Fabrication of ultra-clean copper surface to minimize field emission dark currents

The current from copper electrodes treated by four different types of surface cleaning procedures was measured under DC high field gradient condition. The best results were obtained by using the electrode rinsed with ultra-pure water after diamond turning. A field gradient of 47 MV/m was achieved with dark current at the level of 1 nA, and the microscopic field enhancement factor was estimated to be a very low value of 56. The dark current from this electrode was dependent only on the field gradient at the cathode and not affected by the total voltage applied to the gap. In this case the surface would be covered by a Cu2O layer, which creates few secondary ions by the electron bombardment. On the other hand, a large total voltage effect and a large vacuum increase were observed for the electro-polished electrode. Cu(OH)2, which would be formed at the copper surface during the electro-polishing process, would emit H2O molecules during the electron bombardment.