Review of the Book “High-Energy Phenomena in Electric Discharges in Dense Gases” by Leonid P. Babich

The book “High-Energy Phenomena in Electric Discharges in Dense Gases” by Leonid Babich provides a comprehensive review of high-energy processes in high-pressure (dense) gases in the range of tens to hundreds of Torr involving so-called “runaway electrons (REs)” whose energies can range up to MeVs. This monograph is of timely relevance, well-conceived and written concisely and clearly by one of the leading experts in the field. Prof. Babich takes the reader on a journey that begins with the early (1930s) experimental observations of high-energy charged particles and penetrating radiation in thunderstorm clouds to the current state-of-the-art in theory, experiment, simulation, and application and closes the loop with a chapter on current research of high-energy phenomena in natural environments such as thunderstorm clouds and lightning storms. The author has been a very active researcher himself in this field and his knowledge of the pertinent literature is second-to-none. This is a book on a largely underappreciated topic of significant scientific merit written by an expert in the field for an audience that spans a wide range from graduate students to postdocs to academics and active researchers in the field. Even people with little background in the field can use sections of the book to get a first impression of the richness of high-energy phenomena driven by REs in electric discharges in dense gases.

The book consists of 6 chapters. The first two chapters provide an introduction into the subject matter and review the historical development of the field from the early beginnings in the 1930s to about 1970. The organization of the rest of the book (chapters 3-6) reflects a preference by the author to deal separately with theoretical descriptions and concepts and with experiments in the laboratory and in nature. This is justified, although one could imagine a different structure of this part of the book, one that uses the various phenomena as the headings and then provides a comprehensive discussion of theoretical, experimental, and simulation work of the respective phenomenon. I might have preferred the second approach, but this is merely a matter of personal preference. Prof. Babich’s organization of chapters 3 through 6 of his book is methodical, sensible, and can be justified topically and scientifically. Chapter 3 brings the reader up to speed with contemporary concepts and theoretical descriptions of REs and chapter 4 applies these descriptions to the concrete and very important phenomenon of breakdown in discharges in dense gases governed by REs. Chapter 5 is devoted to an in-depth review of laboratory studies starting with experiments carried out in the 1970s. Chapter 6 complements chapter five by describing more recent experiments after 1970 that investigated natural high-energy phenomena, primarily phenomena associated with activities in thunderstorm clouds and near or in lightning storms.

In chapter 1, Prof Babich gives the reader a very illustrative and hands-on introduction into gas discharges and the physical processes in discharges stressing the important role of electron-driven processes. Here he also introduces the concept of ‘runaway electrons, REs” following a historical path starting with Wilson’s hypothesis (1925) of the possibility of non-local processes, e.g. electrons accelerated to very high energies in the lower terrestrial atmosphere by the high electric fields in thunderstorm clouds. Prof. Babich makes the important observation that high-energy phenomena in dense gas discharges involving REs had long been considered an oddity of little basic scientific interest and that this opinion was partly due to the difficulty in carrying out controlled laboratory experiments that could verify theoretical predictions because of the high energies and the short, nanosecond time scales involved. Likewise, the scale and complexity of experiments carried out in nature in the vicinity of a violent thunderstorm made such studies not any easier than laboratory studies and limited the availability of reliable and reproducible data for a long time. In the remaining part of chapter 1, in addition to outlining the rest of the book, the author begins to develop the idea that REs not only participate in many interesting phenomena in discharges in dense gases, but that the simple fact that high-energy REs exist in media as dense as the lower terrestrial atmosphere is intriguing and remarkable in itself.

In chapter 2, Prof. Babich traces the beginning of the concept of REs to Wilson (1925) who postulated that charged particles in a very dense gas can be accelerated to very high energies under certain circumstances by a strong external field rather than be thermalized quickly through (the high frequency of) collisions with neutral atoms in the dense gas. Wilson went on to predict that this process could lead to nuclear reactions and to the generation of penetrating radiation in the space domains of the atmosphere where the electric fields of thunderclouds could be felt. The chapter proceeds to describe in great detail the early experimental attempts to verify Wilson’s hypothesis starting in the 1930s. The author manages to give the reader a very good and illustrative understanding of the experimental challenges and difficulties that faced the early experimentalists when making measurements near thunderstorms. Prof. Babich also points out the ambiguities in the data that were collected under such trying circumstances. Next, the chapter describes a few early attempts to detect penetrating radiation generated by REs in laboratory experiments and ends with the first direct observation of REs in a laboratory experiment in the late 1960s. Throughout the chapter, the author impresses on the reader the notion that experimental studies were challenging and faced enormous difficulties resulting from interferences and from the extreme conditions and short time scales under which the data had to be collected and that the results were often not conclusive or subject to easy criticism.

In chapter 3, Prof. Babich introduces the reader to the current status of the theoretical description of REs. He begins with the hypothesis that there can be only two definitions of REs, a microscopic one and a macroscopic one and that each definition is representative of the level at which the dynamic behavior of the electron motion in a background medium under the influence of an external field is considered. At the microscopic level, the description is essentially stochastic and no well-quantified criterion for runaway can be given. At this level, Prof. Babich cites the seminal work of Kunhardt and co-workers who give the definition of an REs as “a RE is an electron that does not circulate through all energy states available to it at a given reduced field strength, but on average moves towards higher energies”. The author rightfully labels this statement the most general and most properly formulated definition of the runaway phenomenon. In contrast, any macroscopic description becomes deterministic and is based on the equations of moments for the time-dependent distribution function or on the one-particle equations of motion or the energy balance equation. In this picture, a drag force on the electrons from the side of the background medium is used to describe the electron-atom interactions. Electrons that overcome a definite threshold energy can continue to gain energy from the external field and become runaways. The chapter proceeds to introduce the deterministic treatment in detail and explains the notion of the runaway energy threshold. The second part of the chapter is devoted to the kinetic energy approach, which is particularly suitable for the description of an electron ensemble with a large number of particles. This chapter is well-written and covers in great detail the general approaches to the description of REs in various environments. It is obvious to the reader familiar with the subject matter, that Prof. Babich and his co-workers have made significant research contributions to the general theory of REs and that the state-of-the art in this field has benefited tremendously from the work of his group.

Chapter 4 is devoted to the critical and very important issue of breakdown and discharge formation in dense gases initiated by REs. The author starts by stressing that various models describing electrical breakdown in dense gases differ dramatically in many details, even in very essential aspects, but that they all have one feature in common, i.e., they are all local models, that is the values of quantities such as the electron energy, the electron velocity, etc. are determined by the electric field at the same point in space. Prof. Babich then develops the general principles of classical breakdown based on the local approach. The chapter proceeds to introduce non-local models as an essential requirement to describe situations where REs participate in the breakdown in dense gases. Specifically, the author discusses in detail the non-local breakdown models developed in his group and by Kunhardt and co-workers. This may be a comparatively short chapter in the book, but it is scientifically profound, topically very encompassing and comprehensive, and it leaves the reader with a sense of deep insight into the microscopic details of electrical breakdown in dense gases in the presence of REs.

Chapter 5 is devoted to a in-depth summary of laboratory studies of high-energy phenomena in pulsed discharges in dense gases carried out since the early 1970s. This is by far the longest chapter in the book. The author has aimed for and achieved complete coverage of the various experiments. The chapter is written in a very dense form and the reader runs the risk of getting lost in too many details. A first reading of this chapter left me with a sensation of too much of a good thing. There is a tendency that the reader will not be able to distinguish the really significant experiments from those that yielded less important or inconclusive results. In my opinion, less would have been more in this chapter. I would have preferred a selection of the most important experiments with a clear presentation and discussion the significance of their findings. The chapter is rich in important findings and conclusions and describes numerous instances where theoretical and experimental work coupled with simulations revealed important scientific findings and truly advanced the knowledge in the field. It is unfortunate that the historic approach of the author sometimes sidetracks the reader before closing the loop. Nevertheless, the author has provided perhaps the most comprehensive account of laboratory experiments after 1970 in this field, which is quite a remarkable achievement.

In chapter 6, the author returns to experiments in natural environments, i.e., to phenomena associated with thunderstorm activities. This chapter closes the loop started in chapter 2 in terms of experiments in natural environments. The author describes experiments relating to RE effects and X-ray production from thunderstorms and the associated lightning phenomena. Both ground-based experiments and flight experiments are reviewed in detail and the data collected and the conclusions drawn from the data are put into the context of current theoretical models. This is a very well written chapter that illustrates the progress in instrumentation and experimental techniques that has been made since the days of the early experiments and how this progress facilitates the collection of more reliable data. The last section of this chapter is devoted to neutron production by lightning, which is a small area of activity, but one of very high scientific importance in various fields. Neutron production in natural environments are of interest because the neutrons provide quantitative information about the processes in which they are created and because the possible transmutations that they can produce provide tools to the understanding of other physical and chemical processes, in particular processes related to the history of the universe. Again, the author provides a description of the experiments that meticulously bears witness to the details, the experimental difficulties, and the ambiguity of some of the data that were collected, analyzed, and compared to predictions from theory and simulation.

The book ends rather abruptly after chapter 6 with no section providing a summary and/or an outlook. This is somewhat of a shortcoming as the reader reaches a point where it would be helpful to provide some assessment of the status of the field. This assessment can either be “this field is mature, no further basic scientific research is needed” or “ this is what we know so far, these are the remaining challenges”. I clearly feel that the second approach would be the more appropriate one, as the field has not reached maturity by a long shot. It would have been nice, however, if the author had offered his own views as to the current status and the future challenges in this field.

In summary, this is a well conceived and clearly written book on a subject of considerable scientific merit and timely relevance by an author who is an expert in the field and who has a love for elaborating meticulously on the details without sacrificing the big picture (perhaps with the exception of chapter 5). A diverse audience will benefit from Prof. Babich’s profound knowledge and his ability to share it with readers of a broad range of prior exposure to this topic. The only serious criticism that I have (in addition to a minor difference of opinion with the author about the level of detail provided in chapter 5) is that book ends too abruptly and leaves the reader guessing as to the future challenges in the field.

Hoboken, December 2002

Kurt H. Becker, PhD
Professor of Physics
Director, Department of Physics & Engineering Physics
Stevens Institute of Technology
Hoboken, NJ 07030, USA

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