Breakdown in Electrical Feedthroughs- Effects of Gases, Vacuum, Dielectric Surfaces and Gap Spacings on the Dielectric Withstanding Voltage
Vacuum feedthrough connectors are designed to pass current, impress voltage or connect
power across a sealed boundary. These electrical products provide an insulated connection
between atmospheric pressure on the air-side and high and ultrahigh vacuum on the other.
Most common gases and vacuum are insulating. The breakdown voltage in air is approximately
3MV/m. The maximum field strength without breakdown in vacuum is approximately ten times
that or approximately 30MV/m. These limits are as measured through space – electrode to
electrode and are used to ensure the safe high-voltage design of electrical feedthroughs.
If there is any material besides air or high vacuum between the electrode surfaces, the limits
are not the same. A few examples are illustrative: Fused Silica has a dielectric strength of
around 600MV/m.
Theoretically, a fused silica plate 0.1mm thick could withstand 60kV. And Teflon film has a
dielectric strength of over 150MV/m, so a .01mm thick Teflon coating could withstand 1.5kV.
In contrast, the dielectric strengths of many gases are much lower than that of air. Consequently,
if one is attempting to make electrical connections within gas-filled chambers, the high
voltage standoff can be much lower. In some arrangements, both electrodes are in contact with
lower dielectric strength material. The dielectric strength in Argon at one-atmosphere pressure
is only 0.6MV/m and in Neon is only 0.06MV/m. The electrode gap spacing in Neon at one-
atmosphere would need to be 50 times that in air to hold off the same voltage! Similarly, the
electrode spacing in Argon would require five times that in the air.
The condition “gas-filled” includes partial vacuum (p > 10 -4 Torr) which can lead to a
breakdown condition in gases known as “glow discharge” that can occur at relatively low
voltages. The dielectric strength of air at the Paschen minimum 327 V. This low voltage breakdown
condition corresponds with the reduced pressure-gap minimum of 0.567 Torr-cm. If the
residual gas is air, the spacing would need to be much greater than 0.567 m at 0.01 Torr in
order to hold off 327V. Compare this very large spacing to the 10 micron gap required to hold
off 327 V at in high vacuum (p < 10 -4 Torr). At this partial pressure of 0.01 Torr the spacing is
50,000 to 100,000 times greater to hold off the same voltage. The breakdown voltage of other
gases like helium, argon, neon, and nitrogen are all a little lower than in residual air alone under
the glow discharge condition. Glow discharge breakdown can occur during vacuum system
roughing. During pump down, the pressure of the vacuum vessel will drop to the Paschen
minimum voltage limit sometime in the process of achieving high-vacuum. Disconnecting all
high voltage sources during vacuum system pump down will avoid this surprisingly frequent
problem.
