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Theory of Molecular Fluids Volume 2: Applications, by Christopher G. Gray, Keith E. Gubbins, and Christopher G. Joslin

of instabilities are prerequisites for designing a fusion reactor that can generate electrical power. The confinement time depends on plasma density and temperature and hence on effective heating overcoming the various cooling processes. There is presently no analytical model for confinement time. As presented here, through a combination of theory, empirical scaling laws and experimental data, the known performance of one tokamak helps to predict the performance of future designs. For example, the Hmode of operation, observed some years ago on the ASDEX tokamak, greatly increased the confinement time, and is now an important option on other machines, but it can lead into adverse effects, such as increased impurity levels (which cool the plasma) and high transient thermal loads on plasma-facing components. Experiments and computer models are used together to explain these complex transitions and the onset of turbulence. Similarly, the additional plasma heating necessary for ignition requires combinations of model and experiment for the ITER machine to be a successful scale-up from present machines. The different options are clearly explained, with some of the engineering challenges. Experimental tokamaks are able to initiate and probe instabilities, of which there is a ‘zoo’ of different types, each described and analysed in some detail, but a future power reactor will have to avoid their onset which can arise from current and pressure gradients in adverse magnetic fields. The most damaging would be disruptive modes that abruptly destroy the plasma and arise through various preconditions. Edge localised modes, ELMs, associated with the desirable H-mode operation, greatly heat up the divertor surfaces where the plasma is terminated, but some ELM control is possible. At this stage it will be appreciated that interactions between plasma and surfaces present a serious materials challenge to avoid erosion, fuel (tritium) retention and impurity cooling of the fusion reaction. The physics of the plasma edge (sheath region) is perhaps more tractable than plasma instabilities but requires a combination of physics, chemistry and engineering for its control. The basic choice of plasma-facing materials lies between high Z and low Z elements: refractory metals resist erosion and melting, but can introduce impurities into the plasma; graphite and carbon composites have the required thermal load management but erode both chemically and physically, and beryllium introduces toxicity. As explained here, ITER will use a compromise of C, Be and W for different parts of the chamber walls, despite the known problems mentioned above and despite the likelihood of forming uncharacterised ternary compounds when the elements mix through vapour transport. Materials choices affect tokamak design and ITER will provide needed data for the design of the intended demonstration power reactor, DEMO. Plasma diagnostics are an essential part of tokamak experiments, to enable theory and simulation to match actual conditions. A chapter describes the basic measurements and some of the more sophisticated spectroscopies, which provide data on particle distributions and temperatures. The final chapters offer a catalogue of working tokamaks around the world both small and large, from the pioneer machines to the latest constructions, omitting other plasma devices that feed into the fusion programme. Special features are briefly described and key achievements are summarised. The five largest tokamaks are given more prominence as they operate around the breakeven threshold. Their experimental milestones include 410 MW of fusion power from deuterium–tritium fusion in JET, pellet refuelling, Be wall operation, the divertor as a means of controlling impurity particles, disruption observations and control. These have particular interest for the future of ITER, which is designed to achieve ignition and to operate in different scenarios. The concluding remarks are optimistic for the future of fusion in tokamaks and suggest that the economics of a fusion reactor will be commercially acceptable. This is an exciting topic with the prospect of ITER leading to further progress in developing a clean source of electricity. Progressing from pure plasma physics to practical tokamak machines, via matters of plasma confinement, plasma heating, plasma stability andmaterials, there is sufficient in this text to satisfy anyone coming fresh to the field, but with concise summaries of data and computer simulations that will be valuable support for anyone already working in one or other specialist aspect. I highly recommend this book to all in the fusion community, whether student or practitioner.