The design of the LIRA setup [32, 33] proposed by VNIITF associates is one version of a high-powered pulsed nuclear-laser facilities. It is presumed that the LIRA setup (like the OKUYaN) consists of two blocks: a rather large subcritical block made from laser elements in the form of long thin-walled pipes, and two relatively small “initiating” pulsed reactors disposed inside this block (Fig. 6.16).

The reactor block contains two DRAKON pulsed solution reactors operating synchronously (half-width of pulse duration is 2 ms, energy release in the core is 30 MJ). The ELIR reactor [29], which was operated for many years at VNIITF, was

used as the basis of this reactor. The basic calculated characteristics of the DRAKON reactor are given in study [33].

The LIRA setup laser block consists of seven identical amplification laser assemblies, each of which consists of 19 laser elements. An eighth assembly includes eight laser elements, one of which is a master oscillator. The laser elements are enclosed in an aluminum tube 300-mm diameter, and are arranged it at intervals of 60 mm. The pipe is filled with water and surrounded by graphite, forming a laser assembly (Fig. 6.17). The outside dimensions of the assembly are 43 x 43 cm in cross section, with a length of 3.8 m. On the outside, the laser assemblies are surrounded by a neutron polyethylene moderator 10-cm thick.

Each laser element consists of three concentric tubes. A 6 ^m thick layer of metallic 235U is deposited to the inside surface of the first, innermost, tube, which is 48 mm in diameter. The two outer aluminum tubes have a 12 ^m layer of 235U between them and constitute a simulator, which is necessary to obtain a neutron multiplication coefficient in the laser block of kef < 0.9. The total mass of the uranium in the laser setup is around 20 kg.

To optimize the laser block design, calculations were carried out to study the effect on energy release of different neutron moderators in the assemblies (poly­ethylene, graphite) and water in the space between the laser elements and the absorbent screens. Calculations were also used to determine the temperature of the uranium layers and the gas pressure in the laser element after the pulse.

The optical circuit of the LIRA setup is shown in Fig. 6.18. One of the laser elements of the eighth laser assembly is a master oscillator. Laser radiation from the master oscillator is split into seven beams of identical power and directed to the other seven laser elements of this same assembly for preliminary amplification. Then the laser radiation is expanded using telescopes to a diameter of 300 mm and

goes to the other amplifier assemblies. After amplification, the beam aperture is reduced to 100 mm and output past withdrawn beyond the biological shielding.

In the opinion of the authors of [32, 33], when a He-Ar-Xe mixture (A = 2.03 ^m) is used as the laser medium, the specific energy deposition to the gas medium is about 1 J/cm3, while the specific energy deposition is ~8 mJ/cm3 for a lasing pulse duration of 5 ms. Under these conditions, the full energy of laser radiation of the LIRA setup can reach ~4.5 kJ.

In study [34], experimental investigations of a multichannel laser assembly on the BARS-5 pulsed reactor [29] were carried out (Figs. 6.19 and 6.20). The assembly consists of a package of 19 stainless steel tubes with an internal diameter of 15 mm and wall thickness of 0.2 mm. Each of the tubes held thin-walled aluminum tubes with a layer of 235U3O8 (layer thickness 2.5 mg/cm2, layer length 1,050 mm). The specific power deposition of the gas mixtures He-Ar-Xe (A = 2.03 ^m) and He-Ar (A = 1.79 ^m) reached 5 kW/cm3 at a helium pressure of 3 atm. The half-width pulse duration of the thermal neutrons was about 130 ^s.

Fig. 6.20 Photo of end of laser assembly [34]

Under these conditions, laser radiation energy from the entire assembly was 9.9 J (2.03 ^m) and 10.3 J (1.78 ^m). The output power was around 30 kW.

Higher energy parameters were recently obtained in experiments [35] when a multichannel laser assembly was irradiated by neutrons using the reactor facility BARS-5 + RUN-2 which consists of BARS-5 pulsed reactor and RUN-2 neutron amplifier (see reference [29]). One end of laser assembly was located between the two cores of the BARS-5 reactor and the other one was located inside the RUN-2 core (see Fig. 6.21).

The laser assembly in reference [35] consists of the 37 stainless steel tubes with a wall thickness of about 0.1 mm and a length of 1.5 m. The end of the laser assembly had approximately the same design as shown in Fig. 6.20. Inside the each tube, the ten aluminum cylinders were placed with the inner diameter of 19.5 mm which were covered by thin layers of 235U3O8 (with a layer thickness of 2.5 mg/cm2). To

enhance the thermal-neutron flux, this laser assembly was located inside a cylin­drical polyethylene neutron moderator with the wall thickness of 60 mm.

As described in reference [35], investigations were carried out using a He-Ar-Xe (700:700:1) mixture at 4 atm pressure. Lasing occurs on a transition in the Xe atom (A = 2.03 pm). As an example of the result, Fig. 6.22 shows the oscillogram of one experiment. These experiments demonstrated record energy parameters for NPLs excited with help of pulsed reactors: laser output energy of 520 J with a laser pulse duration of about 400 ps (laser power output is 1.3 MW, n ~3 %). Such high laser energy parameters are attributed to the large active laser volume (16l) and high specific input power deposition (~1.5 kW/cm3) which was distributed rather uni­formly along the length of laser tubes.

Study [36] assessed the maximal possible energy parameters of pulsed nuclear — laser devices designed by an “initiating reactor-subcritical laser block” scheme. With allowance for the laser media efficiencies obtained up to now of ni ~ 1 % (for example, for xenon NPLs) and the limitations associated with the operating modes of the laser element and the specific features of neutron pulse in such a system, it is possible to obtain a full laser radiation energy of ~2.5 MJ for a pulse duration of ~5 ms. Characteristic dimensions of a laser block that includes 104-105 laser elements with a total active volume of ~250 m3 is 7-10 m. Three pulsed TRIGA — or ACPR-type reactors [7] with an energy release in the reactor core of 100-200 MJ can be used as “initiating” reactors.