Brief History of Plasma Physics

When blood is cleared of its various corpuscles there remains a transparent liquid, which was termed plasma (after the ancient Greek word polutonikogreekpl'asma, which means ``that which is formed or molded'') by the great Czech medical scientist Johannes Purkinje (1787-1869). The American Nobel laureate chemist Irving Langmuir first used this term to describe an ionized gas in 1927--Langmuir was reminded of the way that blood plasma carries red and white corpuscles by the way that an electrified fluid carries electrons and ions. Langmuir, along with his colleague Lewi Tonks, was investigating the physics and chemistry of tungsten-filament light-bulbs, with a view to finding a way to greatly extend the lifetime of the filament (a goal that he eventually achieved). In the process, he developed the theory of plasma sheaths--the boundary layers that form between plasmas and solid surfaces. (See Section 4.16.) He also discovered that certain regions of a plasma discharge tube exhibit periodic variations of the electron density, which we nowadays term Langmuir waves. (See Section 8.2.) This was the genesis of plasma physics. Interestingly enough, Langmuir's research nowadays forms the theoretical basis of most plasma processing techniques for fabricating integrated circuits (Lieberman and Lichtenberg 2005). After Langmuir, plasma research gradually spread in other directions, of which five are particularly significant. Firstly, the development of radio broadcasting in the early twentieth century led to the discovery of the Earth's ionosphere--a layer of partially ionized gas in the upper atmosphere that reflects radio waves, and is responsible for the fact that radio signals can be received on the surface of the Earth when the transmitter lies over the horizon. Unfortunately, the ionosphere also occasionally absorbs and distorts radio waves. For instance, the Earth's magnetic field causes waves with different polarizations (relative to the orientation of the magnetic field) to propagate at different velocities, an effect that can give rise to ``ghost signals'' (in other words, signals that arrive a little before, or a little after, the main signal). In order to understand, and possibly correct, some of the deficiencies in radio communication, various scientists, such as E.V. Appleton and K.G. Budden, systematically developed the theory of electromagnetic wave propagation through nonuniform magnetized plasmas (Budden 1985). Secondly, in the first half of the twentieth century, astrophysicists recognized that much of the universe consists of plasma, and, thus, that a better understanding of astrophysical phenomena requires a better grasp of plasma physics. The pioneer in this field was the Swedish Nobel laureate Hannes Alfvén, who around 1940 developed the theory of magnetohydrodynamics, or MHD, in which plasma is treated essentially as a conducting fluid (Cowling 1957a). This theory has been successfully employed to investigate sunspots, solar flares, the solar wind, star formation, and a host of other topics in astrophysics (Kallenrode 2010). Two topics of particular interest in MHD theory are magnetic reconnection and dynamo theory. (See Sections 7.9-7.17.) Magnetic reconnection is a process by which magnetic field-lines suddenly change their topology. It can give rise to the rapid conversion of a great deal of magnetic energy into thermal energy, as well as the acceleration of some charged particles to extremely high energies, and is thought to be the basic mechanism behind solar flares (Priest 1984; Priest and Forbes 2007). Dynamo theory studies how the motion of an MHD fluid can give rise to the generation of a macroscopic magnetic field. This process is important because the terrestrial and solar magnetic fields would both decay away comparatively rapidly (in astrophysical terms) were they not maintained by dynamo action (Kulsrud 2004). The Earth's magnetic field is maintained by the motion of its molten core, which can be treated as an MHD fluid to a reasonable approximation. Thirdly, the creation of the hydrogen bomb in 1952 generated a great deal of interest in controlled thermonuclear fusion as a possible power source for the future (Fowler 1997). At first, this research was carried out secretly, and independently, by the United States, the Soviet Union, Great Britain, and France. However, thermonuclear fusion research was declassified in 1958, leading to the publication of a number of immensely important and influential papers in the late 1950s and the early 1960s. Broadly speaking, theoretical plasma physics first emerged as a mathematically rigorous discipline in these years. Not surprisingly, fusion physicists are mostly concerned with understanding how a thermonuclear plasma can be trapped--in most cases by a magnetic field--and investigating the many plasma instabilities that may allow it to escape (Freidberg 2008). Fourthly, in 1958 James A. Van Allen discovered the so-called Van Allen radiation belts surrounding the Earth, using data transmitted by the U.S. Explorer satellite. This discovery marked the start of the systematic exploration of the Earth's magnetosphere via satellite observations, and opened up the field of space plasma physics (Baumjohan and Treumann 1996). Finally, the development of high powered lasers in the 1960s opened up the field of laser plasma physics (Kruer 2003). When a high powered laser beam strikes a solid target, material is immediately ablated, and a plasma forms at the boundary between the beam and the target. Laser plasmas tend to have fairly extreme properties (for instance, densities characteristic of solids) not found in more conventional plasmas. A major application of laser plasma physics is the approach to fusion energy known as inertial confinement fusion. In this approach, tightly focused laser beams are used to implode a small solid target until the densities and temperatures characteristic of nuclear fusion (which are similar to those at the center of a hydrogen bomb) are achieved (Atzeni and Meyer-ter-Vehn 2009). Another interesting application of laser plasma physics is the use of the extremely strong electric fields generated when a high intensity laser pulse passes through a plasma to accelerate charged particles (Joshi 2006). High-energy physicists hope to use plasma acceleration techniques to dramatically reduce the size and cost of particle accelerators.