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

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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.