Slippery stuff at the cutting edge

Plasma is famously (and etymologically) slippery stuff, and describing its properties to undergraduates is an elusive task. For many students, a course on plasmas provides the first rude introduction to the compromises and evasions when physics has to be applied to environments where it is imperative to describe complicated systems defined by engineering desiderata rather than introduce some clever mathematical ansatz that enables us to address the problem we can solve, instead of the one that we must.

In Introduction to Plasma Physics, Robert Goldston and Paul Rutherford, two distinguished plasma physicists working at the Princeton Plasma Physics Laboratory, have converted their university lectures into a half-year course for final-year undergraduates. With the addition of supplementary material included in the text, the course becomes suitable for postgraduates preparing to do research in plasma physics. Although most of the content is standard, the organisation is novel. There are six units dealing with single particle motion, plasmas as fluids, collisionless processes in plasmas, waves in a fluid plasma, instabilities in a fluid plasma and kinetic theory of plasmas. Plasma wave modes are left for a long while and the book is half done before the plasma frequency is introduced. Also there is significant emphasis on developing an understanding of plasmas by asking where individual particles go, and the Vlasov equation is deliberately delayed.

The choice of material is heavily influenced by the peculiar properties of Tokamaks. Magnetoionic theory, planetary magnetospheres, inertial confinement and so on are barely mentioned. The textbook has yet to be written that emphasises the physics for its own sake and embraces extreme behaviour and broad application. This is perhaps a pity at a time when the fusion programme has stumbled and other applications of the same underlying principles are in the ascendant. In few instances is the reader told why people working with real plasmas became interested in or explore tearing modes, drift waves, Landau damping etc. The broader issues raised by the history of research into controlled thermonuclear fusion are ignored. For many students, having a good appreciation of the context comes before understanding the concepts. There is one refreshing innovation though which explains Hamiltonian maps with simple examples, graphically illustrated using a computer program supplied on disk. The mathematical development ought to make few demands on the student who has mastered introductory courses on classical dynamics, electromagnetic theory, vector calculus and complex variables. The steps between equations are short and the students' basic understanding and technical prowess is not stretched. Carrying out these transcriptions from the original scores is no mean accomplishment and the authors are to be congratulated on their treatment especially of MHD stability. Introduction to Plasma Physics can be recommended for both final-year undergraduates and those intent upon learning the basis of research into fusion plasmas.

Roger Blandford is professor of theoretical astrophysics, California Institute of Technology.