1094 C. Llewellyn Smith and S. Cowley kinetic energy to fuse. This gas is in fact a plasma since, above a few thousands degrees, inter-atomic collisions knock the electrons out of the atoms to form a mixture of separated nuclei and electrons. In the temperature range 100–200 million degrees Celsius the fusion power per unit volume (PFusion) from a D–T plasma is given roughly by PFusion ∼ 0.08P2 (MWm−3), (3.4) where P is the plasma pressure in atmospheres. It is immediately clear from equation (3.4) that the D–T reaction produces commercially viable power densities (megawatts per metre cubed) at practically sustainable pressures (a few atmospheres). The remarkably large D–T cross section puts fusion power within reach! Many conceptual schemes for using reactions (3.1)–(3.3) to generate power have been proposed. However, the tokamak (see below) is the only device to have produced signiﬁcant fusion (on JET and the Tokamak Fusion Test Reactor (TFTR) in the USA). Moreover, careful studies of power plants based on the tokamak show that they provide a promising route to commercial fusion power (see below and EU Studies (2004)). In this paper, we focus mainly on the tokamak approach. We will not make any remarks about the inertial approach to fusion since it is, at present, a generation behind magnetic fusion. 4. What will a fusion power station look like? Figure 1 shows the conceptual layout of a tokamak fusion power station. Fusion power stations will be similar to existing thermal power stations, but with a different furnace and fuel. We describe a typical design (roughly model B of the fusion power plant designs (EU Studies 2004)) with approximate numbers to set the scale. At the centre is a fusion burning D–T plasma with a volume of approximately 2000 m3 conﬁned in a ‘toroidal’ (doughnut shaped) chamber by a strong magnetic ﬁeld (approx. 5–6 T). D and T are fed into the core and heated to over 100 million degrees Celsius—typically there is of the order of a gram of fuel in the device at any moment. The D–T reaction (equation (3.1)) produces about 3.6 GW of fusion power. The helium nuclei (alpha particles) from the fusion reactions are conﬁned by the magnetic ﬁeld. They are slowed down by collisions on the deuterium and tritium thereby depositing about 700 MW of the fusion power in the plasma. This heating sustains the plasma temperature against leakage of energy across the magnetic ﬁeld. The neutrons produced by the fusion reaction (3.1) escape the magnetic bottle and penetrate the surrounding structure, known as the blanket, which will be about 1 m thick. In the blanket, the neutrons collide with lithium and breed tritium through the reactions in equations (3.2) and (3.3). Most of the tritium is produced by the reaction with 6Li (equation (3.2)). Since natural lithium is about 7.5 per cent 6Li, some blanket designs envisage enhancing the 6Li. There are also various competing reaction channels, which do not produce tritium, but in many cases produce additional neutrons that in turn can Phil. Trans. R. Soc. A (2010)