A vacuum variable capacitor uses a high vacuum as the dielectric instead of air
or other insulating material. This allows for a higher voltage rating and/or capacitance
value using a smaller total volume. In addition to the higher voltage rating a vacuum
dielectric greatly reduces the chance of arcing between the plates. There are several
different designs in vacuum variables, the most common geometry of the capacitor
plates is usually inter-meshed concentric cylinders. The meshed cylinders are contained
within a ceramic vacuum envelope, similar to an electron tube. Metal bellows are
used to maintain vacuum seal while allowing positional control for the moving parts
of the capacitor.
What is vacuum?
Vacuum is the state of gas with an smaller molecular density than that of air. (Air
pressure: 2.7×1019＞ high vacuum: 2.7× 1010 piece/cm3). Since the gas molecule density
is extremely low in a high vacuum, the charged particles (gas molecules) coming
out of the cathode hardly collide with the residual gas molecules. The vacuum controls
the breeding of charged particles after collision without reducing the dielectric
breakdown voltage. Therefore, the vacuum has the insulating performance ten times
greater than that of air.