Q-factor optimization for high-beta 650 MHz cavities for PIP-II

TL;DR

This paper investigates surface treatments, especially nitrogen doping, to maximize Q-factors in high-beta 650 MHz superconducting cavities, demonstrating significant improvements in cavity performance for particle accelerators.

Contribution

It provides new insights into surface treatment protocols, including nitrogen doping and cold electropolishing, to enhance Q-factors and quench fields in multicell cavities at this frequency.

Findings

Nitrogen doping is essential for high Q-factors at medium RF fields.

Adding cold electropolishing after N-doping increases quench field gradients.

High Q-factors achieved in five-cell cavities demonstrate effective surface treatment protocols.

Abstract

High Q-factors are of utmost importance to minimize losses of superconducting radio-frequency cavities deployed in continuous wave particle accelerators. This study elucidates the surface treatment that can maximize the Q-factors in high-beta 650 MHz elliptical niobium cavities. State-of-the-art surface treatments are applied in many single-cell cavities, and surface resistance studies are performed to understand the microwave dissipation at this unexplored frequency. The nitrogen doping treatment is confirmed to be necessary to maximize the Q-factors at medium RF fields. We applied this treatment in five-cell high-beta 650 MHz cavities and demonstrated that extremely high Q-factors were obtained at medium RF fields with this treatment. We also demonstrated that adding a cold electropolishing step after N-doping is crucial to push the quench field of multicell cavities to higher gradients.