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The economic viability of fusion power plants such as UK Industrial Fusion Solutions Spherical Tokamak for Energy Production (STEP) [1] is a function of their size, toroidal field (TF) strength and availability throughout their operating lifetime [2]. This optimisation has led STEP to adopt a compact design featuring TF cables comprising a stack of rare-earth barium copper oxide (REBCO) coated conductors (CC) as the current carriers for their magnets, given its excellent current carrying capacity at low temperature and high field [3].
During operation, the magnets of the STEP tokamak will need to operate at high current density to produce the strong fields required for efficient deuterium - tritium fusion. STEP consists of several electromagnets, each of which will generate a magnetic vector that will impinge on the magnet itself and adjacent electromagnets and this will change over the course of a plasma pulse. Given that the TF coil is to use REBCO as its current carrier and that it has been reported that REBCO current carrying capacity is a function of temperature (T), field strength (B), and field angle (θ, φ) [4], [5], knowing how the magnetic vector field impinging on the REBCO in the TF coil’s cable changes is required to accurately determine the margin between the operating current and the limit of the cable’s current carrying capacity, defined here as the critical current per unit width of a single tape (Icpw) multiplied by the number of tapes in the cable’s tape stack.
In this work, the STEP magnet design was analysed to determine how the magnetic field vector impinging on each cable of the STEP TF coil varies along its length. This is compared to Icpw(B,θ,φ) data acquired for a REBCO CC made by Faraday Factory using a multi-axis sample goniometer [6], [7]. The goal was thereby to determine if the operating current of the TF coil exceeds the Icpw for each cable at any point along its length. Results of the magnetic field vector analysis at different moments during a STEP plasma pulse allow the cable sections with the lowest margins to be identified and resulting design recommendations to be made.
[1] Chapman et al. 2024. doi: 10.1098/rsta.2023.0416
[2] Sorbom et al. 2015. doi: 10.1016/j.fusengdes.2015.07.008
[3] Wimbush, Strickland 2017. doi: 10.1109/TASC.2016.2628700
[4] Zhou et al. 2025. doi: 10.1088/1361-6668/ad9ad6
[5] Wimbush, Strickland 2022. doi: 10.1088/1361-6668/ac4172
[6] Hopkins et al. 2012. doi: 10.1016/j.phpro.2012.06.171
[7] Wiegand 2010. PhD Thesis, Cambridge University
