For some time now there has been an interest in the nop. linear interaction of electromagnetic waves in plasma [1,2]. But only in the last few years has the theory of nonlinear wave inter action effects undergone such vigorous development as to result in the formulation of clear phYSical concepts regarding the mech anisms of interaction. This development has been engendered by attempts to solve many of the plasma-physical problems accom panying the tremendous growth of experimental research [3]. The importance of nonlinear effects in modern plasma physics is dis cussed in detail in Chap. I. At this point we merely stress the fact that today the analysis of nonlinear effects is a practical ne cessity in any experiment involving plasma instabilities. We should also point out that plasma instabilities can assert them selves extensively in solids (solid state plasma) and play an im portant part in the study of cosmic plasma. Consequently, the problems of nonlinear wave interaction in plasma are of concern to those working in widely different areas of physics. Yet it is difficult to assimilate the results of investigations on nonlinear effects, owing to the complicated way in which the results of orig inal research are presented. In the present book the author hopes in some measure to fill the need for a text on the physics of non linear effects that is accessible to a fairly general audience.

In recent years the significant progress in satellite-based observations of plasma states and associated electromagnetic phenomena in space has resulted in the accumulation of much evidence of various plasma instabilities. Today plasma instabilities are believed to be responsible for electromagnetic radiation as well as for many of the macroscopic dynamics of plasmas in space. Most students who begin to study plasma physics are intrigued by the unstable nature of plasmas compared with other states of matter; however, they often become frustrated because there are so many in stabilities. Such frustration explains in part why there is no textbook which treats this subject exclusively. A description of plasma instabilities in a systematic way is nontrivial and takes a pertinacious effort. This book is an attempt to provide a basic introduction on the subject and covers most of the important instabilities. However, the author must apologize for any omission of references to contributions of individuals who deserve more credit. The reader is assumed to have a general knowledge of plasma physics obtainable in an undergraduate course. The book is intended to be used as a reference text on the subject of plasma instabilities at the under graduate level as well as for a text in a special course in graduate school. Because the book is part of a series on physics and chemistry in space, emphasis is placed on plasma instabilities relevant in space plasmas.

A variety of nonlinear effects occur in a plasma. First, there are the wave steepening effects which can occur in any fluid in which the propagation speed depends upon the wave-amplitude. In a dispersive medium this can lead to classes of nonlinear waves which may have stationary solutions like solitons and shocks. Because the plasma also acts like an inherently nonlinear dielectric resonant interactions among waves lead to exchange of energy among them. Further, an electromagnetic wave interacting with a plasma may parametrically excite other waves in the plasma. A large-amplitude Langmuir wave undergoes a modulational instability which arises through local depressions in plasma density and the corresponding increases in the energy density of the wave electric field. Whereas a field collapse occurs in two and three dimensions, in a one-dimensional case, spatially localized stationary field structures called Langmuir solitons can result. Many other plasma waves like upper-hybrid waves, lower-hybrid waves etc. can also undergo a modulational instability and produce localized field structures. A new type of nonlinear effect comes into play when an electromagnetic wave propagating through a plasma is strong enough to drive the electrons to relativistic speeds. This leads to a propagation of an electromagnetic wave in a normally overdense plasma, and the coupling of the electromagnetic wave to a Langmuir wave in the plasma. The relativistic mass variation of the electrons moving in an intense electromagnetic wave can also lead to a modulational instability of the latter.

A Nobel Foundation Symposium on the subject: "Nonlinear Effects 1n Plasmas", was held at Aspenasgarden, Lerum, in the G6teborg area of Sweden from June 11-17, 1976. The Symposium was the 36th in the series of Nobel Foundation Symposia, which have been held mainly within the areas of physics, chemistry, medicine, literature and peace prizes. Some 30 leading experts from the United States, Soviet Union, Japan and Western Europe attended the Symposium. The purpose of the Symposium was to discuss various topics in the field of modern plasma physics. We had to select from this vast area of active research a suitable common theme with a great number of new and interesting contributions. We decided to devote our Sym posium in particular to nonlinear effects in plasmas and to emphasize some areas where important developments seemed to be taking place. In recent years basic theory and experiments in nonlinear plasma physics have been stimulated largely by the need for an energy source based on fusion of light nuclei. In many laboratories all over the world attempts are being made to come closer to the final goal by studying magnetically confined plasmas and systems of inertial con finement. Heating of plasmas to fusion temperatures remains a key problem. There are good reasons to believe that the nonlinear effects in plasmas will play an important role for fusion, a long-range program which is still largely in its basic research phase.