Synchrotron Radiation and Free‐Electron Lasers. Principles of Coherent X‐ray Generation. By Kwang‐Je Kim, Artur Braun and Zhirong Huang. Cambridge University Press, 2017. Pp. 298. Price GBP 89.99, hardback, ISBN 978‐1107162617.

When I represented the European Synchrotron Radiation Facility (ESRF) Science Advisory Committee at the ESRF Machine Advisory Committee meetings in the early 1990s I had the privilege to meet many of the leading accelerator physicists of the time. ESRF was the first third-generation synchrotron radiation (SR) source in the world and experience of such X-ray light sources and the emission properties in practice was naturally limited. Machine physicists at the meetings took an approach which to me was an amalgam of scientist and of engineer; the latter tempering science theory and experiments with what I call safe parameter specifications of the engineer (‘safety factors’). Thus, I saw undulator X-ray emission spectra tabled for discussion consisting only of the fundamental and third harmonic, and also assuming a minimum undulator magnet gap of 20 mm. I commented that “couldn’t the higher harmonics be added?” and the reply was that they could not be sure of the precision of the periodicity of the placement of each successive magnet, to which I replied ‘it seems like the perfection of a crystal and trying to achieve higher diffraction orders’. Their reply was that shimming of magnets to extend to higher harmonics might be possible. With respect to the undulator minimum gap of 20 mm, smaller undulator gaps were to prove possible, i.e. while still retaining safe electron injection. It is with such a background that I really looked forward to reviewing this book, and which I have enjoyed. I learned in detail from the book that the discipline of SR machine physics experience has come a long way, and has been firmly extended into the domain of X-ray viable free-electron lasers (FELs). The back cover of the book promises ‘a comprehensive resource for graduate students, researchers and practitioners who design, manage or use X-ray facilities’. This book, to say the least, is for the mathematically trained. It certainly brought back memories of the mathematical physics courses of my undergraduate years at York University as well as, of course, resonating with many aspects of my work as a SR facility user and provider, through the different stages of my scientific career. The authors’ view of what is the most interesting aspect of SR in their book was quite revealing; for example, on page 33, ‘modern synchrotron facilities, often referred to as light sources, use high-quality electron beams to provide a high-brightness flux of photons in the soft to hard X-ray region of the spectrum that is not accessible with other sources’. They do not actually say ‘fully tunable’ or ‘polychromatic’. Nor can one find these words in the contents list or subject index. Undulator radiation and their emission lines are a key aspect, I agree, but these emission lines are moved around by varying the magnet gap so as to achieve tunability. Fig. 2.1 does explicitly show the polychromatic nature of the bending magnet and the wiggler though. The figure could be improved by showing numbers on the y-axis, i.e. to emphasize the higher intensity of the wiggler. Also the wavelength shifter as a type of SR source could have been illustrated. The strength of this book is, as I remarked earlier, the extensive mathematical treatments, but in turn the accessibility of the book text to SR and FEL facility users relies on figures like Fig. 2.1. The authors’ book subtitle stresses the principles of coherent X-ray generation and this is indeed brought out in various aspects such as longitudinal and transverse coherence ISSN 1600-5775