# Avoiding the Formation of Bubbles and Pits in Buffered Chemical Polishing for the Niobium Superconducting Cavity Through Adjusting the Acid Ratio

## TL;DR

This paper presents a new buffered chemical polishing method for niobium superconducting cavities that prevents surface pits and improves performance.

## Contribution

A novel acid ratio optimization in buffered chemical polishing eliminates W-shaped pits and improves RF performance in niobium cavities.

## Key findings

- An optimal acid ratio was determined to eliminate W-shaped pits during buffered chemical polishing.
- Niobium cavities polished with the new method showed excellent RF performance and high accelerating gradients.
- A simplified BCP recipe using only HF and HNO3 can achieve pit-free polishing without H3PO4.

## Abstract

Buffered chemical polishing (BCP) is an important and widely used polishing technique for superconducting radio-frequency (SRF) cavities made of niobium. A common problem encountered during BCP is the formation of bubbles and W-shaped pits on the cavity surface, which may severely limit the RF performance. We report a method to address the problem of W-shaped pits through optimizing the BCP acid ratio. We systematically investigate the effect of the BCP acid ratio through sample and cavity BCP experiments and determine an optimal ratio for the three acids. The new BCP recipe with the optimal acid ratio is verified through the development of niobium cavities with several different shapes, which are shown to be free of pits and demonstrate excellent RF performance; notably, several 3.9 GHz nine-cell cavities present unprecedented accelerating gradients. Furthermore, the findings suggest a simple pit-free BCP recipe that does not require H3PO4, using only HF and HNO3. The method proposed in this study is also appropriate for suppressing pit formation with other acid mixtures or when polishing other metal objects.