Review. The path to fusion power 1093 Clearly, fusion’s promise is huge. The fact that a tiny amount of lithium can produce so much electricity, without any production of CO2 or air pollution, is sufﬁcient reason to develop fusion urgently even if success is not 100 per cent certain. There are no known or expected obstacles but there are certainly challenges. The most obvious challenge—holding fusion fuel at temperatures 10 times hotter than the centre of the sun—has in fact been surmounted. Experiments at the Joint European Torus (JET) in the UK regularly achieve such conditions. Indeed, JET has produced 16 MW of fusion power (Keilhacker et al. 1999), and the International Tokamak Experimental Reactor (ITER; the international fusion experiment being constructed in France) is expected to produce 500 MW for hundreds of seconds at a time. So fusion works. The big question is: when will it be made to work reliably and economically on the scale of a power station? Before attempting to answer, we consider the following questions. What is fusion? What will a fusion power station look like? What has been achieved? What are the outstanding science and technological challenges? We will then argue that attaining the ﬁrst fusion electricity in about 30 years (the EU goal) is challenging but possible. Every attempt should be made to reduce the timescale further. 3. What is fusion? The fusion reaction of primary interest as a source of power on Earth involves two isotopes of hydrogen (deuterium, D = 2H, and tritium, T = 3H) fusing to form helium and a neutron, D + T −→ 4He + n + energy (17.6 MeV). Energy is liberated because helium-4 is very tightly bound: it takes the form of kinetic energy, shared 14.1 MeV/3.5 MeV between the neutron and the helium-4 nucleus. This reaction has a cross section about a hundred times larger than any other fusion reaction at energies of interest. Tritium is unstable with an approximately 12 year half-life and thus it has to be bred from lithium using the neutron from the fusion reaction (3.1) and the reactions (3.1) (3.2) (3.3) and 6Li + n −→ T + 4He + (4.8 MeV) 7Li + n −→ T + 4He + n + (−2.47 MeV). Thus, the fuels are deuterium and lithium. The reaction (3.2) dominates (3.3) in many designs even though 6Li is only approximately 7.5 per cent of natural lithium. One kilogram of natural lithium yields about 3 × 1013 J or equivalent to 9 × 105 l of petrol. To initiate the fusion reaction (3.1), the charged deuterium and tritium nuclei must get close enough for the strong nuclear force to act and bind the helium nucleus together. To get this close, they must approach each other fast enough to overcome their mutual electrical repulsion. Thus, a gas of deuterium and tritium must be heated to over 100 million degrees Celsius (10 KeV)—10 times hotter than the core of the sun—for sufﬁcient nuclei to have the required Phil. Trans. R. Soc. A (2010)