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Topic

Characterization of SRF Cavity Materials with Radioactive Beam Based Techniques for Gradient Enhancement

Department of Physics and Astronomy

Date & location

• Wednesday, December 17, 2025
• 10:00 A.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Tobias Junginger, Department of Physics and Astronomy, ӣ��Ӱ�� (Co-Supervisor)
• Prof. Robert Laxdal, Department of Physics and Astronomy, UVic (Co-Supervisor)
• Dr. Andrew McFarlane, Department of Chemistry, University of British Columbia (Outside Member)
• Dr. Ryan McFadden, Accelerator Division, TRIUMF (Outside Member)

External Examiner

• Dr. Marc Wenskat, Staff Scientist R&D, Physics, Deutsches Elektronen-Synchrotron DESY

Chair of Oral Examination

• Dr. Karen Courtney, School of Health Information Science, UVic

Abstract

Superconducting radio frequency (SRF) cavities accelerate charged particle beams by transferring radio frequency (RF) energy. When operated near resonance, the applied RF power generates strong electromagnetic fields, with the electric field driving beam acceleration and the accompanying magnetic field running along the cavity surface. If this magnetic field becomes too large, the superconductor quenches, transitioning to the normal state and dissipating the stored energy. State-of-the-art Nb cavities are limited by their superheating field (𝐵_sh), which defines the maximum sustainable surface field in the superconductor’s Meissner state [1, 2]. Since applied magnetic field (𝐵₀) scales with the accelerating gradient (𝐸_acc) (i.e., the energy gain per unit length), increasing the 𝐵_sh limit allows a given beam energy to be achieved with a shorter linear accelerator, thereby reducing both the overall length and cost.

Coating Nb with thin superconducting layers of larger penetration depth 𝜆 than Nb, with or without insulating spacers (i.e., forming superconductor-superconductor (SS) or superconductor-insulator-superconductor (SIS) heterostructures), can enhance the attainable 𝐸_acc by sustaining the Meissner state beyond each layer’s 𝐵_sh, supported by reduced surface currents and interfacial barriers. To investigate how these coatings modify local magnetic behavior, “exotic” ion-implanted 𝛽-detected spin spectroscopies (i.e., muon spin rotation (𝜇SR), low energy muon spin rotation (LE-𝜇SR) and 𝛽-detected nuclear magnetic resonance (𝛽NMR)) were used to probe the internal fields within such heterostructures.

Previous 𝜇SR studies have indicated the presence of an interface barrier to flux penetration at the boundary between two superconductors, while low-temperature baking (LTB) treatments showed apparent increases in the vortex penetration field (𝜇₀𝐻_vp) where surface pinning effects could not be excluded. Motivated by these findings, 𝜇SR measurements were performed on Nb₃Sn(2 μm)/Nb and LTB-treated Nb samples to distinguish between interface-barrier and pinning contributions. Using thin Ag foils as energy moderators for the implanted muon spin probes, depth profiling in the 10 μm-100 μm range of an SS bilayer revealed a 𝜇₀𝐻_vp consistent with Nb’s metastable superheating field (𝜇₀𝐻_sh), indicating a surface barrier that delays flux entry. In contrast, the LTB-treated Nb exhibited a 𝜇₀𝐻_vp close to the lower critical field lower critical field (𝜇₀𝐻_c1) of Nb, with values increasing with implantation depth, provides clear evidence for surface-localized flux pinning.

Further optimization of multilayer coatings requires understanding how layer thickness influences screening currents. To this end, LE-𝜇SR measurements probe nanoscale (depths ≲150 nm) Meissner screening in Nb_1−xTi_xN(𝑥 nm)/Nb bilayers with surface-layer thicknesses 𝑥 = 50, 80, and 160 in applied fields of ≤ 25mT. The results confirm strong current suppression in the surface layer, consistent with theoretical predictions, and reveal bipartite field profiles well described by London theory. The extracted penetration depth 𝜆_Nb1−xTi_xN agrees with bulk values, establishing the optimal coating thickness for maximizing the vortex penetration field. These findings emphasize the importance of multilayered superconducting/insulating stacks for achieving the highest 𝐸_acc.

To further investigate multilayer enhancement mechanisms, the superconducting and normal-state properties of a thin-film Nb_0.75Ti_0.25N(91 nm) in aNb_0.75Ti_0.25N (91 nm)/AlN(4 nm)/Nb SIS heterostructure were studied using 𝛽NMRin the vortex state. Resonance spectra displayed broad, symmetric lineshapes at all temperatures, with additional broadening below critical temperature (𝑇_c) ∼15K, yielding penetration depth (𝜆) and upper critical field (𝐵_c2) values consistent with literature. Spin-lattice relaxation (SLR) data exhibited metallic Korringa behavior at low temperatures, modified below 𝑇_c by a Hebel-Slichter peak characterized by a superconducting gap and modest Dynes-like broadening, confirming the superconductor’s strong-coupling behaviour. Possible sources of high-𝑇 SLR dynamics are suggested.

Finally, this characterization of the superconducting layer in the SIS heterostructure establishes a foundation for future studies of the Meissner–vortex phase transition to directly test field enhancement. The high-parallel-field 𝛽NMR spectrometer [3] has been upgraded for such transition measurements, increasing the maximum 𝐵₀ from 24mT to ∼200mT (comparable to the lower critical field (𝐵_c1) of Nb) parallel to the sample surface, enabling investigations under realistic SRF cavity conditions.