Influence of the electrode surface condition on the breakdown of liquid helium

AC breakdown strength of liquid helium (4.2 K) in the gap range from 0.5 mm to 3 mm is studied with various electrode materials. The electrodes are spheres 50 mm in diameter and the materials are niobium, steel and brass. The breakdown strength varies from 15 kV/mm to 55 kV/mm, depending on the condition of the electrode surface. This includes roughness as well as nicrocracks and oxide layer. The statistical distribution of 10 to 20 individual breakdown values is found to give a normal Gaussian function against the logarithm of the voltage. Plots of the breakdown voltage versus gap distance give a linear relation in all cases.