Statistical characteristics of electrical breakdown in saturated superfluid helium

Superfluid liquid helium (HeII) with a lower temperature than 2 K is an attractive coolant for large high field superconducting magnets because it has far better heat transfer properties and increases critical current of superconductor. This paper deals with electrical breakdown characteristics of saturated HeII at 1.85 K and the statistical analysis based on the Weibull distribution function and the Weak-link theory. A special attention is paid to the size effects on the breakdown characteristics. Comparing the obtained results with those of normal liquid helium (HeI), it is shown that the dc breakdown strength of HeII is much more sensitive to the electrode area than HeI. Consideration on the breakdown time lag under a pulsed stress reveals that the breakdown triggering electron is produced by field emission from the cathode surface as in the case of HeI.