Motion observation of particles between electrodes and subsequent breakdown phenomena in vacuum

The influence of particles on breakdown and pre-breakdown phenomena in vacuum was experimentally investigated. The Cu sphere particles were injected artificially between 10-mm gap plane electrodes made of Cu and the movement of particles was measured by a high speed camera under AC and DC voltage application. The correlation among the motion of particles, the field emission current and the breakdown phenomena was examined. Under the application of high voltage between the electrodes, most particles bounced up and down between the electrodes. However, some particles attached to the electrode after the collision to the electrode and detached from the electrode after the reversal of voltage polarity. It is natural to say that those attached particles were covered with insulating oxide layer. In most cases the speed of attached particles exceeded that of bounced particles and the impact speed of attached particles to the opposite electrode exceeded 50m/s with 10-mm gap and 100-kVp applied voltage. These high speed particles detached from the electrode at various voltage phase, even at around 0-phase. Discharges induced by collision of these attached particles were also observed.