Field and Frequency Dependence of the Surface Resistance of Superconducting Microwave Resonators for Particle Accelerators

Abstract

For decades, the cutting-edge of niobium superconducting radio-frequency cavities for use in charged particle acceleration was limited to operating frequencies up to 1.5 GHz resulting from the strong frequency dependence of the surface resistance which limited achievable accelerating fields and increased cryogenic power costs to unreasonable levels. Recent advancements such as nitrogen-doping, nitrogen-infusion, and niobium(III)-tin coating of niobium cavities have created an opportunity for the use of smaller, higher frequency cavities by de- creasing surface resistance and, therefore, reducing cryogenic costs. Nitrogen-doping and infusion surface treatments introduced an astounding effect called 'anti-Q-slope' where the cavity quality factor increases with increasing accelerating field allowing quality factors to be achieved that were not previously possible. This dissertation will discuss the field dependence of the surface resistance of impurity-infused cavities at fixed frequency (1.3 GHz) and temperature (2.0 K), the design and commissioning of apparatus to perform electropolishing, high pressure rinsing, and radio-frequency testing of high frequency cavities (3.9 and 5.2 GHz), a model that uses data of the field and frequency dependence of the surface resistance to find the optimal operating field, frequency, and temperature that minimizes cryogenic power load per unit beam energy for accelerator design.