DOE OSTI2014
The minimization of cost and energy consumption of future particle accelerators, both large and small, depends upon the development of new materials for the active surfaces of superconducting RF (SRF) accelerating structures. SRF properties are inherently a surface phenomenon as the RF only penetrates the London penetration depth ?, typically between 20 and 400 nm depending on the material. The greatest potential for dramatic new performance capabilities lies with methods and materials which deliberately produce the sub-micron-thick critical surface layer in a controlled way. Thus, two opportunities arise for the use of SRF thin ?lms as single layer superconductor or multilayer Superconductor-Insulator-Superconductor structures: SRF properties are inherently a surface phenomenon, and when other technological processes are optimized, the fundamental limit to the maximum supportable RF ?eld amplitude is understood to be the ?eld at which the magnetic ?ux ?rst penetrates into the surface, Hc1. Niobium, the material most exploited for SRF accelerator applications, has Hc1 ? 170 mT, which yields a maximum acceler?ating gradient of less than 50 MV/m. Thus, new materials must be investigated to push this limit to the desired range. Although many compounds have shown higher critical superconducting transition temperature (Tc), their critical ?eld (Hc1) is lower than niobium. To circumvent this issue, a Super?conductor/Insulator/Superconductor (S-I-S) multilayer ?lms approach has been proposed to achieve performance in excess of that of bulk Nb. In this system, the critical ?eld is enhanced and the ?uxon penetration delayed by using, on top of the Nb surface, a superposition of superconductor layers, each less than a penetration depth in thickness, separated by a dielectric layer a few nanometers thick. For bulk type-II superconducting materials, vortex entry occurs at surface magnetic ?eld Hc1. The key aspect of Gurevich's analysis is the recognition that vortex entry is inhibited if the surface layer thick?ness (d) of the superconductor is less than the penetration depth, ?. The overlayer provides magnetic screening of the underlying Nb with the attenuation factor exp(d/ ?0). The Nb surface can remain in the Meissner state at ?elds much higher than in bulk due to the increase of the parallel Hc1 in a thin ?lm. This could potentially lead to further improvement in RF cavity performance using the bene?t of the higher-Tc superconductors without being limited by their lower Hc1. Niobium on copper (Nb/Cu) technology for superconducting cavities has proven over the years to be a viable alternative to bulk niobium. However the deposition techniques used for cavities, mainly magnetron sputtering, have not yielded, so far, SRF surfaces suitable for high ?eld performance. High quality ?lms can be grown using methods of energetic condensation, such as Electron Cy?clotron Resonance (ECR) Nb ion source in UHV and High Power Impulse Magnetron Sputtering (HiP?IMS) with energetic ions only and neutral atoms signi?cantly blocked. The main advantages of these techniques are the production of higher ?ux of ions with controllable incident angle and kinetic energy and the absence of macroparticle production. The project in general, is focusing onto understanding the physics underlying the RF superconduc?tivity in layered structures and developing SRF material ?lms suitable as substrate and S-I-S multilayer structures. In ?rst hand, the primary focus is to understand the physics governing the relationship be?tween growth conditions, ?lm microstructure and RF performance for SRF materials produced by energetic condensation. The deposition by energetic condensation in UHV via ECR and HiPIMS are studied for Nb and B1 superconducting compounds on substrates with various lattice match and sur?face conditions (in-situ etching, high energy assisted atomic annealing, deposition temperature). The general energy-temperature zone model developed by André Anders is explored to de?ne the optimized coating conditions for ?lms suitable for single layer and multilayer SRF structures. The contribution of both the ion incident energy and the substrate lattice energy on the nucleation of the SRF materials considered, and some insulating materials (AlN and MgO) and the in?uence of the coating conditions on the resulting materials, superconducting and/or RF properties will be investigated. Attention is be given to minimize the residual stress (intrinsic and thermal) with the choice of lattice match between the ?lm and substrate materials, the coating temperature, and to evaluate the coating methods used in terms of conformality for 3D structures. In second hand, as a proof of concept, the coating condi?tions developed will be used to prepare multi-layer superconductor/insulator/superconductor (S-I-S) ?lms having high quality factor and high gradient breakthrough potential. A thick Nb ?lm (> 1µm) could be used as a substrate. Particular attention is given to the interface between superconducting and insulator ?lms. For this work, various material analyses techniques have been used to character?ize each type of thin ?lm material: electron backscatter di?raction (EBSD), X-ray di?raction (XRD), high-resolution scanning electron microscopy (SEM), transmission electron microscopy (TEM), stylus and AFM pro?lometry, secondary ion mass spectroscopy (SIMS), scanning auger microscopy (SAM). Superconducting and RF measurements are also performed (RRR, Tc, and low-temperature SRF sur- face impedance, SQUID-magnetometry). The insight gained from this project on the physics involved in RF superconductivity of layered structures open the way to enable major reductions in both capital and operating costs associated with future particle accelerators across the spectrum from low foot?print compact machines to energy frontier facilities. Realization of quality thin ?lm SRF cavities may dramatically change the cost structure of acceleration systems and signi?cantly improve performance consistency and reliability.
VALENTE-FELICIANO, Anne-Marie↗