Plasma Processing of Large Curved Surfaces for SRF Cavity Modification

TL;DR

This paper explores plasma-based surface modification of niobium for SRF cavities, focusing on how various parameters affect etching efficiency and surface quality, demonstrating promising results with large curved surfaces.

Contribution

It introduces a plasma processing method for large curved SRF cavity surfaces, analyzing parameter effects and demonstrating effective etching in a non-planar geometry.

Findings

High etching rates achieved with optimized electrode shape

Surface roughness can be controlled through process parameters

Potential for improved SRF cavity performance

Abstract

Plasma based surface modification of niobium is a promising alternative to wet etching of superconducting radio frequency (SRF) cavities. The development of the technology based on Cl2/Ar plasma etching has to address several crucial parameters which influence the etching rate and surface roughness, and eventually, determine cavity performance. This includes dependence of the process on the frequency of the RF generator, gas pressure, power level, the driven (inner) electrode configuration, and the chlorine concentration in the gas mixture during plasma processing. To demonstrate surface layer removal in the asymmetric non-planar geometry, we are using a simple cylindrical cavity with 8 ports symmetrically distributed over the cylinder. The ports are used for diagnosing the plasma parameters and as holders for the samples to be etched. The etching rate is highly correlated with the shape of the inner electrode, radio-frequency (RF) circuit elements, chlorine concentration in the Cl2/Ar gas mixtures, residence time of reactive species and temperature of the cavity. Using cylindrical electrodes with variable radius, large-surface ring-shaped samples and d.c. bias implementation in the external circuit we have demonstrated substantial average etching rates and outlined the possibility to optimize plasma properties with respect to maximum surface processing effect.