Apparatus and method for plasma processing of SRF cavities

TL;DR

This paper presents a novel apparatus and method for plasma etching of the inner surfaces of SRF cavities, addressing non-uniform etch rates through synchronized movement of gas and electrodes, verified on a test structure.

Contribution

It introduces a segmented plasma generation technique with electrode shaping to improve uniformity in plasma etching of complex SRF cavity geometries.

Findings

Achieved uniform etching on cylindrical structures with variable diameter.

Demonstrated effective plasma processing using a test pillbox cavity.

Validated the method with niobium samples showing consistent etch rates.

Abstract

An apparatus and a method are described for plasma etching of the inner surface of superconducting radio frequency (SRF) cavities. Accelerator SRF cavities are formed into a variable-diameter cylindrical structure made of bulk niobium, for resonant generation of the particle accelerating field. The etch rate non-uniformity due to depletion of the radicals has been overcome by the simultaneous movement of the gas flow inlet and the inner electrode. An effective shape of the inner electrode to reduce the plasma asymmetry for the coaxial cylindrical rf plasma reactor is determined and implemented in the cavity processing method. The processing was accomplished by moving axially the inner electrode and the gas flow inlet in a step-wise way to establish segmented plasma columns. The test structure was a pillbox cavity made of steel of similar dimension to the standard SRF cavity. This was adopted to experimentally verify the plasma surface reaction on cylindrical structures with variable diameter using the segmented plasma generation approach. The pill box cavity is filled with niobium ring- and disk-type samples and the etch rate of these samples was measured.