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

摘要

A new search for a permanent electric dipole moment (EDM) of the neutron isunderway using ultracold neutrons produced and held in a bath of superfluidhelium. Attaining the target sensitivity requires maintaining an electric fieldof several tens of kilovolts per centimeter across the experimental cell, whichis nominally 7.5 cm wide and will contain about 4 liters of superfluid. Theelectrical properties of liquid helium are expected to be sufficient to meetthe design goals, but little is known about these properties for volumes andelectrode spacings appropriate to the EDM experiment. Furthermore, directapplication of the necessary voltages from an external source to theexperimental test cell is impractical. An apparatus to amplify voltages in theliquid helium environment and to test the electrical properties of the liquidfor large volumes and electrode spacings has been constructed. The deviceconsists of a large-area parallel plate capacitor immersed in a 200 literliquid helium dewar. Preliminary results show the breakdown strength of normalstate liquid helium is at least 90 kV/cm at these volumes, at the helium vaporpressure corresponding to 4.38 K. These fields hold for more than 11 hours withleakage currents less than 170 pA (about 20% of the maximum tolerable in theEDM experiment). The system is also found to be robust against anticipatedradiation backgrounds. Preliminary results for superfluid show that fields ofat least 30 kV/cm can be sustained at the volumes required for the EDMexperiment, about 60% of the design goal. These results are likely limited bythe low pressure that must be maintained above the superfluid bath.