高真空中のガス吸着とMicrodischarge発生

抄録

A Pyrex glass vacuum chamber was evacuated with an oil diffusion pump to a pressure of 3×10^-4 Pa and CO₂ or O₂ gas was introduced into the chamber. The pressure of the introduced gas was maintained at a constant value between 10^-3 Pa and 5×10^-1 Pa for 2 or 4 minutes and pumped down once again to a vacuum of 3×10^-4 Pa. After this, an impulse voltage slightly lower than the breakdown voltage of a test gap (0.2mm), was applied and the microdischarge (MD) current was measured.
The onset of MD occurs when the applied impulse voltage exceeds a threshold voltage and the peak current of MD increases with increasing gas pressure introduced. But, there was no difference in the generation characteristics of MD between CO₂ and O₂ gas introduction. The components of the residual gas at after re-pump-out of the introduced gas were analyzed using a quadruple mass spectrometer. It was found there was no large difference in the components of the residual gas between CO₂ and O₂ introduction and the main component was H₂O in both cases. As other components, N₂, O₂, H₂, CO₂, hydrocarbons and vacuum pump oil were also left.
MD with large peak current over 4mA were measured only for the first application of the impulse and they for the subsequent applications were very small. A larger part of the adsorbed gas molecules on the electrodes desorbs together with the generation of the first MD. But the number of ions responsible for the first MD current was much less than the estimated total number of the adsorbed gas molecules on the effective area of the electrodes. Most of the adsorbed gas molecules was considered to desorb as neutral molecules.
A model of MD was proposed to explain the experimental results as follows. The generation of MD requires multi-layers of gas molecules on the electrodes and a lot of gas molecules are released at one time by the bombardment of field emitted electrons or the stimulation of field desorbed ions. In the cloud of sufficient density of neutral gas, cumulative ionization by electron collision can occur and it transfers MD current.