Initiation of electric breakdown of wide interelectrode gaps in high vacuum is generally attributed to macroparticles, freely moving in vacuum, due to their interactions with electrodes. This approach is known as a clump hypothesis of Cranberg. There were proposed many modifications of the hypothesis. Chatterton's result, that breakdowns are initiated by macroparticles, moving from an anode to a cathode, is adopted in the paper. This paper discusses ways of free macroparticle generation in real conditions of vacuum electric devices and physical processes occurring at the particle interaction with an electrode. As a result, a model of vacuum breakdown initiation was suggested, including release of loosely bonded particles by electric field, their acceleration to an opposite electrode, impacts onto the electrode and formation of potentially electron emitting centers. Some published observations and data on breakdowns in centimeter gaps with long delays (tens of microseconds) indirectly indicate short-term existence of emission centers immediately prior breakdown, and thus they support the model. Mechanism of the emission center formation by plastic deformation of material in the impact zone in presence of electric field is discussed. A final stage of breakdown initiation involves emission current interaction with the anode, leading to the anode thermal instability.
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