Experimental and Theoretical Studies of Fundamental Limits of Surface Resistance of Superconductors for Next Generation Particle Accelerators

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Superconducting Radio Frequency (SRF) cavities have been used in a variety of particle accelerators for quite some time. The ulra-low electrical resistivity of a superconductor material allows a radio frequency resonator (needed for an accelerator) to obtain a high quality factor (Q) which allows energy storage with very low loss. Future accelerators, important in many areas of science and medicine, will benefit from advances such as the techniques in this award. This award is for advanced studies of superconductors used in SRF accelerators. Superconductors used in SRF are limited by the fundamental surface resistance. The next significant breakthrough in the SRF cavity technology is only possible if the physics of surface resistance is fully understood. The main goal of this research is to provide an understanding of the origin of the various contributions to the power dissipation in superconducting niobium and in particular the origin of the non-linear behavior as a function of the radio frequency field. This award includes integrated experimental and theoretical parts to address the unresolved issues of the surface resistance of superconductors at high radio frequency field, and in particular how the surface composition and structure can be modified and tuned in order to reduce surface resistance. The theoretical work will guide the experimental work which, in turn, will provide feedback to further develop the theory. This research will be performed by two graduate students and thus contribute to the education of accelerator scientists, who are in extremely short supply. The successful completion of this work will be very beneficial for the ongoing R&D of high performance SRF cavities. Addressing the outstanding problems of the residual surface resistance and the increase of the quality factor Q with the radio frequency field magnitude can help guide the ongoing R&D of SRF cavities and evaluate possibilities of promising new materials as possible SRF materials. This work will have a significant impact on the application of the SRF technology in general, and to accelerators operating in continuous wave mode in particular.In this award some of the unresolved issues in our understanding of the surface resistance of superconductors will be addressed, in particular: (1) the physics of the residual resistance, which can exceed 20% of the theoretical contribution; (2) mechanisms of the observed increase of Q with the field and the possibilities of using this effect to extend the increase of Q to higher fields to boost the cavity performance; (3) possibilities of increasing the Q factor by managing impurities; and (4) making SRF materials other than Niobium useful for low field accelerator applications at low temperature.