Material Aspects of superconducting cavities beyond state of the art

亚原子物理实验依赖于粒子加速器提供的高能带电粒子。辐射空腔将电磁能转化为动能。超导技术在世界范围内用于需要高效率的腔体，例如TRIUMF的ARIEL和ISAC-II直线加速器以及加拿大光源的储存环。先进的表面和热处理已经推动了最先进的椭圆形腔体的性能，该椭圆形腔体被优化以加速电子接近功率耗散和最大加速梯度方面的基本材料限制，后者是每单位长度的带电粒子的能量增益。腔由典型厚度为几mm的铌金属片制成，而RF屏蔽电流仅在薄层内流动，因为超导电流在伦敦穿透的长度尺度上呈指数衰减，在干净铌的情况下为32 nm。通过在真空或低压气体环境中烘烤腔体，可实现最佳性能。这些处理通过扩散过程改变材料性质，例如最外层的氧、氢和空位浓度。根据所需的性能特征，选择不同的定制处理。例如，不同的处理产生最高的加速梯度或最低的损失。 几乎所有未来资助和拟议的大型加速器设施将需要超导腔，最大的项目是国际直线对撞机（ILC）需要约17000腔。如果获得资助，ILC将使用铌腔，并进行优化的表面处理。未来的升级可能会使用铌以外的超导体。优化铌处理和开发SRF应用的新材料都需要深入的材料科学研究，这里提出。大多数SRF研究都是在优化的椭圆铌腔上进行的，以加速电子。除了在真空中120 ℃下烘烤之外，还没有系统地研究用于核物理实验所需的相对缓慢的重粒子的同轴腔的高级表面处理。同轴腔具有不同的场配置，并且在较低的RF频率下工作。从椭圆腔到同轴腔的治疗的直接转换是不可能的，因为所获得的性能增强的确切机制以及损耗如何随RF频率缩放还没有完全理解。 一个协调的方法，包括腔测试和见证样品制备在TRIUMF的SRF基础设施与材料科学研究在UVIC的建议，以获得更好地了解在射频暴露下的超导体的损耗机制。该提案旨在利用两个研究所现有的基础设施，在这一领域建立一个新的研究小组。

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.