Effects of clumps and ion bombardment on electrical breakdown in vacuum

The formation of emitting sites on highly polished stainless steel and titanium electrodes in vacuum is investigated. It is shown that during the first application of an electric field between broad-area electrodes clumps damage the cathode surface. These clumps originate both from the cathode and the anode. If a spark-conditioned cathode with a field enhancement factor β=40 is placed opposite a new anode, an electric field of 6·5×10 6 V m −1 is enough to damage the cathode, resulting in a field enhancement factor of 100. It is demonstrated that conditioning of electrodes by sparks results from the detachment of more or less loosely bound particles from the electrodes. For small electrode separations ( d <1 mm) clumps initiate a breakdown if the microscopic field strength β V / d after the impact on the cathode is higher than a critical field strength F b , which equals similar 7×10 9 V m −1 . The field enhancement factor β can be decreased by ion bombardment, but the breakdown voltage is the same as after spark conditioning. It is shown that after conditioning by ion bombardment clumps are responsible for the onset of a spark.