On the field dependent surface resistance of niobium on copper cavities

TL;DR

This study investigates the field-dependent surface resistance of niobium-coated copper superconducting cavities, revealing that local heating is minimal and that cooldown speed and baking influence performance.

Contribution

It provides new insights into the thermal behavior and surface resistance dynamics of niobium on copper cavities, including effects of cooldown and baking.

Findings

Inner surface heating is minimal during operation.

Fast cooldown reduces surface resistance.

Low temperature baking has detrimental effects.

Abstract

The surface resistance Rs of superconducting cavities prepared by sputter coating a thin niobium film on a copper substrate increases significantly stronger with the applied RF field compared to cavities of bulk material. A possible cause is that due to the thermal boundary resistance between the copper substrate and the niobium film Rs is enhanced due to global heating of the inner cavity wall. Introducing helium gas in the cavity and measuring its pressure as a function of applied field allowed to conclude that the inner surface of the cavity is heated up by only 60+/-60 mK when Rs increases with Eacc by 100 nOhm. This is more than one order of magnitude less than what one would expect from global heating. Additionally the effect of cooldown speed and low temperature baking have been investigated in the framework of these experiments. It is shown that for current state of the art niobium on copper cavities there is only a detrimental effect of low temperature baking. A fast cooldown results in a lowered Rs.