 vacuum spark gap
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johnf
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Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
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Good vacuum is related to mean free path ie the distance between gas molecules. The longer the path between molecules the better the vacuum. Current flow in a vacuum can result from either ion flow or electron flow. Ion flow happens when gas atoms are ionised and are repelled / attracted and tend to happen at poor vacuums ie low mean free path. Electron flow needs either field emmision or thermionc emmision and requires a good vacuum ie long mean free path as electrons will hit gas molecules ionising them but reducing current flow.
Now down to your question of a switch.
Yes a good vacuum without field or thermionic emitters will withstand many tens of kV per inch but will be hard to switch as there is nothing to breakdown and carry charge.
A cheap pressurised gas mix of 25% co2 and N2 75% at 20 atmospheres will withstand 20Mv per meter and if it does break down the gap readily ionises to provide an almost lossless channel for the current ie big dI/DT and resets quickly.
We use the gas mix at work on our two linear accelerators 6Mv and 3Mv and tank sparks from terminal to earth are rare but are over very quickly so as to not influence our measurements. SF5 would also work but we stay clear of it as if you do spark in it it breaks down to sulphur and flourine gas the later is not good for most metals/ insulators
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