Influence of Temperature and Gas Impurities on Laser Characteristics

In experiments with NPLs based on pulsed reactors, the specific power deposition q < 5 kW/cm3 (specific energy deposition up to 1 J/cm3), which can lead to a marked increase in the temperature of the gas medium, of up to ~1,000 °K at pressures of ~1 atm. An increase in the temperature in the process of pulse pumping can influence the output power and possibly be the reason for a cessation of lasing until the moment of attainment of the pumping pulse maximum for some mixtures (for example, see [8, 45, 66]). Marked heating of the active medium can occur in powerful stationary nuclear-laser devices, so that a study of the influence of heating temperature of the NPL active medium on their characteristics is of great interest.

The first experiments in this field were carried out at VNIITF for NPLs operating on transitions of the Xe atom. By now the influence of temperature on NPL characteristics has been studied for mixtures of He-Xe, He-Ar-Xe, Ar-Xe [33, 67, 68]; 3He-Ar-Xe [69]; Ar-Xe, He-Ar-Xe, Ne-Ar-Xe [70] and Ar-Xe, He-Ar-Xe, He-Ar, He-Ne-Ar [71]. In all the experiments, an electric heater was used to change the temperature of the gas media. In the experiments, a marked reduction (by a factor of ~2) in the energy parameters was recorded at comparatively low temper­atures (see Table 3.7). Substantially different data were obtained in study [72], in

which no changes were noted in the laser parameters up to a temperature of 900 K for an electroionization laser using a mixture of 3He-Ar-Xe at a pressure of 1.5 atm with preliminary ionization of the gas medium by nuclear reaction products 3He(n, p)3H.

Analysis of the data of Table 3.7 shows that the influence of temperature on the NPL parameters depends on the pressure (gas density), composition of the gas medium, and laser wavelength. The reasons for this influence have not been finally ascertained to this point, and are the subject of discussion.

The most likely reasons considered for the reduction in the energy laser param­eters with the growth in temperature were the following: the influence of temper­ature on the processes of formation of the population inversion, for example, collisional “quenching” and “mixing” of laser levels by the atoms of the buffer gas [69]; the reduction in the rate of formation of heteronuclear ArXe+ ions and “mixing” of laser levels by electrons [70]; and the destruction of ions ArXe+ during collisions with atoms of the buffer gas [73].

Another possible cause is contamination of the laser medium by outside gas impurities (in particular water vapor) as a result of their desorption from the cell walls as the temperature increases [68, 74, 75]. The electron attachment to mole­cules of H2O can lead to a decrease in the concentration of electrons, and conse­quently to a reduction in the populating rate of the upper laser level as a result of reduction in the rate of recombination processes.

Interesting information was obtained in studies [76, 77], in which, given the excitation of lasers using mixtures of Ar-Xe (A = 1.73; 2.03 pm) and He-Ar-Xe (A = 1.73 pm) by a beam of 32S9+ ions with an energy of 100 MeV (duration of rectangular pulses 20-50 ps, pulse repetition frequency 30-45 Hz), it was possible to study the separate influence of the temperature of the medium and H2O impu­rities on the laser parameters. The energy parameters of a laser using an Ar-Xe mixture decrease both with an increase in the active medium without H2O impu­rities, and with an increase in the content of H2O at constant room temperature. Thus for the mixture Ar-Xe (Р = 0.16 atm; 0.3 % Xe), the output power decreases by a factor of 2 when the temperature increases to 400 K. When there is an increase in the water vapor content in the Ar-Xe mixture (Р = 0.5 atm; 0.5 % Xe), a similar reduction is observed when the concentration of H2O is about 1 x 1015 cm-3. For mixtures of He-Ar-Xe (Р = 0.2 atm; 23 % He; 0.3 % Xe), an output power reduction by a factor of 2 occurs at a temperature of 520 K. In the opinion of the authors of [76, 77], the influence of water vapors lies not only in the reduction of electron concentration, but also in the collisional “quenching” of the upper laser level 5d[3/2]10 of the Xe atom, because the rate constant of the “quenching” process is very high and amounts to 4 x 10~9 cm3 s-1. The output power reduction observed in the experiments [77] with growth in temperature for mixtures without impurities of H2O testifies that the gas temperature influences the processes of formation of the inverse population of laser levels, which were noted previously.

Apart from the water vapors, the NPL active media also contain the impurities N2, CO2, O2, H2, etc., which can appear both because of an inadequate degree of evacuation of the laser cells and the desorption processes of these impurities from the cell walls. At high power depositions (q > 10 kW/cm3), which are achieved with the use of electron beams, the addition of molecule impurities sometimes leads to an increase in output power, which is due primarily to the reduction in the electron temperature [78]. In the case of NPLs operating at q < 5 kW/cm3, the presence of molecular impurities leads to a reduction in the energy parameters and an increase in the laser thresholds [79].

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