ABSTRACT
Compared to other dielectric materials, an ideal vacuum has, in principle, the highest possible dielectric strength, because there are no carriers to conduct the current. In a vacuum below 10–2 Pa, less than 3 × 1012 molecules per cubic centimeter are present and the length of the free path is in the order of meters. In such a vacuum, an electron may cross the electrode gap of few centimeters without any collision with particles. Therefore, in the absence of the multiplication process, a vacuum behaves as an ideal insulator. Nevertheless, in actual practice the existence of metallic and insulating surfaces within the vacuum and the presence of adsorbed gases and oil vapors contaminates the vacuum. Therefore, the vacuum has a breakdown threshold, since the electric field intensity ultimately generates charge carriers by literally pulling them from surrounding electrodes and contaminants. However, with a proper design, vacuum insulation can still exceed the breakdown strength of most dielectrics. This is why vacuum insulation has found widespread use where very high electric field intensities are needed.
