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

超导射频(SRF)腔已经在各种粒子加速器中使用了相当长的一段时间。超导体材料的超低电阻率使射频谐振器(加速器所需)获得高品质因数(Q)，从而允许以非常低的损耗存储能量。未来的加速器在科学和医学的许多领域都很重要，它们将受益于本奖项中的技术进步。该奖项是对用于SRF加速器的超导体的高级研究。在SRF中使用的超导体受到基本表面电阻的限制。只有在充分了解表面电阻的物理原理的情况下，SRF腔技术才有可能取得下一个重大突破。这项研究的主要目的是了解超导Nb中功率损耗的各种贡献的来源，特别是作为射频场函数的非线性行为的来源。该奖项包括综合实验和理论部分，以解决尚未解决的超导体在高频领域的表面电阻问题，特别是如何修改和调整表面成分和结构，以降低表面电阻。理论工作将指导实验工作，实验工作将为进一步发展理论提供反馈。这项研究将由两名研究生进行，从而为加速器科学家的教育做出贡献，而加速器科学家非常短缺。这项工作的顺利完成将对正在进行的高性能SRF腔的研发非常有利。解决剩余表面电阻和品质因数Q随射频场大小增加的突出问题，有助于指导SRF腔的正在进行的研发，并评估有希望的新材料作为可能的SRF材料的可能性。这项工作将对SRF技术的普遍应用，特别是对工作在连续波模式下的加速器的应用产生重大影响。在本奖项中，我们将讨论一些在我们对超导体表面电阻的理解中尚未解决的问题，特别是：(1)剩余电阻的物理，它可以超过理论贡献的20%；(2)观察到的Q随场增加的机制以及利用这种效应将Q增加到更高的场以提高腔性能的可能性；(3)通过管理杂质来增加Q因子的可能性；以及(4)使Nb以外的SRF材料在低温下可用于低场加速器应用。

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.