High voltage and electrode system for a cryogenic experiment to search for the neutron electric dipole moment

TL;DR

The paper addresses the challenge of attaining a high and stable electric field inside 0.4 K liquid helium to search for the neutron EDM with a sensitivity near $d_n \sim 10^{-28} e\cdot cm$. It develops an in-situ HV system based on Cavallo's multiplier, designs measurement-cell electrodes with low field-enhancement, and identifies electrode materials (Cu-Ge coated PMMA, silicon bronze, SiC) that meet non-magnetic, neutron-activation, and resistivity requirements. It introduces a physics-driven framework combining the Fluctuation-Dissipation theorem and COMSOL modeling to quantify magnetic Johnson noise and eddy-current heating, setting concrete resistivity targets. The results demonstrate feasible HV operation in LHe, quantify breakdown probabilities via a hazard-function approach, and provide a path forward to full-scale construction, with implications for achieving unprecedented nEDM sensitivity in cryogenic environments.

Abstract

The cryogenic approach to the search for the neutron electric dipole moment--performing the experiment in superfluid liquid helium--holds promise for a substantial increase in sensitivity, potentially enabling a sensitivity level of $10^{-28}$ e-cm. A crucial component in realizing such an experiment is the high voltage and electrode system capable of providing an electric field of 75 kV/cm. This, in turn, requires an electric potential of 635 kV to be applied to the high voltage electrode, while simultaneously satisfying other experimental constraints, such as those on heat load and magnetic noise requirements. This paper describes the outcome of a comprehensive development program addressing these challenges. It outlines the system requirements, discusses new insights into relevant physical phenomena, and details selected technical solutions with their corresponding experimental demonstrations and expected performance. The results collectively demonstrate the successful development of the necessary technology for the high-voltage and electrode system for this approach.

Figures (17)

• Figure 1: A detailed view of the components of the central detector system within the composite central volume.
• Figure 2: An example of COMSOL geometries used to calculate the magnetic Johnson noise from the electrodes at the SQUID loops using the method described in Ref. Phan2024 based on the F-D theorem. Here "MC" stands for "measurement cell".
• Figure 3: Top panel: SQUID pickup loop arrangement that use both axial (circles) and planar (rectangles) gradiometers. Bottom panel: expected magnetic field signal detected by each gradiometer. The same color is used for the expected signal and the corresponding gradiometer.
• Figure 4: Calculated magnetic Johnson noise for axial SQUID gradiometers.
• Figure 5: (a) A plot showing typical breakdown voltage distributions measured in Ref. Phan2021. (b) Hazard function obtained from the measured breakdown voltage distribution. The hazard function is well described by the Fowler-Nordheim equation for field emission. (c) The hazard function determined from the measured breakdown field distribution can be used to predict the breakdown field distribution for electrodes of different sizes. The prediction made this way is compared to data in the literature, showing an excellent agreement. (d) The breakdown field depends on the pressure of the LHe.