Detection of low-velocity microparticles occurring in an ultra-high-vacuum test chamber

In the year 1953 Cranberg¹ proposed his clump theory to explain the breakdown of a vacuum gap. According to this theory loosely adhering charged aggregates of matter called clumps or micro-particles are released from the electrodes under the action of electrostatic force associated with the high field, and travel to the cathode gaining energy during the transit. The impingement of the clump on the opposite electrode is expected to vaporize enough electrode material into the gap to create a breakdown. Slivkov² supplemented the above hypothesis by including the possibility of evaporation of the particle itself. While there is enough experimental evidence^3–5 to indicate that the presence of such micron-sized clumps do initiate breakdown of a high-vacuum gap, there are no reports as yet, concerning the detection of any naturally occurring microparticles in a vacuum gap. In a 0. 5 cm vacuum gap operating near breakdown voltages (≃ 150 kV) particles which attain supersonic velocities will be of submicron size. A sub-micron size particle is unlikely to provide sufficient metal vapor on impact for a breakdown to occur. On the other hand, particles with very low velocities (V ≃ 0.5 m/s) should be large enough to be seen with the naked eye and are thus unlikely to be present in a vacuum apparatus. But microparticles with velocities 1–50 m/s are small enough to be easily detected but are large enough to initiate a breakdown. The experiment described below is designed to determine the presence or absence of such microparticles in an ultra-high-vacuum test chamber. Moreover provisions have also been made to elucidate the charge velocity and mass of such a particle as well as to provide an estimate of the number of particles that could emanate under any given conditions.