The basic plasma processes occurring in a single-component gas mixture were examined previously in this chapter. Most often binary mixtures A-B are used as the active medium, where A is a buffer gas with a high ionization and excitation potential, and B is a laser additive with lower ionization and excitation potential. A diagram of the main plasma processes in a binary mixture is shown in Fig. 4.16.

The basic channels to transfer energy from ions and atoms of the buffer gas A to the atoms of the additive B are the charge-transfer process A+(Aj)+B! (B+) *+A (2A), the Penning reaction A* + B! (B+)*+ A + e (if the energy of the excited atom A* is greater than the ionization potential of atom B), or transfer of excitation A* + B! B*+A.

In high-pressure plasma, the molecular ions and Aj, Bj AB+ are the main ion types. These are formed as a result of three-body processes A+(B+) + 2A(A, B)! Aj (Bj, AB+)+A. Neutralization of plasma occurs as a result of recombination pro­cesses, among which, depending on the specific conditions, either the processes of three-body recombination of atomic ions A+(B+) + 2e(e, A)! A*(B*) + e(A), or the processes of dissociative recombination Aj(Bj, AB+) + e! A*(B*)+ A(B, A) predominate.

Kinetic equations, which represent the balance of rates of formation and decay of individual plasma components, are used to calculate the plasma parameters, and subsequently, the laser characteristics. In certain models, the number of considered plasmochemical reactions reaches several hundred (see [51], for example). For a satisfactory description of plasma phenomena relating to calculation of the charac­teristics of a specific laser, it is entirely sufficient to use 10-15 main processes for binary mixtures. In this regard, for the calculations it is sometimes helpful to use so-called “small” models, in which only the basic plasma processes are included. The results of calculation of the plasma parameters for mixtures He-Xe and Ar-Xe using “small” models are provided next as an example [52].

Calculations of the concentrations of electrons and ions and the electron tem­perature depending on the partial pressure of Xe are performed for the experimental conditions [53, 54], in which He-Xe and Ar-Xe NPLs excited by uranium fission fragments with a neutron pulse duration of 4 ms were studied. The specific power deposition at the neutron pulse maximum for PHe = 2 atm and PAr = 0.5 atm was around 20 W/cm3. The plasma processes that were included in the kinetic models are shown in Tables 4.10 and 4.11. The processes were selected as a result of preliminary analysis of the characteristic times of several tens of plasmochemical reactions with the participation of atomic and molecular ions of rare gases, as well as excited atoms and molecules.

The kinetic equations were supplemented with electron energy balance equa­tions. For the He-Xe mixture, this equation included the following processes of electron energy change: (a) ionization of the gas, as a result of which subthreshold electrons with an energy ee = 0.31V; = 7.6 eV arise at a rate off = q/w;; (b) the Penning process (1), as a result of which electrons with an energy of 8.5 eV are produced; (c) inelastic collisions of electrons with molecular ions HeJ, which can transfer a maximal energy of 2.4 eV to the electrons [62]; (d) elastic collisions of

electrons with atoms of helium, as a result of which electron thermalization occurs. For the Ar-Xe mixture, the electron energy balance equation includes the processes similar to those cited in points (a), (c), and (d) for the mixture He-Xe.

Characteristic times of all plasma processes are much shorter than the pumping pulse durations, so a quasistationary mode is established in the plasma. The results of calculations of ion and electron concentrations and the electron temperature for the mixtures He-Xe and Ar-Xe at the neutron pulse maximum are shown in Fig. 4.17. With the increase in Xe partial pressure, there is a change in the ion composition of the plasma: concentrations of atomic ions and molecular ions of the buffer gas (HeJ, ArJ) decrease, while the concentration of the molecular ions XeJ increases. The low concentration of heteronuclear ions ArXe+ (Fig. 4.17b) is explained by their effective destruction as a result of collisions with atoms of Ar and Xe. Partial pressures of Xe for the mixtures He-Xe and Ar-Xe, at which the maximal output laser powers are achieved, are 1-2 Torr [53, 54]. In this case, as follows from Fig. 4.17 data, the process of dissociative recombination of molecular ions XeJ with electrons is the basic channel of formation of excited atoms Xe*.

In calculating plasma characteristics for a He-Xe mixture, the processes of formation and decay of heteronuclear ions HeXe+ were not taken into account. The dissociation energy of heteronuclear ions decreases with an increase in the difference in masses of the atoms making them up. Thus for the ions ArXe+ and HeAr+ the dissociation energy is 0.18 eV and 0.026 eV, respectively [63], while for the ion HeXe+ it is <0.02 eV. Consequently, HeXe+ ions are effectively destroyed in collisions with atoms, and their equilibrium concentration is insignificant. Mass — spectrometry measurements [64], which registered different heteronuclear ions except for the ion HeXe+, can serve as a confirmation of this.

Analogous calculations of plasma parameters using the “small” model were carried out for He-Ar and He-Kr mixtures [65]. As in the case of the He-Xe mixture, the processes of formation and decay of heteronuclear ions HeAr+ and HeKr+ were not taken into account. One of the significant differences between mixtures based on helium and mixtures in which the buffer gases are Ne, Ar, or Kr, is the appreciable role of the processes of collisional-radiative and three-body recombi­nation of atomic ions in the processes of plasma neutralization and formation of excited atoms (Fig. 4.17a). In mixtures based on Ne, Ar, or Kr, these processes are virtually entirely suppressed because of the high electron temperature.

The main problem that has to be resolved as a result of analysis of plasma processes in NPL gas media is identification of channels for the population of upper and lower laser levels, as well as determination of their populating rates.