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Tokamaks rely upon a centre solenoid to initiate and warm up the plasma at the beginning of each pulse [1]. A conventional centre solenoid consisting of hollow copper conductors and insulated with epoxy resin-impregnated glass and kapton tape was used in JET [2]. In the larger ITER, however, it was decided that only a superconducting coil could fulfil the flux requirements. NbTi cooled down to the boiling temperature of helium is used in this case [3]. The introduction of Nb3Sn and/or high temperature superconductors appear as the only viable options for the even bigger DEMO [4]. Nevertheless, cryogenically cooled materials that exhibit conventional conduction behaviour could still be a feasible solution for smaller devices that promise to deliver a simpler design for plasma confinement and heating. This is the case of the spherical tokamaks, where the bootstrap current greatly contributes to plasma heating, and hence less externally applied flux variation is needed. Moreover, in this kind of devices the leaner centre pylon limits the neutron shielding options, and neutron triggered effects would reduce the endurance of delicate materials such as HTS. In this sense, this contribution will present an optimisation procedure, from the point of view of the purity of the material, of conventional, cryogenically cooled centre solenoids. It will show how higher purities do not always imply a reduction in losses [5]. The effect of neutron radiation will also be accounted for [6]. Finally, a simple method to connect current feeders to the centre solenoid will also be presented.
[1] P. Rutherford, “The Tokamak: 1955-80,” Nuclear Fusion, vol. 20, no. 9, pp. 1086-1092, 1980.
[2] J. R. Last, D. Cacaut, A. P. Pratt, J. C. Rauch, P. J. Ferry, U. Arensmann and A. Alvarez, “Poloidal coils and transformer core for JET - Design and manufacture,” IEEE Transactions on Magnetics, vol. 17, no. 5, pp. 1923-1926, 1981.
[3] B. Lim, F. Simon, Y. Ilyin, C. Y. Gung, J. Smith, Y. H. Hsu, C. Luongo, C. Jong and N. Mitchell, “Design of the ITER PF coils,” IEEE Transactions on Applied Superconductivity, vol. 21, no. 3, pp. 1918-1921, June 2011.
[4] M. Kumar, K. Sedlak, X. Sarasola and P. Bruzzone, “Preliminary design of DEMO PF coils based on EU DEMO 2018 baseline,” IEEE Transactions on Applied Superconductivity, vol. 30, no. 4, p. 4201505, June 2020.
[5] H. Zhang, A. C. Smith, M. F. Iacchetti, P. M. Tuohy, A.-V. Rusu, S. M. Harrison and V. Climente-Alarcon, “Optimizing loss performance in aluminum litz wires for cryogenic electrical machines,” IEEE Transactions on Energy Conversion, vol. 40, no. 3, pp. 2399 - 2409, September 2025.
[6] H. R. Brager, “Effects of neutron radiation to 63 dpa on the properties of various commercial copper alloys,” Westinghouse Hanford Company, 1986.
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