High voltage insulation in vacuum

Voltage withstand is due to the dielectric nature of the materials separating two regions of space at different electrical potentials, preventing the flow of charges from one region to the other. Generally, this potential difference is transmitted or generated between two electrodes. However, it can occur naturally in the atmosphere between two clouds, or between clouds and the Earth, leading – under certain conditions – to the generation of lightning. This means that atmospheric gas is not a very good electrical insulator, although it can withstand several tens of kilovolts per metre [D 2 530] .

One way of improving high-voltage resistance between two electrodes is to lower the pressure to well below atmospheric pressure, in other words, to immerse these electrodes in a vacuum. In fact, despite the very high kinetic energy recovered by charged particles (essentially free-state electrons) from the electric field, the probability of collision of these energetic particles with the other residual gas molecules present in the inter-electrode space decreases exponentially as the pressure is lowered. As a result, vacuum is insulating when subjected to a very intense electric field, and in some cases can exceed the properties of solid or liquid dielectrics.

The importance of holding very high voltages (a few megavolts or so) over a few centimeters is a very strong constraint on high-voltage lines. To prevent arcing, most of these lines have the inter-electrode space filled with dielectric liquid or electronegative gases (e.g. SF ₆ ) at high pressure (~ 10 bar). If only for environmental reasons, we need to examine alternative possibilities for transmitting high powers over long distances, especially if DC voltage is involved, in order to power gas pedals, superconducting coils, etc.

As soon as the electric field between the two electrodes becomes very strong, new phenomena appear on the surface of the electrodes, the best known of which is field-induced electron emission. If multiplication of these free electrons is possible, then the system can degenerate into a vacuum arc [D 2 820][D 2 870] .

The aim of this article is to give an overview of most of the fundamental mechanisms that condition the onset of vacuum breakdown, and to present advances...