Chemical Lasers

The interest in chemical lasers [173] may be explained by the fact that for them, in CW mode, high output powers of up to several megawatts have been obtained [174]. Various methods of acting on the active medium are used to initiate chemical lasers: gas discharge, photoinitiation, fast electron beams, у radiation from a nuclear explosion, etc.

Initiation of chemical lasers using HF(DF) molecules with ionizing radiation is one of the methods of creating powerful IR lasers in a spectral range of 2.7-4.4 qm. Marked successes in this area were achieved in pulsed mode using electron beams and the у radiation from nuclear explosions as the source of initiation. In the first case, the output energy of the HF laser using the mixture H2-F2-O2-NF3 with a chain reaction reached 4.5 kJ (n = 226 %) at the pulse duration of 50 ns [175], and in the second case, for the mixture SF6-H2 with a non-chain reaction, it reached 70 kJ (n ~5 %) in ~10 ns pulse (see Chap. 12).

When chemical lasers are initiated by nuclear radiation with a pulse duration >100 qs, which is typical of pulsed reactors, the specific power deposition is not great, so it is hard to ensure high rates of dissociation of the fluorine-containing substances, and accordingly, the rate of formation of the excited molecules HF*. As a result, HF molecules accumulate in their ground state, and effectively quench molecules of HF* in collisions. This circumstance leads to an increase in the laser threshold and a reduction in efficiency.

The possibility of creating chemical NPLs using pulsed reactors was examined in several studies. The first of them [176], carried out in 1970 and published in 1989, examined the mixtures 235UF6-H2 and 235UF6-H2-F2. It was concluded that for the first mixture, ФгА = 5 x 1014 cm-2 s-1, while for the second, ФгА « 1017 cm-2 s-1. Study [176] briefly discussed one version of chemical NPL at the mixture 235UF6-D2-F2-CO2 (A = 10.6 qm), with transfer of energy from the excited DF* molecules to the CO2 molecules. Study [177] examined several problems associated with development of the chemical laser DF-CO2, intended for experiments with the reactors Godiva-IV (t1/2~30 qs) and SKUA (T1/2 < 400 qs).

The 235UF6-H2 mixture is interesting in that at present it is perhaps the most realistic NPL active medium in which the “fissionable” material 235UF6 can be a component part of the laser medium. Study [178] calculated the gain for this mixture depending on the duration of the neutron pulse, the neutron flux density, the pressure and composition of the mixture, the temperature of the medium, and the dimensions of the laser cell. In contrast to other NPLs, which usually operate for an interval of time in which the thermal neutron flux density exceeds the threshold value, the characteristics of NPLs using the mixture 235UF6-H2 depend on the duration and shape of the neutron pulse. It was shown that for typical reactor pulse parameters (Фтах < 1017 cm-2 s-1, t1/2 = 30-200 ^s), the laser threshold is achieved at ФгА < 1014 cm-2 s-1, and the small-signal gain can be >5 x 10-3 cm-1.

When mixtures based on H2-F2 with a chain reaction are used, it is possible to obtain a laser efficiency of greater than 100 % owing to the chemical energy stored in the active medium [173]. The kinetics of one such laser based on the mixture He-F2-H2-O2, initiated by nuclear radiation, was examined in study [179] for experimental conditions of the EBR-L reactor. Calculations showed the possibility of achieving lasing at a number of vibrational transitions of the HF molecule. Lasing can occur at the leading edge of the reactor pulse, and for the optimal mixture He-F2-H2-O2 (12:2:1:2) at a pressure of 2 atm, the duration of the laser radiation is ~10 ^s with an efficiency of up to 100 %.

Unfortunately, the few experiments attempting to create chemical NPLs did not yield positive results. Such experiments were carried, for example, with the HPRR fast pulsed reactor (t1/2~50 ^s) using the mixtures 235UF6-H2 (Р = 0.07-0.8 atm) and 3He-F2-H2 (Р = 0.4 atm) [180]. The thermal-neutron flux density inside a laser cell 46 cm in length was >1017 cm-2 s-1. Measurements of the chemical compo­sition of mixtures showed that the concentration of HF molecules formed in the pulse ([HF]/[UF6] > 70 %) is entirely sufficient to achieve the laser threshold. In the opinion of the authors of [180], the lack of lasing may be explained by contamina­tion of the active medium with impurities due to the increased temperature of the laser cell or an insufficiently high appearance rate of atomic fluorine. Analogous experiments performed at VNIIEF in 1973 with the BIR-2-pulsed reactor (t1/2~100 ^s) with mixtures 3He-SF6-D2 and 3He-SF6-D2-CO2 also did not yield positive results.

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