Speaker
Description
We present a modeling framework to evaluate the feasibility of using a close-coupled-secondary (CCS) for quench protection of stellarator magnets. Foreseen compact stellarator fusion reactors feature non-planar high-temperature-superconductor (HTS) coils with a large stored magnet energy, on the order of 100 GJ. Safely extracting this energy in case of a quench requires avoiding excessive temperature, temperature gradients, voltage, and mechanical stress.
CCS, a well-established technique in superconducting magnets [1,2,3], enhances quench detection and protection by enabling lower quench detection voltage thresholds through inductive noise reduction [4]. Additionally, the inductive transfer of current from the HTS primary coil to the metallic secondary reduces localized heating in quenching sections. Furthermore, the induced current in the secondary generates heat, which transfers thermally to the primary and triggers additional quenches (thermal quench-back [1]). This effect is highly dependent on the local magnetic field magnitude and orientation, which vary significantly across the stellarator coil geometry.
To assess CCS for stellarator quench protection, we employ a novel coupled model comprising: 1) an electrical network, 2) a 1D current-sharing model for the HTS primary, and 3) a 3D thermal model. By incorporating a 1D magnetic field profile along the conductor, we quantify localized heating in the secondary, heavily influenced by magnetoresistance. The model also captures the thermal quench-back effect, where thermal contact between the secondary and primary creates additional normal zones. This occurs in the high field regions, where the critical current of the primary is low and the resistive heating in the secondary is high. A limitation of the model is that it can only represent insulated cables due to the 1D current sharing assumption.
We show that incorporating a CCS can significantly help the safe extraction of the stored magnetic energy in a stellarator relative to protection methods such as solely external extraction in combination with balanced voltage taps. The CCS helps distribute energy across the primary, the secondary, and across external dump resistors, enabling better control of voltages and hotspot temperatures. We identify key design trade-offs, including allowable voltages and thermal limits, and highlight an optimal balance between the metallic stabilizer in the primary and the space allocated for the secondary.
[1] https://doi.org/10.1016/0011-2275(84)90049-3
[2] Wilson, M.N. (1983) Superconducting Magnets. Clarendon Press, United Kingdom
[3] DOI: 10.1109/TASC.2015.2510078
[4] DOI: 10.1109/TASC.2016.2529838
