Speaker
Description
The STEP (Spherical Tokamak for Energy Production) program in UKAEA aims to develop a large-scale high-temperature superconducting (HTS) toroidal field (TF) model coil over four years, with plans to scale it up to a full-size TF coil. The TF model coil (TFMC), with approximate dimensions of 1m × 2m and a terminal current reaching up to 100kA, serves as part of the risk retirement program for the TF coil development. This work focuses on the quench protection design of TFMC using partial insulation, which has the potential to prevent avalanche quench triggered by hotspots and to reduce overall voltage during the quench discharge process, thereby enhancing operational safety and stability.
Simulating 3D quench behaviour in partially insulated coils presents several challenges, such as demanding mesh quality, capturing leakage currents in the insulation and the current-sharing process governed by the E-J characteristics of HTS materials. To enable rapid iteration of the TFMC quench design, the STEP team collaborated with Quanscient from Finland to simulate the quench behaviour of TFMC using their Allsolve platform. It incorporates an in-house developed H-Ф formulation tailored for superconducting applications, offering several distinct advantages over conventional H-Ф formulations:
- Automatic cohomology cut generation for applying net currents and
solving for unknown/unconstrained ones. No additional surfaces
(elsewhere called ‘thin cuts’ or ‘cohomology cuts’) required in
geometry. - Direct access to the total voltage (dual to the net current) during
the simulation, including both the inductive and the resistive
voltage.
Using these unique features and the cloud computing resources with extensive and parallel processing of the Allsolve platform, we simulated various quench events in TFMC, such as sudden power-offs and critical current (Ic) drops, optimizing the design of the partially insulated material, including its optimal electrical resistivity. Then, the impact of remountable joints on the quench process of partially insulated TF coils was also investigated. Finally, we examined the influence of mesh size on quench simulation outcomes, particularly its effect on the accuracy of the temperature propagation profile. While achieving a very fine mesh is often impractical in 3D HTS simulations, it remains crucial to assess the implications of coarser meshes on the reliability of the results.