1104 C. Llewellyn Smith and S. Cowley generated in a fusion reactor. These neutrons will provide estimated displacement rates (in steel) of 50, 20 and 1 displacements per atom per year over volumes of, respectively, 0.1, 0.5 and 6 l. The priority at IFMIF will be to fully test the relatively conventional materials that are likely to be used in early fusion power plants, but it is very important also to push forward the development of advanced materials (such as Si–C composites) that would allow higher blanket temperatures and hence greater efﬁciency in generating electricity. The European Union and Japan are currently spending ¤150 M on ﬁnal design and prototyping for IFMIF. However, there is no commitment to proceed to construction, and no site has been agreed. 10. What next? (a) The ﬁnal technology stages—component test facility and DEMO As any nuclear engineer knows there is much more to a working reactor and commercial power generation than a demonstration of scientiﬁc feasibility. Several components of the future fusion reactor—in particular, the systems that convert neutron power to electrical power—have yet to be tested at any scale. We believe that testing is essential and a CTF should be built (here we add an element to the usual fast-track scenario) to test wall and blanket components at full power. In our opinion a CTF will be needed before DEMO. But in any case it will be needed in parallel with and beyond DEMO to optimize components for succeeding reactors. The ﬁnal technology integration must take place in a demonstration reactor, DEMO. The fast track calls for construction of DEMO (the ﬁrst power plant, albeit an experimental one) to begin in 20 years. (i) Component test facility A CTF would be a compact, affordable (¤1B), driven fusion device that can deliver reactor-level neutron and heat ﬂuxes over many square metres. It would not be required to make net power. Instead, it would be dedicated to testing whole components of the blanket and wall at full power for many years. The materials supporting the blanket must retain structural integrity under these very challenging conditions. Several blanket designs will face their ﬁrst nuclear tests in the later stages of ITER operation. The spherical tokamak is the prime candidate for a CTF. Culham has pioneered spherical tokamaks and currently operates MAST (the MegaAmp Spherical Tokamak) that has achieved near fusion plasma conditions at very modest scale. The National Spherical Tokamak Experiment (NSTX) at Princeton in the USA also operates at about the MAST scale. Both Culham (Voss et al. 2008) and a team in the USA (Peng et al. 2005) have developed conceptual designs of CTFs based on spherical tokamaks. These designs are compact and affordable. A pragmatic approach to hasten fusion would be to build CTFs in parallel with ITER. A vigorous programme of wall and blanket development on these test facilities coupled with ITER’s programme could pave the way for the ﬁrst demonstration reactors in the 2030s. There are no plans for a CTF in the current international programme. But there should be. Phil. Trans. R. Soc. A (2010)