Magnetic diffusion models for FAST toroidal magnet coils

FAST is a project of a compact high-field tokamak (up to 8.5 T) with copper coils ( either for toroidal and poloidal fields) operated at cryogenic temperatures (30 °K) to get a higher electrical conductivity of the copper and to permit an adiabatic duty cycle, i.e. the coils cooling is carried out between two consecutive pulses, to minimize the coils size. The Toroidal Field (TF) system is provided by 18 coils, each composed by 14 copper plates [1]. Differently from other high field tokamaks, every plate is suitably worked out in order to realise 3 turns in radial direction, so that the current supply is reduced at the expense of a greater voltage, obtaining so a better balance between the current and the voltage required to the electric supply system. It´s the first time that this particular choice is made, at less for a medium/large size tokamak. Then a careful analysis of the effects due to the magnetic diffusion, during the electromagnetic transients, is needed for this particular configuration of the toroidal coils, also because of the high electrical conductivity of the copper at the envisaged operative temperatures. In fact the need of analyzing the transient electromagnetic phases is due to the key role that magnetic diffusion plays in the coil electric resistivity, not only controlling the density current diffusion and then the ohmic heating in the copper, but also trough the magneto-resistive effect. Of course the resultant instant magnetic field and the density current determine the electromagnetic forces distribution, that together the attained temperatures are the load conditions that have to be evaluated for the mechanical stability of the toroidal coils system. A very unfavourable distribution of these loads, with a strong concentration in a particular area during a transient phase, can result in a local overcoming of the maximum admissible stress in the copper. With this aim in mind a 3D model of the three-layer plate has been developed by t- - he multiphysics code ™COMSOL, to ascertain if the magnetic diffusion could create, during the transient phases, dangerous stress conditions due to the localized concentration of electromagnetic forces or to the large thermal differential conditions, caused by a strong distribution unevenness of the current density. Because a similar model can´t take in account all the effects of the toroidal geometry, even an axi-symmetric 2D model for the entire toroidal system was used, in which a number of corrections in the parameters system were applied, to overcome the artefacts introduced by the axissymmetry of the model that, without these corrections, can´ t describe realistically the discrete nature of the 18 toroidal coils assembly. This 2D corrected model, together with the 3D model of only one plate, instead of all the fourteen constituent a coil, have proved to be a valuable tool to perform transient time analysis in contained elaboration time and without particular hardware requirements, because of their simplicity and reduced size. In the following the main results gained by the analysis with these two models will be showed, after a brief introduction about the hypothesis and the assumptions underlying the models set-up.