INTRODUCTION

The amplification of laser radiation in plasmas is attractive for two reasons. First, in contrast to a solid, liquid or gas, the aggregate state of a plasma does not change at high densities of the pump energy and, therefore, it should be possible to build plasma lasers with a considerably higher energy output than the output of lasers using other media. (Berger et al.) Secondly, plasmas provide provide means for efficient population of electron-excited atomic, ionic, and molecular levels which can be used to generate short-wavelength radiation. In this way, it should be possible to generate coherent radiation not only in the visible or ultraviolet range but also in the x-ray range.

Gudzenko and Shelepin drew attention in 1963 to a recombining plasma as a potential active medium. In that paper and in several later communications they showed that the recombination flux between excited states of atoms and molecules in a dense plasma can ensure a population inversion and a fairly high gain; lasers utilizing recombining plasmas have since been called plasma lasers.

The development of plasma lasers was also stimulated by the appearance of lasers utilizing various forms of gas discharges. In these lasers, the amplification occurs due to the ionization of a gas and it is usual to call them gas lasers. (Mir, 1968) We shall retain the terminology in spite of the fact that the degree of ionization of the amplifying medium in high power gas lasers is now considerable.

Thus we may distinguish between two types of lasers utilizing ionized gases:

  1. Gas lasers in which the active medium amplifies the radiation because of ionization.
  2. Plasma lasers in which the amplifying medium is a recombining plasma.
The current terminology reflect, in particular, the fact that a gas is transformed into plasma in the gas lasers and a plasma is transformed into a gas in the plasma lasers. Moreover, under recombination conditions, the plasma properties of an 'overionized' medium are manifested more strongly than in the case of ionization. The qualitative differences between the the plasma and gas lasers is the deviation of the active medium from thermodynamic equilibrium in opposite directions. In a gas laser, the electrons are overheated and the temperature of the free electrons Te is higher than the equilibrium temperature Ti at which the degree of ionization is equal to that actually observed, whereas, in a plasma laser, the electrons are supercooled: Te is less than Ti. This qualitative difference determines in each specific case the method used to produce an amplifying medium. For example, the pulsed gas laser use the leading edges of heating-field pulses, whereas the plasma lasers utilize the afterglow (Sec.5); in the case of gas laser, an electron beam enters a rarefied medium, whereas, in the case of a plasma laser, a beam enters a dense medium (Sec.6 and 8), and so on.

The problem of building an efficient plasma laser can be reduced to two tasks:

  1. The establishment of a sufficiently rapid depopulation of the lower lasing level.
  2. The generation of a rapidly recombining dense plasma.

Next Section: Recombination mechanisms of population inversion of atomic and ionic levels.