Nitrogen and argon doping of niobium for superconducting radio frequency cavities: a pathway to highly efficient accelerating structures

Abstract

We report a surface treatment that systematically improves the quality factor of niobium radio frequency cavities beyond the expected limit for niobium. A combination of annealing in a partial pressure of nitrogen or argon gas and subsequent electropolishing of the niobium cavity surface leads to unprecedented low values of the microwave surface resistance, and an improvement in the efficiency of the accelerating structures up to a factor of 3, reducing the cryogenic load of superconducting cavities for both pulsed and continuous duty cycles. The field dependence of the surface resistance is reversed compared to standardly treated niobium. * Corresponding author: annag@fnal.gov

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

“…4,5 This field-dependent reduction in surface resistance causes an increase in Q 0 ; throughout the remainder of this work, we will use the term "positive Q-slope" to differentiate it from the traditional negative Q-slope in the medium-and high-field regions (the positive Q-slope effect is also commonly referred to as the "anti-Q-slope"). 4,6,7 Figure 1 shows examples of this positive Q-slope, medium-field Q-slope, and high-field Qslope. In the field of SRF, the positive Q-slope phenomenon was discovered in 2013 by Grassellino et al at Fermilab in niobium doped with nitrogen and by Dhakal et al at Jefferson Laboratory.…”

“…In the field of SRF, the positive Q-slope phenomenon was discovered in 2013 by Grassellino et al at Fermilab in niobium doped with nitrogen and by Dhakal et al at Jefferson Laboratory. 4,5 In this context, "doping" entails treating an SRF material in a furnace with a low-pressure dopant gas atmosphere (nitrogen-doped bulk niobium is the only material considered in this work). Impurity-doping SRF cavities in this way causes both this reverse dependence of R BCS on the field strength and a decrease in the low-field surface resistance, making the technology highly appealing for new accelerators requiring very efficient SRF cavities.…”

Maximum screening fields of superconducting multilayer structures

“…4 High-field rf performance of the Nb cavities can be boosted by depositing not only materials with higher H s but also alloyed Nb-I-Nb multilayers which can increase H m and benefit from a significant raise of Q(H) with H in a wide field region. [17][18][19][20] A polycrystalline Nb multilayer may be…”

“…The lack of radiation losses and vortex dissipation in the Nb cavities (clean Nb has the highest lower critical field H c1 ≃ 180 mT among type-II superconductors) enables one to probe the high-field nonlinear quasiparticle conductivity 17 which can be tuned by alloying the surface with impurities. [18][19][20] The screening field limit of Nb could be exceeded by using s-wave superconductors with higher H c and the critical temperature T c , but such materials are prone to the dissipative penetration of vortices at H ≃ H c1 < H N b c1 . To address this problem it was proposed to coat the Nb cavities with multilayers of thin superconductors (S) with high H c > H N b c separated by dielectric (I) layers 11 (see Fig.…”

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

“…High Q 0 reduces the considerable costs for cryogenics-both infrastructure and AC wall power for the cryogenic plant-required to cool cavities operating with high duty factor. Treatments such as nitrogen-doping 1 have been invented to substantially improve nominal quality factors, but Q 0 can be strongly degraded by trapped magnetic flux.…”