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。射频腔将电磁转换为动能。超导技术在全球范围内被用于需要高效率的腔体，例如在TRIUMF的Ariel和ISAC-II直线加速器以及加拿大光源的储存环中。先进的表面处理和热处理已将经过优化以加速电子的最先进椭圆腔的性能推向接近基本材料限制的功率消耗和最大加速梯度，后者是单位长度带电粒子的能量增益。腔体由典型厚度为几毫米的Nb金属片组成，而RF屏蔽电流只在薄层内流动，因为超导电流在伦敦穿透的长度范围内呈指数衰减，如果是干净的Nb，其长度范围为32 nm。优化的性能是通过在真空或低压气体气氛中烘烤型腔来实现的。这些处理通过扩散过程改变了材料的性质，如氧、氢和最外层的空位浓度。根据所需的性能特征，可选择不同的量身定制的治疗方法。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都将需要超导腔，其中最大的项目是国际直线对撞机(ILC)，需要大约17000个腔。如果获得资金，ILC将使用表面处理优化的铌腔，具体情况待定。未来的升级可能会使用Nb以外的超导体。优化Nb处理和开发用于SRF应用的新材料都需要深入的材料科学研究，这里提出了这一点。大多数SRF研究都是在优化为加速电子的椭圆形Nb腔上进行的。除了在120℃的真空中烘焙之外，先进的表面处理还没有在核物理实验所需的为相对较慢的重粒子优化的同轴腔体上进行系统研究。同轴腔具有不同的场配置，并且工作在较低的射频频率。直接将处理从椭圆形腔转换为同轴腔是不可能的，因为获得的性能增强的确切机制以及损耗如何随射频频率变化尚不完全清楚。为了更好地理解超导体在射频辐照下的损耗机制，提出了一种由TRIUMF的SRF基础设施的空腔测试和见证样品制备与UVic的材料科学研究相结合的方法。该提案寻求在这一领域建立一个新的研究小组，受益于这两个研究所现有的基础设施。

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.