Multipactor Breakdown is a failure mode of an RF component that only occurs under conditions of high vacuum, where a certain frequency distance product condition exists between the inner and outer conductors and where a sufficiently large RF electric field strength exists. In a high vacuum environment, an electron may have a free path longer than the electrode separation distance. When this electron collides with the electrode it may release secondary electrons. If both frequency and the distance between inner and outer conductor are favorable, the secondary electrons will be accelerated by the electromagnetic field. Large electron densities rapidly build up and breakdown results. At very low and very high frequencies Multipactor Breakdown is impossible. Multipactor Breakdown can also occur between the conductor and the insulator. A multipactor discharge itself adsorbs little power, but once initiated it can cause increased outgassing from materials within components, which may lead to a gas discharge and total failure. To prevent this event, the microwave components should have vent holes of sufficient size to allow the gasses to escape at a known rate. Multipactor Breakdown also results in increased heating within the cable or connector, noise generation, harmonic distortion and intermodulation (when multiple frequency RF signals are applied).

For every vacuum application the power handling should be calculated individually. The worst frequencies for Multipactor Breakdown are between 500 MHz and 2.5 GHz. At low voltage levels (less than 20 V) and low average power (less than 8 W), Multipactor Breakdown is theoretically impossible.

In Ionization Breakdown, secondary electrons are produced through collisions between electrons and gas molecules. Ionization Breakdown occurs at pressures higher than those for multipaction. Like Multipactor Breakdown, Ionization Breakdown is not possible at very low and very high frequencies and low power levels.

However, Ionization Breakdown is considerably more complex than Multipactor Breakdown because of the additional dependence on pressure and the type of gas (if other than air).

Astrolab, Inc. has designed, manufactured and delivered several high performance cable assemblies for use in high power, high altitude and space environments. These products were tested by an independent laboratory to determine if any failures due to Ionization and/or Multipactor Breakdown would occur.

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