High temperature superconductors for fusion applications and new developments for the HTS CroCo conductor design

Abstract

Recent improvements in the superconducting performance and technical maturity of high-temperature-superconductors (HTS) lead to considerations of using HTS in future fusion magnets in addition to the presently used low-temperature superconductors (LTS). Compact high field magnet systems entirely made of HTS for compact tokamaks, hybrid HTS-LTS magnets for enhanced operational capabilities (e.g. larger flux swing) in conventional tokamaks and future stellarator devices with HTS magnet systems are presently investigated. An overview of recent concepts, developments and designs of HTS-based future fusion magnet systems is given and a number of physical and technical challenges will be addressed, too: In particular, the so-called second generation HTS Rare-Earth-Barium-Copper-Oxide (REBCO) shows an excellent superconducting performance over a wide range of magnetic fields and temperatures. The layered architecture of these technical REBCO conductors and the ceramic nature of REBCO lead to mechanical challenges at high Lorentz forces, which is relevant for cable-in-conduit-conductors (CICC) of future fusion magnets due to the high currents and high field strengths. Different high-current cable concepts for future fusion magnets are discussed. High loads are not only a challenge for potential future HTS conductors but also for the conductor jackets and the coil casing of the magnet systems. An increase of the numbers of load cycles on the path towards a future fusion power plant requires superior mechanical strength. The high critical temperature of HTS materials allows to design HTS CICC with a noticeably higher minimum quench energy and temperature margin compared to LTS conductors – but leads inevitably to substantially slower quench propagation velocity and challenges related to quench detection. Recent results from numerical modeling and experimental investigation of quench in high-current HTS CrossConductor (HTS CroCo) based CICC are addressed.