  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.
