Reduction of the field emission dark current from copper surface in a high gradient DC field

Field emission dark current is a most serious problem for a long lifetime operation of a photocathode-type RF-gun. It is especially true, if a GaAs with NEA surface shall be used as the photocathode of RF-gun to produce spin polarized electrons. To find a solution for this problem, a plan to construct an RF-gun cavity with an extremely low dark current has been started as an international collaboration of CERN/KEK-Nagoya/SLAC. As a part of this collaboration, a study was started to reduce the DC dark current from a clean copper surface. At the first step, various surface treatment methods have been examined by making test samples from Class-1 OFHC copper. Impurities on the surface were inspected by methods of SEM and AES. The surface treatments such as ozonized ultrapure water rinsing, high pressure pure water rinsing and electro-polishing have been tested. Then, at the second step, electro-polished electrodes were fabricated to measure field emission current from its surface under the DC high gradient field. Preliminary results are described in this report. (author)