Progress on Electrode Designs and Tests to Generate High Electric Field in Superfluid Liquid Helium-4 for the SNS nEDM Experiment

The SNS nEDM experiment is aiming to search for the neutron electric dipole moment (EDM) with ultracold neutrons (UCNs) stored in superfluid liquid helium-4 at the Spallation Neutron Source (SNS) in Oak Ridge National Laboratory, with a goal sensitivity of 3x10$^{-28}$e-cm, an improvement of two orders of magnitude over the current limit. To achieve the goal sensitivity of the EDM searches,it relies on the stable application of a 75 kV/cm electric field across the UCN storage space without breakdowns in superfluid liquid helium-4. The electrode designs and tests on various geometries and coating materials are undergoing. The high voltage tests in superfluid helium-4 are performed in a specially-designed apparatus at temperatures as low as 0.4 K and pressures between saturated vapor pressure (SVP) of liquid helium and 1 atm.These tests would help in a better understanding of the electric breakdown phenomenon in liquid helium-4. In this talk, we will present the latest progress on electrode designs and tests,and their implications of findings that affect the design of the SNS nEDM experiment.