Low cost vibrating sample magnetometer to study materials for superconducting radiofrequency cavities

国际直线对撞机（ILC）需要大约17000个由超导材料铌制成的超导射频腔。目前，铌腔在可实现的加速梯度和质量因子方面接近其基本极限。后者是衡量低温效率的指标，而加速梯度是单位长度的能量增益，并最终设定了所需能量输出的线性加速器长度的极限。目前，采用普通导电技术可以获得最大的加速梯度，但其代价是占空比和效率较低。因此，对于ILC的能量升级，目前正在考虑正常和超导的选择。超导腔需要在无磁通的迈斯纳状态下运行，以避免磁涡的耗散。由于加速梯度与表面磁场成正比，第一涡穿透场对加速梯度设置了限制。利用薄膜超导体、超导体与绝缘体的纳米薄层交替或在真空或低压气体气氛下热处理铌的几种途径已被提出。在所有情况下，目标是在更高的场中维持迈斯纳状态（无涡流超导状态），而不是块Nb技术。一些研究表明，层状结构之间的界面为通量穿透提供了额外的能量屏障[T]。久保Supercond。科学。抛光工艺。vol. 30, p. 023001(2017)]以及通过邻近效应恢复缺陷附近阶参量的稳定性[j]。Junginger等人。Supercond。科学。抛光工艺。， vol. 30, p. 125012(2017)]。阐明层状超导体的机制和有效性将需要探测界面附近每层的局部磁相和相同样品上的第一通量穿透场。使用直流方法进行样品研究可以更快、更准确地测量基本限制场，而不受射频表面条件的影响，也不需要构建整个腔体。为了研究每一层的磁相，NSERC通过项目发现拨款资助了beta-SRF设施。本课题要求研制振动样品磁强计，用于测量同一样品上的第一涡穿透场，以预测可实现的最大加速梯度。与商业上可用的仪器相比，在大学开发仪器将会便宜得多，并为学生开发和操作该仪器提供了很大的机会。

The International Linear Collider (ILC) requires about 17000 superconducting radiofrequency cavities made of the superconducting material niobium. Nowadays niobium cavities perform close to their fundamental limit in terms of achievable accelerating gradient and quality factor. The latter is a measure of the cryogenic efficiency while the accelerating gradient is the energy gain per unit length and ultimately sets the limit to the length of a linear accelerator for a required energy output. Currently, largest accelerating gradients are achieved with normal conducting technology but at the cost of low duty cycle and efficiency. Therefore, for an energy upgrade of ILC normal and superconducting options are currently being considered. Superconducting cavities need to be operated in a flux free Meissner state to avoid dissipation from magnetic vortices. As the accelerating gradient is directly proportional to the surface magnetic field the field of first vortex penetration sets the limitation for the accelerating gradient. Several routes have been proposed using thin film novel superconductors, alternating nanometer thin layers of superconductors and insulators or Nb heat treated under vacuum or in low pressure gas atmosphere. In all cases, the goal is to sustain the Meissner state (vortex free superconducting state) at a higher field compared to bulk Nb technology. Several studies have suggested that the interface between the layered structure provides an additional energy barrier for flux penetration [T. Kubo Supercond. Sci. Technol. vol. 30, p. 023001 (2017)] as well as recovering the stability of the order parameter near defects via the proximity effect [T. Junginger et al. Supercond. Sci. Technol., vol. 30, p. 125012 (2017)]. Elucidating the mechanism and effectiveness of layered superconductors will require probing the local magnetic phase at each layer near the interface and the field of first flux penetration on identical samples. Sample studies with DC methods can allow a quicker and more accurate measurement of the fundamental limiting field without being affected by RF surface conditions and without building entire cavities. To study the magnetic phase at each layer NSERC has funded the beta-SRF facility through a project discovery grant. Here funding is requested to develop a vibrating sample magnetometer to measure the field of first vortex penetration on the same samples to predict the maximum achievable accelerating gradient. Compared to a commercially available instrument developing the instrument at the university will be significantly cheaper and give great opportunities for students developing and operating this instrument.