# Surface Processing and Discharge-Conditioning of High Voltage Electrodes for the Ra EDM Experiment

**Authors:** Roy A. Ready, Gordon Arrowsmith-Kron, Kevin G. Bailey, Dominic Battaglia, Michael Bishof, Daniel Coulter, Matthew R. Dietrich, Ruoyu Fang, Brian Hanley, Jake Huneau, Sean Kennedy, Peyton Lalain, Benjamin Loseth, Kellen McGee, Peter Mueller, Thomas P. O'Connor, Jordan O'Kronley, Adam Powers, Tenzin Rabga, Andrew Sanchez  
**arXiv:** [2102.11029](https://arxiv.org/abs/2102.11029)  
**Date:** 2021-02-17  
**Summary page:** https://www.summarizepaper.com/en/paper/2102.11029v2/

## Key points

- The Ra EDM experiment aims to search for the atomic electric dipole moment of $^{225}$Ra using high voltage electrodes
- The electrodes are identical, plane-parallel structures with a primary high gradient surface area of 200 mm$^2$
- Surface decontamination and processing techniques borrowed from accelerator physics literature were employed to polish and clean the electrodes
- Niobium and titanium electrodes were used, with niobium electrodes undergoing discharge-conditioning with electric field strengths up to $+52.5$ kV/mm and $-51.5$ kV/mm across electrode gaps ranging from 0.4 mm to 2.5 mm
- One pair of large-grain niobium electrodes successfully underwent discharge-conditioning and validation processes, operating at $\pm 20$ kV/mm with a steady-state leakage current not exceeding 25 pA ($1\sigma$) and an average frequency of discharges per hour at $98 \pm 19$
- These niobium electrodes replaced a copper electrode pair in the Ra EDM experimental setup, operating at an electric field strength approximately 3.1 times larger than that achievable with copper electrodes
- The ultimate limitation of the niobium electrodes is the maximum output capacity (30 kV) of the bipolar power supply utilized in the experiment
- Meticulous surface processing and discharge-conditioning techniques borrowed from accelerator physics literature were successful in preparing and validating the niobium electrodes for use in the Ra EDM experiment, demonstrating improved performance compared to copper counterparts when operating at higher electric fields strengths up to 30 kV/mm

## Summary

The Ra EDM experiment aims to search for the atomic electric dipole moment of $^{225}$Ra using a pair of high voltage electrodes. These electrodes are identical, plane-parallel structures with a primary high gradient surface area of 200 mm$^2$, allowing them to generate reversible DC electric fields. The statistical sensitivity of the experiment is directly proportional to the strength of the electric field in the electrode gap. To ensure optimal performance, the research team employed surface decontamination and processing techniques borrowed from accelerator physics literature. They applied these techniques to polish and clean a set of newly fabricated large-grain niobium and grade-2 titanium electrodes. Three pairs of niobium electrodes and one pair of titanium electrodes were subjected to discharge-conditioning using a custom high voltage test station. During discharge-conditioning, the niobium electrodes experienced electric field strengths as high as $+52.5$ kV/mm and $-51.5$ kV/mm across electrode gaps ranging from 0.4 mm to 2.5 mm. One pair of large-grain niobium electrodes successfully underwent discharge-conditioning and validation processes, demonstrating their ability to operate at $\pm 20$ kV/mm with a steady-state leakage current not exceeding 25 pA ($1\sigma$). Additionally, this pair exhibited an average frequency of discharges per hour at $98 \pm 19$. Subsequently, these niobium electrodes replaced a copper electrode pair in the Ra EDM experimental setup and were revalidated for operation at $\pm 20$ kV/mm. Comparatively, the performance of the niobium electrodes surpassed that of legacy copper electrodes by operating at an electric field strength approximately 3.1 times larger than that achievable with copper electrodes; however their ultimate limitation lies in the maximum output capacity (30 kV) of the bipolar power supply utilized in the experiment. In summary, through meticulous surface processing and discharge-conditioning techniques borrowed from accelerator physics literature, the research team successfully prepared and validated a set of large-grain niobium electrodes for use in the Ra EDM experiment which demonstrated improved performance compared to copper counterparts when operating at higher electric fields strengths up to 30 kV/mm limited by their power supply's maximum output capacity . The findings highlight the importance of electrode quality and conditioning in enhancing sensitivity and accuracy when searching for atomic electric dipole moments experiments such as Ra EDM .