1106 C. Llewellyn Smith and S. Cowley role in the energy mix in the second part of the century. The possible role of fusion in Europe from now to the end of this century was studied by the Netherlands Energy Institute in 1998 (Lako et al. 1998). Some of the assumptions in this study no longer look reasonable (e.g. that in 2100 the cost of oil would be $30 a barrel!), although others still look sensible (e.g. the assumed cost of fusion power). All such scenario modelling is of course subject to enormous uncertainties, and should be seen as an exploration of what might happen—not a prediction of what will happen. That said, the results are rather robust. If an unlimited coal supply is assumed, and no constraints are put on consumption, coal will be dominant. On the other hand, if atmospheric CO2 was assumed to be limited to below 600 ppm or a carbon tax of $30 per tonne or more was assumed, fusion was found to play an important role. Fission will also play a major role while the price of uranium remains reasonable and in the longer term fast breeder reactors become accepted. The fact is that it is incredibly hard to meet expected energy demand with constraints on carbon (either introduced by society, or due to the increasing scarcity of fossil fuels). Success in developing fusion as an effective large-scale source of power on earth is not guaranteed. However, given the magnitude of the energy challenge, and the relatively small investment that is needed on the (approx. $5 trillion per annum) scale of the energy market, accelerated/fast track development of fusion is fully justiﬁed in view of its enormous potential. With so few other options available to provide the world’s power as the availability (and willingness to use) fossil fuels decreases, we cannot afford not to develop fusion power. Fortunately, it appears to be within our reach. This work was funded by the United Kingdom Engineering and Physical Sciences Research Council under grant EP/G003955 and the European Communities under the contract of Association between EURATOM and CCFE. The views and opinions expressed herein do not necessarily reﬂect those of the European Commission. Appendix A. Power plant studies Four models (A–D) were studied in the European Fusion Development Agreement (EFDA) study as examples of a spectrum of possibilities. Systems codes were used to vary the designs, subject to assigned plasma physics and technology rules and limitations, in order to produce an economic optimum. The near-term models (A and B) are based on modest extrapolations of the relatively conservative design plasma performance of ITER. Models C and D assume progressive improvements in performance, especially in plasma shaping, stability and protection of the ‘divertor’, through which helium ‘ash’ and impurities will be exhausted. Likewise, while model A is based on a conservative choice of materials, models B–D would use increasingly advanced materials and operate at increasingly higher temperatures (which would improve the ‘thermodynamic efﬁciency’ with which they turn fusion power into electricity). The power plant study shows that the cost of fusion-generated electricity decreases with the electrical power output (Pe) approximately as P−0.4. It was e assumed that the maximum output acceptable to the grid would be 1.5 GW. Given the increase in temperature and hence thermodynamic efﬁciency, the size and gross fusion power needed to produce Pe = 1.5 GW decreases from model A Phil. Trans. R. Soc. A (2010)