Pressure and temperature dependence of electrical breakdown in liquid helium*

The SNS nEDM collaboration is developing an experiment to search for the neutron’s electric dipole moment (EDM), using ultracold neutrons (UCNs) stored in superfluid liquid helium, to be run at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. In this experiment, being able to achieve a strong and stable electric field in superfluid liquid helium in the region where UCNs are stored is of critical importance, because in EDM searches in general the sensitivity depends linearly on the strength of the applied electric field. The design goal is to stably apply a DC electric field exceeding 75 kV/cm inside measurement cells whose inner dimension is 40 cm x 7.62 cm x 10.16 cm. However, the phenomenon of electric breakdown in liquid helium is poorly understood, and as such a major R&D effort is under way. As part of this R&D effort, we measured the dependence of the breakdown field on the pressure and temperature of liquid helium using two apparatus, one with ~1cm size electrodes and the other with ~50 cm size electrodes. In this study, we present the results of these measurements and discuss their implications on the mechanism of electrical breakdown in liquid helium.