Review. The path to fusion power 1105 (ii) DEMO—ﬁrst electricity The most comprehensive, power plant conceptual study was completed in 2005, in the framework of the European Fusion Development Agreement (EFDA; EU Studies 2004). This study provided important results on the viability of fusion power, and inputs to the critical path analysis of fusion development described above. The study assumed that the ﬁrst fusion power stations will be based on a conventional (ITER/JET-like) tokamak. Details of this study are given in appendix A. Recent studies in Japan (Tobita et al. 2006) (building on experience from Culham’s spherical tokamak MAST and Princeton’s NSTX) have shown the advantages of power plants with smaller aspect ratio, i.e. somewhere between the conventional and spherical tokamak. Thus, unless ITER produces major adverse surprises, it is likely that DEMO and the ﬁrst commercial power plants will be similar to ITER perhaps with a smaller aspect ratio. The programme outlined here (based on the Fast Track proposal) reﬂects an orderly, relatively low risk, approach. It could be speeded up if greater ﬁnancial risks were taken, e.g. by starting DEMO construction before full power in situ tritium generation and recovery have been demonstrated in CTF. The risks could be reduced—and the timetable perhaps speeded up—by the parallel construction of multiple machines at each stage. In particular, with an Apollo project approach, it would be desirable to start building a low-performance DEMO now, in parallel to proceeding in an orderly way via ITER and IFMIF to a superior DEMO; the lessons from actually constructing a DEMO, and confronting the systems engineering issues involved in building a real power station, would be invaluable. 11. Concluding remarks—when will fusion be commercially viable? Fusion power is still being developed, and will not be available as soon as we would like. Initial estimates of the scientiﬁc challenge of conﬁning plasmas at over 100 million degrees Celsius were clearly overly optimistic. The cost and scale of the development were also underestimated—fusion cannot be achieved at small scale. However, as JET has shown, it is now possible to achieve fusion plasma conditions. Furthermore, it appears that it will be possible to build viable fusion power stations, and it looks as if the cost of fusion power will be reasonable. But time is needed to develop the technology in order to ensure that fusion power will be reliable and economical, and to test under power station conditions the materials that would be used in their construction. High availability is probably the greatest challenge that fusion will face in the future. If anything like 75 per cent is going to be reached relatively quickly, further development of fusion technology and a systems engineering approach (focused on buildability, reliability, operability and maintainability, building on experience from ﬁssion) will have to be adopted very soon. Assuming no major surprises, an orderly fusion development programme— properly organized and funded—could lead to a prototype fusion power station putting electricity into the grid within 30 years, with commercial fusion power following some ten or more years later. Fusion could therefore play an important Phil. Trans. R. Soc. A (2010)