ITER is planned to operate at a nominal fusion power of 500 MWt. If DEMO (the next device after ITER, and the first to generate electricity) is to be a device of approximately similar physical size (and hence cost), its fusion power level has to be increased by about a factor of 4, so that the electrical power potentially delivered to the network will be in the range of 500 MWe, typical of one of today's power stations (albeit rather a small one). The general level of heat fluxes through the walls will be about 4 times higher than in ITER, and plasma performance needs to be improved to gain this 4-fold increase. Calculations show that this performance could be achieved with an ~15% increase in ITER linear dimensions, and an ~30% increase in the plasma density above the nominal expected to be confined by the basic magnetic fields on ITER (this capability can be checked on ITER). It then remains for enough to be learnt on the ITER blanket test beds to allow the DEMO blanket to be designed to withstand 4 times the ITER steady heat loads on those components.
If these systems work successfully on DEMO, DEMO itself can be used as a prototype commercial reactor creating a "fast track" to fusion. This would accelerate the availability of fusion as an energy option by about 20 years. A further step would no doubt subsequently be made for the first-of-a-series commercial-sized fusion power reactor (PROTO), doubling the electrical power by increasing linear machine dimensions by less than 10%, without assuming any improvement in physical behaviour.
Fusion Power Reactor Economics
Challenge 7 - economic viability - incorporates the solutions adopted to resolve all the other challenges. Assuming plant capital cost scales with the tokamak volume, one can expect DEMO capital costs in the region of 14 
/We based on the cost estimates for ITER. Those of PROTO will then be typically 8 /We and, with economies of series production of fusion plants subsequently, capital costs could reduce to ~ 4 /We. This should be compared to today's fission and coal plants at ~ 3 /We and 1.5 /We respectively. However, the capital costs of today's coal plants do not include costs to mitigate environmental damage, nor do any of the above costs include the fuel, operating and decommissioning costs, which for coal are typically comparable to the capital costs and should be lowest for fusion. A more rigorous treatment of potential fusion electricity prices in relation to those of coal and nuclear is given by this model.
Fusion therefore currently looks like it can reach a position of economic competitiveness with other energy sources, that will be available at that time, if things work out well. It is evident that there are a number of challenges to be met and overcome before one can really guarantee that economic viability, and the table shows the overlapping nature in which it is planned to solve the challenges of fusion on the coming devices. Although it is not possible to know if ITER and steps beyond it will throw up further challenges, the timescale predicted from today's perspective seems achievable provided the development programme continues at today's level during the period.
Conclusion
The last 50 years of fusion research and development have continually thrown up new challenges to test the ingenuity and skills of at least two generations of scientists and engineers. So far they have been up to the job and no "show-stoppers" are in view. Nevertheless there are many challenges still to be faced, and there may be some which are unseen from today's perspective, so it is impossible to guarantee the delivery schedule completely. The next step, ITER, which is essential to realising the key technologies of a viable energy source, has to be built and operated first. Only then will it be possible to check with confidence the accuracy of the prediction of Lev Artsimovitch, grandfather of the tokamak, who said in 1972 - "Fusion will be there when society needs it".
Fusion development has many unique aspects:
- it has a very specific goal which will take a long time to reach and which satisfies such a basic human need that it can only be developed by governments as an option they would like to posess - normal rules of commercial development within the world's short-term economic perspective do not apply in a situation where resources are reaching their limits;
- it requires continuity of investigation and passing on of know-how across generations of physicists and engineers, requiring not only continuous levels of funding for the main research line, but also funding to attract and train newcomers to the field on supporting experiments;
- it requires multidisciplinary approaches to the solutions of problems;
- it is the classic example of open research conducted on a world stage and used to bring scientists and engineers together from different backgrounds and political allegiances to share their knowledge openly.
Because of these aspects, many of the challenges originally posed have since been met, and certainly with them the plan and timing for reaching the ultimate goal is as secure as it can be.