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Ceramic Breaks Technical Notes

Occasionally, two parts of a vacuum vessel must be mechanically connected but electrically isolated. Flanged ceramic-metal vacuum breaks placed between these parts provide the necessary insulation.

Where an external part of the vessel is isolated from ground, yet needs a cryogenic liquid or (DI) water supply, narrow diameter ceramic-metal breaks with tube end terminations are used (mounted externally to the vessel).

If the isolated device is inside the vacuum housing, narrow diameter breaks are mounted inside the chamber (attached to normal liquid or cryogen feedthroughs through the wall, see Gas-Liquid Feedthroughs). Alternatively, for the latter application, power feedthroughs with tubular conductors are used (attached outside the vacuum vessel).

Ceramic-metal (vacuum) breaks are available in voltage ratings from 3kV to 40kV, either with weldable ends, KF, or CF flanges. Ceramic-metal (liquid) breaks have 3kV voltage ratings with tube or tube socket ends. When selecting an appropriate feedthrough, note the comments given in the sidebar titled Gas Discharges since they apply to this component, too.

Gas Discharges

A ceramic breaks's maximum voltage rating is determined with an external pressure of 1 atmosphere (air) and an internal pressure of high vacuum. For reasons beyond these Notes' scope, different gases have different breakdown voltages (BVs). For example, a paper from Rensselaer Polytechnic shows helium's BV under some conditions is roughly half that of air.

In general, the simpler the gas's physical structure, the lower its BV. Atomic inert gases, particularly He, have a low BV at most backfill pressures. Complex gas molecules, such as SF6, have high BVs since the many possible bond bending and stretching modes help dissipate energy when the molecule is hit by an electron. That tends to quench ionization and electron multiplication (which causes discharges). SF6 is used as a backfill gas in power transmission lines switches and breakers.

To assess the effects of pressure on a gas's BV, look for references to Paschen's Law. On Wikipedia's reference to the law, there is a link that shows the BV for air declines from 50,000V at 760 Torr to 350V at 0.2 Torr. Clearly, this can seriously affect experiments requiring low gas pressures and high voltages.

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