Development and characterization of Nb3Sn/Al2O3 superconducting multilayers for particle accelerators

Abstract

Superconducting radio-frequency (SRF) resonator cavities provide extremely high quality factors > 1010 at 1–2 GHz and 2 K in large linear accelerators of high-energy particles. The maximum accelerating field of SRF cavities is limited by penetration of vortices into the superconductor. Present state-of-the-art Nb cavities can withstand up to 50 MV/m accelerating gradients and magnetic fields of 200–240 mT which destroy the low-dissipative Meissner state. Achieving higher accelerating gradients requires superconductors with higher thermodynamic critical fields, of which Nb3Sn has emerged as a leading material for the next generation accelerators. To overcome the problem of low vortex penetration field in Nb3Sn, it has been proposed to coat Nb cavities with thin film Nb3Sn multilayers with dielectric interlayers. Here, we report the growth and multi-technique characterization of stoichiometric Nb3Sn/Al2O3 multilayers with good superconducting and RF properties. We developed an adsorption-controlled growth process by co-sputtering Nb and Sn at high temperatures with a high overpressure of Sn. The cross-sectional scanning electron transmission microscope images show no interdiffusion between Al2O3 and Nb3Sn. Low-field RF measurements suggest that our multilayers have quality factor comparable with cavity-grade Nb at 4.2 K. These results provide a materials platform for the development and optimization of high-performance SIS multilayers which could overcome the intrinsic limits of the Nb cavity technology.

Nonlinear Meissner effect in Nb3Sn coplanar resonators

“…( 22), but the orientational dependence of R J (B) is opposite to that of σ ω (B). Thus, GBs can give rise to a strong nonlinearity of the electromagnetic response of polycrystalline films, which can be essential for Nb 3 Sn thin film coatings of high-Q resonator cavities in particle accelerators 28,42,43,53 .…”

“…The fit gives λ(0) = 353 nm, well above the London penetration depth λ(0) ≈ 90 nm for a clean stoichiometric Nb 3 Sn 35 . The latter may result from nonstoichimetric inclusions which cause a slight reduction of T c in our films 27,28 . For λ(0) = 352 nm, w = 15 µm and d = 50 nm, the kinetic inductance L k = µ 0 λ 2 (T )/wd 200 nH/m at 9 K accounts for about 1/3 of the total inductance L = L g + L k with L g = 420.5 nH/m.…”

“…Our polycrystalline films have lateral grain sizes l 2 ∼ 0.1 − 1 µm (see Fig. 2) and local nonstoichiometry causing inhomogeneities of superconducting properties on the same length scales 27,28 . If weakly coupled GBs are regarded as planar Josephson junctions (JJs), each GB has a kinetic inductance 47…”

Next-Generation Superconducting RF Technology based on Advanced Thin Film Technologies and Innovative Materials for Accelerator Enhanced Performance and Energy Reach

“…The outcome of this challenging project is the calculation of the field-dependent quality factor and determination of the conditions under which the extended Q(H) rise can reverse the medium and high-field Q slope up to the breakdown field. This theory can be applicable to new SRF materials like Nb 3 Sn [87] and pnictides, as well as to alloyed Nb multilayers.…”

An Innovative Approach of Surface Polishing for SRF Cavity Applications

“…As mentioned earlier, the best way to evaluate surface quality after metallographic polishing is to measure surface resistance under radio-frequency waves at cryogenic temperatures. However, this type of test is expensive, technically difficult, and shows a very low availability [32][33][34][35]. The final optimization of polishing parameters is not pos- Before producing a new sample for the RF test at a cryogenic temperature additional studies on Q-disease were conducted on the samples after the metallographic procedure.…”