Thus the maximal specific power deposition of gas NPLs is no more than 5 x 103 W/cm3 and is achieved in experiments using pulsed reactors at the maximal possible thermal-neutron flux densities of ~1017 cm-2 s-1. Such a pumping level is substantially lower than when electron and ion beams are used, or in a pulsed gas discharge. [Note that while the pump power density is low, the NPL has a tremen­dous opportunity to achieve enormous energy densities if proper metastable species can be found. This led some later searches to consider lasers involving states like singlet-delta oxygen, 02([1]Д)]. This circumstance hampers the search for laser transitions for NPLs, especially in the visible and ultraviolet spectral ranges, because the unsaturated gain of the laser medium is directly proportional to X[2]. If one considers that the frequency of realization of experiments with pulsed reactors as a rule does not exceed one pulse per day (~100 pulses/year), and pulsed reactors are unique and potentially dangerous, then experiments to find and study active media for NPLs are a complicated and expensive proposition.

Significant assistance in the preliminary selection and study of active media for NPLs is provided by experiments using other, more accessible and safer sources of ionizing radiation operating at high frequency: high current electron and ion accelerators [77, 78], and accelerators with large cross-section beams that have been developed for laser pumping [79]. The kinetics of plasma processes in laser media excited by various types of ionizing radiation (y radiation, fission fragments, and other products of nuclear reactions, fast electrons, and ions) are practically identical, so that laser characteristics depend not on the type of ionizing particles, but on the energy deposition to the laser medium.

At present, electron and ion accelerators operate in a wide range of pulse durations from ~10 ns to stationary mode with electron and ion energies from ~0.1 MeV to ~100 MeV. The specific power depositions of gas media in pulsed mode can reach 109 W/cm1 2 [3] [4] [5] [6] [7] [8] [80], and in stationary mode 10 W/cm3 [81], which makes it possible to model NPL pumping in a wide range of conditions, from nuclear explosions to stationary nuclear reactors.

From investigations with the use of electron and ion beams related to NPLs, one should note the study [10], in which pumping of lasers with mixtures of He-Ne, He-Xe, Ne-Xe, and Ar-Xe at atmospheric pressure by ionizing radiation was carried out for the first time (in this case with an electron beam), as well as studies performed at FIAN [82—84], the Institute of High Current Electronics of the SO RAN [85—87], the Scientific Research Center for Technological Lasers [88, 89], and the Institute of Electrophysics of the Ural Division of the Russian Academy of Sciences [90, 91]. Abroad, the most interesting research was carried out in the United States [92—95], Germany [81, 96], and The Netherlands [97, 98]. A detailed description of gas lasers excited by electron and ion beams is not the purpose of this book. Nonetheless, in the following chapters, in the discussion of NPL character­istics, data acquired using electron and ion beams will be cited as needed.