Propagation of Electromagnetic Waves in and around Plasmas

Abstract

1.1 Historical perspective Interaction between electromagnetic waves and plasmas has been explored for several decades in various fields, such as ionosphere layers and outer space (Ginzburg, 1964), fusion plasmas (Stix, 1962; Swanson, 1989; Nishikawa & Wakatani, 1990), and plasma materialprocessing reactors (Lieberman & Lichtenberg, 1994). When an electromagnetic wave is launched in or near a non-magnetized plasma whose size is much larger than the wavelength, it is transmitted, reflected, or absorbed; these features are determined by a set of the electromagnetic wave frequency, the electron plasma frequency, and the electron elastic collision frequency. These three parameters determine real and imaginary parts of permittivity. In other words, plasmas equivalently behave as conductors or dielectric materials for electromagnetic waves, and these behaviours are controllable by changing complex permittivity, or electron density and gas pressure, which is associated with the electron plasma frequency and the electron elastic collision frequency; this controllability and the time-varying manner for permittivity distinguish plasmas from other electromagnetic media. First of all, in this section, we briefly review the historical perspective of the electromagnetic waves in plasmas, and we point out the reasons why the concept of electromagnetic media composed of plasmas and their discontinuities is focused on in this chapter. Electromagnetic waves in magnetized plasmas have been well investigated for more than half century, aiming at ultimate goals of controlled fusion plasmas for energy production and space plasmas for launching human beings using spacecrafts. In a magnetized plasma, various kinds of wave branches are present from low to high frequency ranges, and change of the external magnetic field induces “walk” on the dispersion curves in a “zoo” of plasma waves. Sometimes a branch leads to another totally-different branch; that is called mode conversion (Stix, 1962; Swanson, 1989). These waves can be launched from the outer side of the plasma, but there are also many inherent waves found as magnetohydrodynamic and micro instabilities (Swanson, 1989; Nishikawa & Wakatani, 1990). Other characteristic features of plasma waves are their nonlinearity; shock waves, solitons, and nonlinear mode conversion originate from the aspects of high-energy-state substance (Swanson 1989). The main focus in this chapter is different from such conventional scientific interests. Before we start our description, one more comment about plasma production for industry should be addressed (Lieberman & Lichtenberg, 1994). Plasma production in fabrication