1100 C. Llewellyn Smith and S. Cowley conventional tokamak: high β (see above) and compactness. This makes it a promising candidate for a cost-effective neutron and plasma source for a component test facility (CTF) for fusion technology development. However, the disadvantage of compactness for a spherical tokamak reactor is that there is insufﬁcient space for superconducting coils. The larger spherical tokamaks, MAST at Culham and NSTX at Princeton, are beginning to reach fusion-scale parameters. The stellarator is a fully three-dimensional toroidal conﬁguration that requires no driven plasma current to conﬁne the plasma—it is inherently steady state. But the three-dimensional coils are very hard to fabricate and the engineering for stellarator reactors remains challenging. A JET-scale stellarator is under construction in Germany. 7. Performance measures—what materials performance has been achieved? The 14 MeV neutron ﬂux across the walls of a reactor is expected to peak at about 2.5 MW m−3. This neutron bombardment will on average displace each atom in nearby parts of the blanket and supporting structures from its equilibrium position some 30 times a year. Displaced atoms normally return to their original conﬁguration (when thermal vibrations bring displaced atoms together with vacancies). Sometimes, however, the vacancies and displaced atoms migrate differently, in which case accumulations at grain boundaries can produce swelling or embrittlement and weaken the material. Because they have much higher energy, fusion neutrons will initiate nuclear reactions that produce helium inside the structural materials about 100 times more copiously, per atomic displacement, than ﬁssion neutrons. There is serious concern that the helium could accumulate and further weaken the structure. Clearly, tests must be done. The neutrons also inevitably produce many other nuclear reactions in the structural material. By careful choice of elements it is possible, however, to ensure that essentially no long-lived radioactive isotopes are produced—such materials are termed low-activation materials. It had been thought that only exotic materials (such as silicon carbide composite ceramics) could survive fusion neutron damage for long periods. The discovery during the 1990s, in tests at ﬁssion reactors, that special low- activation (body centred cubic) steels can probably survive in fusion reactor conditions for around 5 years before they would have to be replaced was, therefore, a very positive and welcome surprise. Silicon carbide composites that could operate at very high temperature (perhaps above 1000◦C), and hence produce power with high thermodynamic efﬁciency, remain attractive in the long term. The so-called plasma-facing materials and the exhaust structure called the divertor (through which particles, impurities and the helium ‘ash’ produced in D–T fusion are exhausted) will be subjected to ﬂuxes of plasma particles and electromagnetic radiation. Typically, reactor designs require particle and radiation ﬂuxes of 500 kW m−2 to the walls and 10 MW m−2 onto the divertor plates. The divertor and plasma-facing wall must resist erosion and survive many years in the hostile reactor environment. Special materials solutions are required and have been proposed for these areas, but they need further development and testing in reactor conditions (see below). Phil. Trans. R. Soc. A (2010)