Ultra-high quality factors in superconducting niobium cavities in ambient magnetic fields up to 190 mG

Impurity-doping is an exciting new technology in the field of SRF, producing cavities with record-high quality factor Q 0 and BCS surface resistance that decreases with increasing RF field. Recent theoretical work has offered a promising explanation for this anti-Q-slope, but the link between the decreasing surface resistance and the short mean free path of doped cavities has remained elusive. In this work we investigate this link, finding that the magnitude of this decrease varies directly with the mean free path: shorter mean free paths correspond with stronger anti-Q-slopes. We draw a theoretical connection between the mean free path and the overheating of the quasiparticles, which leads to the reduction of the anti-Q-slope towards the normal Q-slope of long-mean-free-path cavities. We also investigate the sensitivity of the residual resistance to trapped magnetic flux, a property which is greatly enhanced for doped cavities, and calculate an optimal doping regime for a given amount of trapped flux.

Section: -6mentioning

confidence: 95%

The importance of the electron mean free path for superconducting radio-frequency cavities

Maniscalco

Gonnella

Liepe

2017

“…These recent experiments include the discovery that the fraction of external field that is trapped in the surface of a niobium cavity during cooldown is strongly dependent on the thermal gradient over its surface. 7,8 This flux expulsion may be caused by thermal diffusion forces 9 exceeding forces attracting flux lines to pinning sites (the present case considers forces on flux lines during cooldown from the critical temperature T c ; see Ref.…”

Section: Introductionmentioning

confidence: 99%

Efficient expulsion of magnetic flux in superconducting radiofrequency cavities for high Q applications

Posen

Checchin

Crawford

et al. 2016

Self Cite

Even when cooled through its transition temperature in the presence of an external magnetic field, a superconductor can expel nearly all external magnetic flux. This Letter presents an experimental study to identify the parameters that most strongly influence flux trapping in high purity niobium during cooldown. This is critical to the operation of superconducting radiofrequency cavities, in which trapped flux degrades the quality factor and therefore cryogenic efficiency. Flux expulsion was measured on a large survey of 1.3 GHz cavities prepared in various ways. It is shown that both spatial thermal gradient and high temperature treatment are critical to expelling external magnetic fields, while surface treatment has minimal effect. For the first time, it is shown that a cavity can be converted from poor expulsion behavior to strong expulsion behavior after furnace treatment, resulting in a substantial improvement in quality factor. Future plans are described to build on this result in order to optimize treatment for future cavities.

“…11 Since then, work at Cornell and Fermilab (FNAL) has shown that large spatial temperature gradients are important for maximal flux expulsion during cool down. 12,13 Understanding the dependence of sensitivity of RF losses from trapped flux is crucial now as Q 0 performance and specifications keep increasing. Nearly, all modern cavity preparation techniques use niobium that has a surface RF layer with a lower mean free path than the bulk niobium.…”

mentioning

confidence: 99%

Impact of nitrogen doping of niobium superconducting cavities on the sensitivity of surface resistance to trapped magnetic flux

Gonnella

Kaufman

Liepe

2016

Future particle accelerators such as the SLAC “Linac Coherent Light Source-II” (LCLS-II) and the proposed Cornell Energy Recovery Linac require hundreds of superconducting radio-frequency (SRF) niobium cavities operating in continuous wave mode. In order to achieve economic feasibility of projects such as these, the cavities must achieve a very high intrinsic quality factor (Q0) to keep cryogenic losses within feasible limits. To reach these high Q0's in the case of LCLS-II, nitrogen-doping of niobium cavities has been selected as the cavity preparation technique. When dealing with Q0's greater than 1 × 1010, the effects of ambient magnetic field on Q0 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 °C baked cavities show a sensitivity of residual resistance from trapped magnetic flux of ∼0.6 and ∼0.8 nΩ/mG trapped, respectively, while nitrogen-doped cavities show a higher sensitivity of residual resistance from trapped magnetic flux of ∼1 to 5 nΩ/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 sensitivity of residual resistance to trapped magnetic flux in the dirty limit (ℓ ≪ ξ0), while longer mean free paths lead to lower sensitivity of residual resistance to trapped magnetic flux in the clean limit (ℓ ≫ ξ0). These experimental results are also shown to have good agreement with recent theoretical predictions for pinned vortex lines oscillating in RF fields.