Elizabeth (2019) 'Could high H98-factor commercial tokamak power plants use Nb-Ti toroidal eld coils?', IEEE transactions on applied superconductivity., 29 (5). p. 4200405.

A scaling law for J c in commercial Nb-Ti wire is proposed that describes its magnetic field, temperature and strain dependence. The scaling law is used to fit extensive measurements of the total strand critical current density, J c,TS(B, T, ε), with the applied field orthogonal to the axis of the wire. We present critical current density, heat capacity and resistivity measurements to obtain B c 2 * ( θ ) , which shows clear angular anisotropy. At 4.2 K, the resistivity data show B c 2 * ( B ∥ J ) − B c 2 * ( B ⊥ J ) ≈ 1 T . We also discuss whether the fusion community should consider re-optimising standard commercial Nb-Ti wires that were developed for MRI applications at ~ 5 T, to produce higher J c at say 10T, and higher upper critical fields, perhaps using quaternary Nb-Ti alloys with artificial pinning centres.

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 MW e , a plasma fusion gain Q plasma = 17, a net gain Q net = 1.3, a cost of electricity (COE) of $ 1148 (2021 US) per MW h (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 MW e 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 MW e and 500 MW e 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 MW e 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 ≈ 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. With upgrade magnets the plant is more expensive -first it is cost-optimised for Nb-Ti and then upgraded to REBCO coils. The upgrade increases the net electrical output from 100 to 280 MW e with an ≈ 25 % increase in reactor capital cost. We also evaluate likely advances in fusion technology and find that technologies on the horizon will probably not bring further large reductions in capital cost, and that REBCO magnets are generally stress-limited rather than current density limited. We conclude that: the fusion community should develop high B c2 alloys specifically for fusion applications; superconductors should be tested under operational-like radiation at cryogenic temperatures; and that we should proceed now with detailed design and construction of a prototype fusion power plant that integrates and de-risks all the key technologies including high temperature superconducting cables and joints using remountable training magnets, and hence is the last tokamak before commercialisation of fusion energy.

Rare-earth-barium-copper-oxide (REBCO) coated conductor tapes within next-generation tokamak pilot and power plant magnets will be exposed to broad-spectrum gamma-ray and neutron irradiation concurrently. It has been known since the 1980s that cumulative neutron fluence affects the superconducting properties of REBCO, but the effects of gamma rays are less certain, as are the effects of radiation (of any kind) during current flow. However, the use of superconductors as photon detectors suggests that energetic photons interact directly with the superconducting state, locally destroying superconductivity. Hence, as well as the effect of the overall radiation dose (fluence), the effect of radiation dose rate (flux) on the superconductor’s properties must be quantified to understand how REBCO magnets will perform during fusion magnet operation.

In-situ measurements of the self-field critical current at 77 K, of several REBCO coated conductor tapes were performed during Co-60 gamma ray exposure at a dose rate of 86 Gy min−1. Samples were fully submerged in liquid nitrogen throughout the measurements. No change in the critical current of any sample during or after irradiation was observed within standard error. These are the first reported in-situ measurements of critical current during fusion-relevant gamma irradiation. Two samples were irradiated to a further dose of 208 kGy at room temperature and a second round of in-situ measurements was performed. No change in the critical current of these samples was observed within standard error. This corroborates recent studies, but is in conflict with older literature.