Quench Detection Study for a HTS Model Dipole Magnet

Abstract

Future high-energy physics demands higher magnetic fields to attain higher beam energies and thus needs novel superconductingmaterials in accelerator magnet applications. Current superconductors operate in the temperature range between 1.8 and 4.2 K and attain 8 T in modern operational machines. Meanwhile, prototypes have been built to operate up to 12 T, which approaches the foreseen maximum attainable practical dipole field of 16 T of modern low temperature superconductors. In the framework of the long-term European development project Eucard-2, within the work package ’Future Magnets’, the technology for dipole magnets in the 16 to 20 Tesla range is explored. High Temperature Superconductor (HTS) materials have the ability to provide these high magnetic fields, but are in an early stage of development. In the recent past, significant improvements have been achieved by increasing current density and reducing cost, which implies the conductors’ future potential. The superconducting regime stretches much higher in both temperature and flux density compared to modern Low Temperature Superconductors (LTS), offering a new range of operating regimes. Higher current densities can be reached at the same operating temperature or, equal current densities can be reached at higher temperatures. The increased operating current density reduces the required conductor volume which leads, consequently, to more compact designs, while a higher operating temperature poses less restraints on the cryogenic cooling facilities. This versatility offers great potential for future magnet applications, however complicates thermal stability. Higher current densities and larger thermal margins result in very local normal zones with high peak temperatures endangering the magnet’s integrity. Adequate protection is therefore essential for safe operation especially in high energy density applications, such as multi-strand cables in accelerator dipoles. A quench should be detected sufficiently in advance, allowing time for the protection measures to take effect. Earlier studies showed traditional voltage detection not to be sufficient, causing permanent damage before the quench is identified. This reportwill explore the possibilities for quench detection for HTS magnets, by studying the underlying quench phenomenon in multi-strand Roebel cables using calculations and experiments to discover new possibilities for robust detection methods. Within this thesis, the multi-physics phenomena preliminary to a quench are analysed using a numerical model and various analytic models. To describe the quench behaviour, the stability of the superconductor is analysed as function of the operating current and cable andmagnet properties. Although this analysis focuses on Roebel cable, the locality of HTS quench reduces the influence of the exact cable geometry at hand. As is shown, a quench is predominantly dependent on the local thermal behaviour at the quench location and the macroscopic magnetic and electric properties of the magnet and cable, such as self-induction and interstrand resistance. LTS quench methods are therefore fairly cumbersome for makingHTS quench calculations. Due to its locality, the quench is difficult to detect especially at high current densities. Detection bymeasuring directly the quench itself, e.g. temperature and/or voltage, cannot cover its entire operating domain. However the effects of a quench, in particular the current redistribution, will be proven to have measurable effects on a much larger scale. Based on the model, a novel quench detection design to identify a quench in a preliminary stage is presented. To demonstrate the quench detectionmethod, a prototype is implemented in Feather-M0.4, a HTS racetrack coil with the goal to explore quench behaviour. A compactRIO was used for sensor data acquisition to verify the model and to validate the quench detection. The magnet tests encountered several practical problems for validating the quench detection design, including the ability to reach the higher current densities and artificially quenching themagnet. Although the test only delivered preliminary results, the quench detection design is implemented in FeatherM2 based on its promising potential.