# Investigation of Laser Macro- and Micro-Polishing on Fine-Grained Niobium Material for Superconducting Radio Frequency Cavities

## TL;DR

Laser polishing of niobium sheets improves surface smoothness for use in particle accelerator cavities, but defects can still limit performance.

## Contribution

Demonstrates laser polishing as a viable alternative to conventional methods for preparing niobium surfaces in superconducting cavities.

## Key findings

- Smooth surfaces with up to 1 GV/m acceleration field gradients were achieved without defects.
- Defective regions limited acceleration fields to around 100 MV/m or less.
- Laser polishing can produce surfaces suitable for superconducting radiofrequency cavities.

## Abstract

Fine-grained Nb metal sheets were successively laser macro- and micro-polished for a potential use of the so-prepared materials in superconducting radiofrequency cavities in particle accelerators. The laser-treated Nb surfaces were investigated by a combination of white light interferometry, optical profilometry, electron microscopy with X-ray spectroscopy, and X-ray diffraction to study the influence of the conditions during the laser treatments on the resulting surface topography, the crystallographic structure, and the chemical composition of the material samples. For optimum polishing conditions, smooth, wavy surfaces with a minimum surface roughness could be achieved. However, local defects such as carbon contamination, as well as holes and cracks in the surface, were found. For the different prepared surfaces, the maximum acceleration field gradients, i.e., the onset fields for field emission (EOn), were determined, indicating that for smooth surface regions without defects, EOn may reach values of up to almost 1 GV/m, while for the pristine, rough surface and local defects such as particles and cracks, EOn is limited to values around 100 MV/m or less. The present study suggests that laser polishing should be considered as an alternative to conventional polishing strategies of niobium accelerator cavities.