Investigation of explosive electron emission sites on surface of polished cathodes in vacuum

The field enhancement effect of the surface microstructures of electrodes with two different surface treatments is evaluated, on the basis of observations made by a 3-D laser microscope. The maximum field enhancement factor of the micro-protrusions on the surface after turning or mirror polishing is about 1–5.8. The vacuum gap breakdown strength of the mirror polished electrodes is 1–1.4 times higher than that of the turned electrodes, because the surface grooves are removed. Experiments to test the breakdown strength of 2.5 cm vacuum gaps with the electrodes after the treatments were carried out and the experimental results were found to correspond to the analysis. The results also revealed that it is not enough to induce explosive electron emission and vacuum breakdown just by the field enhancement at the micro-protrusions and grooves. Moreover, the emission sites of mirror polished cathode made of titanium alloy and stainless steel after vacuum breakdown were observed by a scanning electron microscope and an energy dispersive spectrometer. The results show that the proportion of carbon atoms in the damaged area of the stainless steel cathode surface is as high as 40–70%, and most of the pits located at the grain boundaries, which suggests that grain boundaries have important effects on the electron emission and vacuum breakdown.