  All high field superconductors producing magnetic fields above 12 T are
brittle. Nevertheless, they will probably be the materials of choice in
commercial tokamaks because the fusion power density in a tokamak scales as the
fourth power of magnetic field. Here we propose using robust, ductile
superconductors during the reactor commissioning phase in order to avoid
brittle magnet failure while operational safety margins are being established.
Here we use the PROCESS systems code to inform development strategy and to
provide detailed capital-cost-minimised tokamak power plant designs. We propose
building a 'demonstrator' tokamak with an electric power output of 100 MWe, a
plasma fusion gain Qplasma = 17, a net gain Qnet = 1.3, a cost of electricity
(COE) of \$ 1148 (2021 US) per MWh (at 75 % availability) and high temperature
superconducting operational TF magnets producing 5.4 T on-axis and 12.5 T
peak-field. It uses Nb-Ti training magnets and will cost about \$ 9.75 Bn (2021
US). An equivalent 500 MWe plant has a COE of \$ 608 per MW suggesting that
large tokamaks may eventually dominate the commercial market. We consider a
range of designs optimised for capital cost (as the reactors considered are
pilot plants) consisting of both 100 MWe and 500 MWe plants with each of two
approaches for the magnets: training and upgrading. With training magnets, the
plant is cost-optimised for REBCO TF magnets. For a 100 MWe plant, the Nb-Ti
training magnets typically produce 70 % peak field on the toroidal field coils
compared to REBCO magnets, 65 % peak field on the central solenoid and cost
approx. 10 % of the total machine cost. Training magnets could in principle be
reused for each of say 10 subsequent (commercial) machines and hence at 1 %
bring only marginal additional cost.
