The capture and identification of particles in a vacuum gap

Microphotography of electrode surfaces in an uncontaminated vacuum system has revealed that particles from 3 to 40 μm in diameter are removed from stainless steel, aluminium and phosphor-bronze electrodes on application of electric fields significantly below the breakdown stress. A few of these particles have been captured on Mylar film and subjected to x-ray microanalysis. The results showed that neither electrode material nor particles of polishing material were removed at the voltages used. The composition of the captured material suggests that only dust particles were being transferred across the gap, and it is believed that such particles may be instrumental in causing the sporadic or freak breakdowns sometimes observed at relatively low field strengths.