Impact of mechanical constraints on tokamak design and implications for high field power plants
Two analytical models for sizing the toroidal field coils and central solenoid of a tokamak are developed within the D0FUS system code: a pedagogical thin-cylinder model and a refined thick-cylinder and winding packs model. The refined model shows good agreement with six reference machines and the MADE magnet design code. When the high-field design space is explored for DEMO-class power plants (2 GW of fusion power, Q = 40, tplateau = 2 h), pushing the peak field at the Toroidal Field (TF) coil conductor up to Bmax = 20 T, the radial build emerges as the dominant constraint: in fact, in the baseline wedging/316L configuration, no viable design can be found beyond 20 T, making alternative strategies necessary. The primary levers identified are high-strength steels (e.g CHSN01), alternative mechanical architectures (bucking, plug), and reductions of the effective Central Solenoid (CS) flux demand (for example through auxiliary heating during ramp-up), each carrying an impact of the same order of magnitude on the minimum feasible major radius. Secondary optimisations (conductor shape, radial grading) are shown to provide additional but more modest gains. When all favourable levers are combined (CHSN01, bucking, etc.), compact machines (R0 < 4 m) appear feasible. This suggests that, provided one accepts the associated risks of combining new approaches (CHSN01, bucking, etc.), high-temperature superconductors could unlock the path to compact electricity generating tokamaks.