Nb3Sn performance in RF fields is limited to fields far below its superheating critical field. Poor thermal conductivity of Nb3Sn has been speculated to be the reason behind this limit. In order to better understand the contribution of Nb3Sn thermal conductivity to its RF performance, we simulated numerically with Matlab program*(based on SRIMP and HEAT codes) the limiting fields under different realistic conditions. Our simulations indicate that limiting fields observed presently in the experiments with RF fields cannot be explained by the thermal impedance of Nb3Sn alone. The results change significantly in the presence of higher losses due to extrinsic mechanisms.
Background:Nb3Sn is a material of interest due to potential in promising performance and cost reduction. It is implemented as a coating of a thin film inside existing Nb cavities, followed by an annealing in vacuum [1]. The goal is to operate at higher temperatures and higher surface magnetic fields.
A series of experiments were carried out in an effort to develop a simple method for predicting magnetic flux expulsion behavior of high purity niobium used to fabricate superconducting radio frequency (SRF) cavities. Using conventional metallographic characterizations in conjunction with high spatial resolution electron backscattered diffraction-orientation imaging microscopy (EBSD-OIM), we found that the flux expulsion behavior of 1.3 GHz single cell SRF Nb cavities is significantly associated with the grain growth of the Nb material during heat treatment. Most of Nb grains rapidly grew during 900 ºC heat treatment, and likely full-recrystallized with 1000 ºC HT. With comparison of the magnetic flux expulsion ratio (Bsc/Bnc) at ΔT = 5 K, the flux expulsion efficiency of the cavities increases along with increasing of grain size. Most interestingly, 900 ºC HT shows a roughly linear trend that suggests this criterion could be used to predict appropriate heat treatment temperature for sufficient flux expulsion behavior in SRF-grade Nb. This result would be used to see if flux expulsion can be predicted by examining the materials coming from the Nb vendor, prior to cavity fabrication.
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