Prospects for the use of HTS in high field magnets for future accelerator facilities

The enthusiasm that followed discovery of High Temperature Superconductors (HTS) and the initial genuine hope of a replacement technology that could have taken over from conventional Low Temperature Superconductors (LTS) was damped during the years by difficulties in reaching performance levels of competing materials: insufficient current-carrying performance, short piece lengths, and fragility of the brittle oxide superconductors made development of applications slow and limited to demonstrators or devices less demanding from the point of view of conductor performance. However, thanks to a continuous R&D effort, significant progress was made in the past years on the development of cuprate superconductors. Today long lengths of BSCCO 2223 (km range) and REBCO (a more general acronym for YBCO, where RE = Rare Earth) tape (hundreds of meters range) conductor with controlled and homogeneous characteristics are commercially available, and tremendous progress has been made in the development of BSSCO 2212 round wire, where understanding of current limiting mechanism and implementation of a new production process has enabled a breakthrough with record electrical performance. Even if still at the research level, attainments in materials synthesis and theoretical understanding of physical properties of iron-based materials have increased optimism in the possibility of their development into practical superconductors. High upper critical field and low anisotropy are attractive features for potential high field applications of iron-based tapes and wires. Parameters of merit of a superconductor depend on the operating conditions and on the specific requirements given by the application. A review of the potential of HTS for accelerator technology is presented, with a focus on the presently achieved performance of materials and on the perceived needs for further development for use in future accelerator facilities.