This text concerns the basic elementary physics of plasmas, which are a special class of gases made up of a large number of electrons and ionized atoms and molecules, in addition to neutral atoms and molecules as are present in a normal (non-ionized) gas. The most important distinction between a plasma and a normal gas is the fact that mutual Coulomb interactions between charged particles are important in the dynamics of a plasma and cannot be disregarded. When a neutral gas is raised to a sufficiently high temperature, or when it is subjected to electric fields of sufficient intensity, the atoms and molecules of the gas may become ionized, electrons being stripped off by collisions as a result of the heightened thermal agitation of the particles. Ionization in gases can also be produced as a result of illumination with ultraviolet light or X-rays, by bombarding the substance with energetic electrons and ions, or in other ways. When a gas is ionized, even to a rather small degree, its dynamical behavior is typically dominated by the electromagnetic forces acting on the free ions and electrons, and it begins to conduct electricity. The charged particles in such an ionized gas interact with electromagnetic fields, and the organized motions of these charge carriers (e.g., electric currents, fluctuations in charge density) can in turn produce electromagnetic fields.

This book is intended to provide a general introduction to plasma phenomena at a level appropriate for advanced undergraduate students or beginning graduate students. The reader is expected to have had exposure to basic electromagnetic principles including Maxwell's equations and the propagation of plane waves in free space. Despite its importance in both science and engineering the body of literature on plasma physics is often not easily accessible to the non-specialist, let alone the beginner. The diversity of topics and applications in plasma physics has created a field that is fragmented by topic-specific assumptions and rarely presented in a unified manner with clarity. In this book we strive to provide a foundation for understanding a wide range of plasma phenomena and applications. The text organization is a successive progression through interconnected physical models, allowing diverse topics to be presented in the context of unifying principles. The presentation of material is intended to be compact yet thorough, giving the reader the necessary tools for further specialized study. We have sought a balance between mathematical rigor championed by theorists and practical considerations important to experimenters and engineers. Considerable effort has been made to provide explanations that yield physical insight and illustrations of concepts through relevent examples from science and technology.

The material presented in this book was initially put together as class notes for the EE356 Elementary Plasma Physics course, newly introduced and taught by one of us (USI) at Stanford University in the spring quarter of 1998.

Introduction

Practical plasmas are typically contained in a vacuum chamber of finite size. When ions and electrons hit the walls, they recombine and are lost. Since electrons have much higher thermal velocities than the ions, one may at first expect that they would be lost faster, leaving the otherwise neutral plasma with a net positive charge. However, because of this tendency, the plasma develops a positive potential with respect to the wall, which prevents most of the electrons from reaching the wall. In equilibrium, a potential gradient arises near the wall so that most of the electrons are reflected back into the plasma, with the number reaching the wall being equal to the corresponding number of positive ions reaching the wall. This potential gradient cannot extend over large sections of the plasma, since Debye shielding (Chapter 1) confines potential variations in a plasma to a layer of the order of several Debye lengths in thickness. This layer of potential gradient, which must exist on all walls with which the plasma is in contact, is called the sheath. Within the sheath, charge neutrality is not preserved, not even approximately, since qΦ changes through the sheath by an amount comparable to kBT. In this chapter, we provide a simple treatment of the formation and structure of the plasma sheath and discuss a simple but very useful device, called the Langmuir probe, which is commonly used to measure electron density and temperature.