Surface damage on stainless steel and molybdenum electrodes caused by electrical breakdown in high vacuum

We have studied the surface damage produced on stainless steel and molybdenum electrodes when electrical breakdown occurs in ultrahigh vacuum. The electrode gap distance was ≊1 mm. In stainless steel, sharp protrusions form on edges of micron-sized pits which are sometimes present on highly polished surfaces. These protrusions may initiate breakdown. Both molybdenum and stainless steel cathodes show surface features similar to the blisters when these metals are irradiated with high doses of hydrogen ions. We propose that the gases dissolved in the electrode material and the high ionic flux produced as a result of outgassing of the electrodes near the breakdown voltage may cause bubble formation and blistering on the cathode surface. These blisters are sites of high-field emission and may burst, leading to crater formation. The craters on the anode surface are formed as a result of field-electron current-induced heating and impact of microparticles originating at the cathode.