An investigation of the magnetic field of full flux penetration of superconducting materials for radio-frequency cavities

Turner, Dan and Burt, Graeme and Junginger, Tobias (2024) An investigation of the magnetic field of full flux penetration of superconducting materials for radio-frequency cavities. PhD thesis, Lancaster University.

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Abstract

The material of choice for current SRF accelerators is bulk Nb which is reaching the theoretical limits in terms of maximum accelerating gradient, Eacc. To increase Eacc, the magnetic field, B, on the accelerating cavity wall, Bsurf, must be increased. There are multiple methods to increase Bsurf such as new, novel materials. One method to increase Eacc is to use multilayer structures, which consist of superconducting thin films (smaller than the London penetration depth) on the surface that screens Bsurf, such that the superconducting substrate will see a reduced B than on the surface. The increased Bsurf results in a larger Eacc, whilst the bulk substrate is still witness to the same B. As the RF performance is related to B, it is appropriate to investigate the response of a superconductor to an external magnetic field. Whilst commercial magnetometry exists, it consists of limitations. These include flux enhancements, sample alignment, and B penetrating through insulating layers of multilayer structures such that the screening effect will not be observed. These limitations can be mitigated, such as using sample geometries with will known geometries such as ellipsoids. Multilayer structures are difficult to deposit in a 3D geometry, thus a magnetometry system must be designed to be able to accommodate planar multilayer samples by applying B from one side of the sample to the other. A field penetration facility has been designed, built and commissioned at Daresbury laboratory. A DC field is applied from one side of the sample using a C-shaped dipole magnet, similar to that of an RF cavity. Hall probes measure both the applied and the penetrated field. The system underwent a rigorous commissioning process which indicated the system has a random error of 0.9 %. The facility has then been used to investigate new materials for SRF applications.