Processes involved in the triggering of vacuum breakdown by low-velocity microparticles

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Abstract
The role of low-velocity microparticles (diameters < 100 μm; velocities ∼ 10–50 m/sec) in inducing breakdown in a high vacuum gap is investigated. On the basis of a simple model, it is shown that as an anode initiated microparticle approaches the cathode, fields sufficient to cause appreciable field emission (> 109 V/m) are possible at the cathode surface. The emission current not only causes partial neutralization of the initial charge on the microparticle, but also raises the temperature (> 2000°K) of the particle surface. As a consequence, a significant increase in gas pressure (∼ 100 Torr) in the microvolume between the cathode and the particle is possible, resulting in a discharge between the two. Such a discharge could act as a trigger for the breakdown of the main gap. Other possible processes that could arise due to the onset of melting or boiling of the bombarded surface, and which may lead to breakdown of the entire gap, are also discussed.