abeles@mhuxm.UUCP (J. Abeles (Bellcore, Murray Hill, NJ)) (02/19/86)
Question: I am aware that the breakdown electric field in air is typically quoted as 10kV/cm. In various dielectrics the breakdown can be higher--400kV/cm in insulating GaAs is a typical figure for that material. My question--what is the breakdown field in a vacuum? What influences it? In extreme high voltages, pair production may occur. From a negatively charged electrode, cold emission can occur sending electrons into the vacuum. Another possible effect is due to the lack of a perfect vacuum, and I recall that in geiger counters a low pressure is purposely selected to get large currents because of special properties in the 1 torr or so range. Do the residual gases always dominate the breakdown process in practically attainable vacua? Or possibly can UHV systems achieve much greater dielectric strengths? Does anyone have a practical answer to these questions, or better yet, a good reference? --Joe Abeles
jp@lanl.ARPA (James Potter) (02/24/86)
Electric field breakdown in vacuum is of great interest to accelerator builders Try looking in specialized journals or proceedings of conferences. Look for van de Graaff stuff for dc, linear accelerators for rf. As a "rough" guide for rf high fields there is a number called the Kilpatrick criterion that scales roughly with the square root of frequency. It is based on some old experiments with oil diffusion pumps and nowadays is considered a conservative estimate of maximum surface field. To set a scale, the Kilpatrick limit at 500 MHz is approximately 20 MV/m. Designs using twice that field level are considered practical with proper preparation of surfaces and with rf conditioning (gotta burn of them high spots!) Note that this is a surface field limitation. There are those who hold, and have data to back it up that the frequency doesn't matter, only the gap. Don't extrapolate my number, above, to many factors of 2 in frequency without going to the literature, since the square root scaling is only approximate. I have a colleague who described a field level as b * V-Kilpatrick, where b is the "bravery" factor. If you plan to stick to field levels less than 1 Kilpatrick, just have smooth and clean surfaces, a good, high quality vacuum and bring the fields up carefully, with some protection to limit hte damage done by the inevitable sparking. It doesn't condition if it doesn't breakdown. Start with short pulses and exten to longer ones, even CW, as necessary.