In NPLs based on mixtures of rare gases excited by the fission fragments of uranium nuclei, the shape of the lasing pulse is usually close to that of the neutron pulse that initiates the fission reaction [4—8, 49]. However, during a number of experiments involving mixtures, the bulk of which were comprised of gases with high atomic numbers (for example, Ar and Kr), the lasing pulse consisted of an irregular series of small peaks on the crest of a common pulse; the total suppression of lasing followed by its restoration was sometimes observed [4, 34—36].

Various explanations exist for the irregular behavior of lasing with an increase in power deposition. For example, in study [50], lasing in an Ar-Xe mixture (A = 1.73 pm) only occurred on the leading edge of the pumping pulse, which in the opinion of the authors of [51], was the result of Xe atom 6s, 6p, and 5d level collisional mixing by the plasma electrons.

Other attempts at explaining this effect are based on the hypothesis of the appearance of refractive index discontinuities that originate within low-intensity longitudinal sound and shock waves in the presence of comparatively short neutron pulses (т ~ 10~3 s) due to a pumping inhomogeneity in the direction of the optical axis. In particular, the cause of these inhomogeneities may be the presence of gas regions directly adjacent to a laser cell’s end windows that are not excited by the fragments. However, the absence of a correlation between the duration of the aforementioned micropeaks, the time intervals that separate them, the geometric dimensions of the cell, and sound velocity in the gas, as well as, more importantly, the disappearance of these peaks with a decrease in gas mixture density when other conditions remain unchanged, have given rise to doubts concerning the correctness of the proposed interpretation.

On the other hand, the peculiarities of the appearance and development of the transverse optical inhomogeneities that lead to the alteration of cavity stability can explain the absence of a time correlation between lasing and pumping pulses. A comparison of the calculated and experimental data cited in [35, 36] confirms this proposition. During the experiments, a rectangular laser cell was used that had two parallel planar uranium layers located close to one another. The calculation procedure was based on the use of the ray matrix technique to analyze the stability of a laser cavity filled with a medium that had a time-varying parabolic refractive index profile.

In studies [35, 36], during gasdynamic calculations of gas density redistribution in a laser cell that were performed while ignoring heat transfer processes, it was assumed that a gas motion only occurs in the direction normal to the uranium layer planes. Thus, the dynamics of altering cavity stability were examined in an approx­imation of the exclusive existence of one-dimensional inhomogeneities that were symmetrical relative to the cell’s central plane. The inhomogeneity of density distribution (and, accordingly, the refractive index) obtained by means of calcula­tions at each studied moment in time were approximated by a polynomial with a finite number of members. This polynomial’s first two coefficients were used for the parabolic representation of the refractive index profile in the entire volume of the cell’s active section. The calculations made it possible to identify the moments in time at which the loss of cavity stability and its subsequent restoration occur. They are in quite accurate agreement with the experimentally observed lasing cessation and resumption times. It is more difficult to carry out the analysis of laser cavity stability described in Chap. 7, Sect. 7.4 (planar uranium layers) and in the second section of this chapter, because it is impossible to regard the inhomogeneity distribution in such a system as homogeneous.