Field, frequency and temperature dependence of the surface resistance of nitrogen diffused niobium superconducting radio frequency cavities
arxiv(2024)
摘要
We report the RF performance of several single-cell superconducting
radio-frequency cavities subjected to low temperature heat treatment in
nitrogen environment. The cavities were treated at temperature 120 - 165 C for
an extended period of time (24 - 48 hours) either in high vacuum or in a low
partial pressure of ultra-pure nitrogen. The improvement in Q0 with a Q-rise
was observed when nitrogen gas was injected at 300 C during the cavity cooldown
from 800 C and held at 165 C, without any degradation in accelerating gradient
over the baseline performance. The treatment was applied to several elliptical
cavities with frequency ranging from 0.75 GHz to 3.0 GHz, showing an improved
quality factor as a result of low temperature nitrogen treatments. The Q-rise
feature is similar to that achieved by nitrogen alloying Nb cavities at higher
temperature, followed by material removal by electropolishing. The surface
modification was confirmed by the change in electronic mean free path and tuned
with the temperature and duration of heat treatment. The decrease of the
temperature-dependent surface resistance with increasing RF field, resulting in
a Q-rise, becomes stronger with increasing frequency and decreasing
temperature. The data suggest a crossover frequency of 0.95 GHz above which the
Q-rise phenomenon occurs at 2 K. Some of these results can be explained
qualitatively with an existing model which calculates the intrinsic field
dependence of the surface resistance, considering both equilibrium and
nonequilibrium quasiparticle distribution functions.
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