High Voltage Test Apparatus for a Neutron EDM Experiment and Lower Limit on the Dielectric Strength of Liquid Helium at Large Volumes

A new search for a permanent electric dipole moment (EDM) of the neutron is underway using ultracold neutrons produced and held in a bath of superfluid helium. Attaining the target sensitivity requires maintaining an electric field of several tens of kilovolts per centimeter across the experimental cell, which is nominally 7.5 cm wide and will contain about 4 liters of superfluid. The electrical properties of liquid helium are expected to be sufficient to meet the design goals, but little is known about these properties for volumes and electrode spacings appropriate to the EDM experiment. Furthermore, direct application of the necessary voltages from an external source to the experimental test cell is impractical. An apparatus to amplify voltages in the liquid helium environment and to test the electrical properties of the liquid for large volumes and electrode spacings has been constructed. The device consists of a large-area parallel plate capacitor immersed in a 200 liter liquid helium dewar. Preliminary results show the breakdown strength of normal state liquid helium is at least 90 kV/cm at these volumes, at the helium vapor pressure corresponding to 4.38 K. These fields hold for more than 11 hours with leakage currents less than 170 pA (about 20% of the maximum tolerable in the EDM experiment). The system is also found to be robust against anticipated radiation backgrounds. Preliminary results for superfluid show that fields of at least 30 kV/cm can be sustained at the volumes required for the EDM experiment, about 60% of the design goal. These results are likely limited by the low pressure that must be maintained above the superfluid bath.