Using a NdYAG laser incident onto a high gradient insulator d a c e at lo, 20,30, and 40. Best la jitter was <1 ns with no degradation of the switch after 500 shots.

This h a preprint of a p a~i n~n d e d forpubliotion in a jotun?l or proceeding. Since changes may be nude before pnbliotion, this preprint is made available with the understanding ttut it will not be ated or reproduced without the permhion of the author. We have measured an 36-A-~m-~ current emission density over the surface area of an 11.4-cm2-area Lead-Titanate-Zmonate @T) ferroelectric cathode with a pulsed andcathode (A-K) potential of 50 kV. We have also observed currents above those pfedicted by classical Child-Langmuir formula for a wide variety of cases. Since a plasma within the A-K gap could also lead to increase current emission we are attempting to measure the properties of the plasma near the cathode surface at emission time. In other measurements, we have observed strong gap currents in the absence of an A-K potenrial. Further, we continue to make brightness measurements of the emitted beam and observe spatially non-uniform emission and large shot-to-shot variation. Measurements show individual beamlets with a brightness as high 10lr Am-2rad-2. 7 DISCLAIMER

The maximum vacuum insulator surface dielectric strength determines the acceleration electric field gradient possible in a short pulse accelerator. Previous work has indicated that higher electric field strengths along the insulator-vacuum interface might be obtained as the pulse duration is decreased [~].. In this work, a 250 kV, single ns wide impulse source was applied to small diameter, segmented insulators samples in a vacuum to evaluate the multi-layer surface dielectric strength of the sample construction.Resonances in the low inductance test geometry were used to obtain unipolar, pulsed electric fields in excess of 100 MV/m on the insulator surface.The sample construction, experimental arrangement and experimental results are presented for the initial data in this work. Modeling of the multi-layer srmcture is discussed and methods of improving insulator surface dielectric strength in a vacuum are proposed.