Sensitivity of Niobium Superconducting Cavities to Trapped Magnetic Flux Dissipation

Abstract

Future particle accelerators such as the the SLAC ``Linac Coherent Light Source-II'' (LCLS-II) and the proposed Cornell Energy Recovery Linac (ERL) require hundreds of superconducting radio-frequency (SRF) cavities operating in continuous wave (CW) mode. In order to achieve economic feasibility of projects such as these, the cavities must achieve a very high intrinsic quality factor ($Q_0$) to keep cryogenic losses within feasible limits. To reach these high $Q_0$'s in the case of LCLS-II, nitrogen-doping has been proposed as a cavity preparation technique. When dealing with $Q_0$'s greater than 1\e{10}, the effects of ambient magnetic field on $Q_0$ become significant. Here we show that the sensitivity to RF losses from trapped magnetic field in a cavity's walls is strongly dependent on the cavity preparation. Specifically, standard electropolished and 120${}^{\circ }$C baked cavities show a residual resistance sensitivity to trapped magnetic flux of $\sim 0.6$ and $\sim 0.8$ n$\Omega$/mG trapped, respectively, while nitrogen-doped cavities show a sensitivity of $\sim$ 1 to 5 n$\Omega$/mG trapped. We show that this difference in sensitivities is directly related to the mean free path of the RF surface layer of the niobium: shorter mean free paths lead to less residual resistance sensitivity to trapped magnetic flux in the dirty limit ($\ell <<{\xi }_0$) while longer mean free paths lead to lower sensitivity in the clean limit ($\ell >>{\xi }_0$). These experimental results are also shown to have good agreement with recent theoretical predictions for pinned vortex lines oscillating in RF fields.

References in corpus (6)

• Nitrogen and argon doping of niobium for superconducting radio frequency cavities: a pathway to highly efficient accelerating structures
• Ultra-high quality factors in superconducting niobium cavities in ambient magnetic fields up to 190 mG
• Dependence of the residual surface resistance of superconducting RF cavities on the cooling dynamics around $T_{\mathrm{c}}$
• Effect of vortex hotspots on the radio-frequency surface resistance of superconductors
• Strong Meissner screening change in superconducting radio frequency cavities due to mild baking
• Nitrogen-Doped 9-Cell Cavity Performance in a Test Cryomodule for LCLS-II

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