Plasma Physics

James E. Drummond

• Publisher: Dover Publications
• Year: 2013
• Print ISBN: 9780486498652
• eBook ISBN: 9780486320588
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

A historic snapshot of the field of plasma physics, this fifty-year-old volume offers an edited collection of papers by pioneering experts in the field. In addition to assisting students in their understanding of the foundations of classical plasma physics, it provides a source of historic context for modern physicists. Highly successful upon its initial publication, this book was the standard text on plasma physics throughout the 1960s and 70s.
Hailed by Science magazine as a "well executed venture," the three-part treatment ranges from basic plasma theory to magnetohydrodynamics and microwave plasma physics. Highlights include Klimontovich's article on quantum plasmas, Buneman's writings on how to distinguish between attenuating and amplifying waves, and Yoler's clear and cogent review of magnetohydrodynamics. Professional atomic and plasma physicists and all students of plasma physics will appreciate this historic resource.

CHAPTER 1

INTRODUCTION (OSCILLATIONS IN PLASMAS)

by J. E. Drummond

1. History of Plasma Oscillations

1.1. The First Problem in “Plasma Physics.” Plasma physics has been a recognizable field of study from about the time (1929) that Tonks and Langmuir noted [1]† the similarity of certain newly discovered oscillations in ionized gases to the oscillations of a jelly plasma. They christened the new oscillations “plasma-electron oscillations” and also gave the name “plasma” to the nearly neutral part of an ionized gas. The term is now taken in the technical sense as meaning “the portion of a material body which is much larger than a Debye length and throughout which charged particles move.” (See pages 2, 12, and 21 for discussion of Debye length.)

Strong internal oscillations in an ionized gas were first proposed by Dittmer [2] to explain the anomalous “high scattering” observed in electric arcs. Langmuir [3] had discovered in arc discharges the presence of electrons with energies considerably greater than the total potential difference across the tube. Dittmer, using Langmuir’s probe technique [4], found the anomaly especially pronounced within a thin region a few millimeters from the filament. He hypothesized the existence of internal oscillations with periods comparable to the transit time of the primary (50-volt) electrons from the filament to the region of maximum scattering: T ∼ 10⁻⁸ sec.

Dittmer was unable to detect these oscillations. However, before his paper was published, Penning [5] published observations of high-frequency radiations from a controlled arc and established a definite correlation between the observed high scattering and the radiations.

In addition to the puzzling high scattering, there are two other anomalies of electrical discharges in gases that are connected with plasma oscillations. One of these is angular scattering of the primary beam a short distance from the cathode in an arc discharge. The most consistent work on this has been done by Emeleus and his students at Queen’s University, Belfast, Northern Ireland [6]. They have shown repeatedly the correlation between this scattering and the existence of the internal oscillations in discharges.

The other phenomenon is sometimes called the Langmuir paradox. It was found that a Maxwellian distribution of electron energies exists to within a very small distance of an insulated wall in a gas discharge. This seemed quite surprising since such a wall acquires a negative charge which collects only high-energy electrons from the gas, thus distorting the Maxwellian distribution of the remaining electrons near the wall. At large distance from the wall the Maxwellian distribution would be reestablished because of energy-transfer collisions between electrons and other charged and neutral particles in the discharge. However, all the known mean free paths for energy transfer were orders of magnitude too large to account for the extremely short distance required for the reestablishment of the equilibrium distribution.

In 1929, Tonks and Langmuir [1] presented their now famous theory of plasma oscillations which provided a single mechanism to explain each of these anomalies. Their theory was based upon a completely uniform macroscopically neutral, zero-temperature model of the plasma. A small displacement of a slab of electrons from their equilibrium position gave rise through coulomb interaction to a restoring force which in first order was linear. Simple harmonic motion resulted with a frequency which has become known as the characteristic plasma-electron frequency, or the Langmuir frequency.

There has been some objection to this latter name since the characteristic-frequency parameter appeared much earlier in the classical work of Lorentz [7]. It should be pointed out, however, that the waves treated by Lorentz were transverse electromagnetic waves, while the oscillations deduced by Tonks and Langmuir were longitudinal waves of entirely different character. In order to emphasize the difference, waves of essentially the Langmuir type have been called electric sound waves, although, as we shall see, this is not an entirely adequate term.

The theory and experiment on plasma oscillations have been significantly extended since the early pioneer work. The restriction to zero temperature was removed by the theoretical work of Landau [8] in 1946. The Langmuir frequency now appe...

Table of contents

• Cover
• Title Page
• Copyright Page
• Contributors
• Preface
• Contents
• Chapter 1. Introduction (Oscillations in Plasmas)
• Part I: Basic Plasma Theory
• Part II: Magnetohydrodynamics
• Part III: Microwave Plasma Physics
• Name Index
• Subject Index