Material Aspects of superconducting cavities beyond state of the art

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Subatomic physics experiments rely on high energy charged particles supplied by particle accelerators. Radiofrequency cavities transform electromagnetic into kinetic energy. Superconducting technology is used worldwide for cavities requiring high efficiency, e.g in the ARIEL and ISAC-II linear accelerators at TRIUMF and the storage ring at the Canadian Light Source. Advanced surface and heat treatments have pushed the performance of state of the art elliptical cavities optimized to accelerate electrons close to fundamental material limitations in terms of power dissipation and maximum accelerating gradient, the latter being the energy gain of charged particles per unit length. Cavities are made of niobium metal sheets with a typical thickness of a few mm while the RF shielding currents only flow within a thin layer as superconducting currents decay exponentially over the length scale of the London penetration which is 32nm in case of clean niobium. Optimized performance is achieved by baking cavities in vacuum or in a low pressure gas atmosphere. These treatments alter the material properties such as the oxygen, hydrogen and vacancy concentrations of the outermost layer through diffusion processes. Depending on the desired performance characteristics different tailored treatments are chosen. For example different treatments yield highest accelerating gradients or lowest losses. Almost all future funded and proposed large scale accelerator facilities will require superconducting cavities, the largest project being the International Linear Collider (ILC) needing about 17000 cavities. If funded, ILC will use niobium cavities with an optimized surface treatment to be determined. Future upgrades will potentially employ superconductors beyond niobium. Optimizing niobium treatments and developing new materials for SRF applications both require in depth material science studies which are proposed here. Most SRF research has been performed on elliptical niobium cavities optimized to accelerate electrons. Advanced surface treatments beyond baking at 120C in vacuum have not been systematically studied on coaxial cavities optimized for relatively slow heavy particles as required for nuclear physics experiments. Coaxial cavities have different field configurations and are operated at lower RF frequencies. A direct translation of treatments from elliptical to coaxial cavities is not possible as the exact mechanisms for the obtained performance enhancements and how losses scale with RF frequency are not completely understood. A coordinated approach consisting of cavity testing and witness sample preparation at TRIUMF's SRF infrastructure with material science investigations at UVic is proposed to gain a better understanding of loss mechanisms in superconductors under radiofrequency exposure. The proposal seeks to establish a new research group in this field benefiting from available infrastructure at both institutes.