In contrast to VNIIEF, where the concept of an autonomous stationary RL is being developed, since 1986, FEI (Obninsk) has been examining the concept of a high — power pulsed laser system based on an optical nuclear-pumped amplifier (OKUYaN) [23]. The “master oscillator-two-pass amplifier with phase conjuga­tion” configuration underlay the work of the OKUYaN-based laser system [23, 24] (Fig. 6.12). The OKUYaN includes reactor and laser blocks. The reactor, which is

Fig. 6.12 Optical scheme of laser system based on OKUYaN [23, 24]:

(1) master laser-oscillator; (2, 4) telescope systems;

(3) optical nuclear-pumped amplifier (OKUYaN);

(5) pulsed reactor core;

(6) Faraday cell; (7) phase conjugation cell;

(8) polarizer

sometimes called an “initiating” reactor, presupposes use of a multicore fast — neutron pulsed reactor. The laser block constitutes a booster subcritical core in which neutrons are multiplied. We note that the first proposal to develop such devices appeared more than 20 years ago (see, for example, review [25]). Require­ments for “initiating” reactors, their arrangement, and the results of calculation of the neutron physical characteristics, are considered in study [26].

At present there is an OKUYaN demonstration model operating at FEI, “Stand B” [23, 24, 27, 28]. It consists of two parts: the so-called “first workplace,” consisting of a two-core BARS-6 reactor and an experimental section for studying the characteristics of individual lasing elements (cells), and the “second work­place.” Organization of experiments for the “first workplace” was examined in Sect. 6.2.

The other part of Stand B (the “second workplace”), which includes the two-core BARS-6 reactor and laser block with a volume of around 2.5 m3, was put in operation in 1999. The design and the neutron-physical characteristics of the BARS-6 reactor were considered in some detail in studies [23, 28]. The BARS-6 reactor, built on the basis of the BARS-5 reactor [7, 29] from the design documen­tation and with the technical assistance of VNIITF (Snezhinsk), has two reactor cores, which are arranged on a platform and can be moved along a rail track to one of the two “workplaces” or to the biological shield, into which the core of the reactor is moved for the period of preparation for experiments.

The relative arrangement of the BARS-6 reactor and laser block is shown in Fig. 6.13. The laser block (shown in Fig. 6.14) is a cylindrical structure with a diameter of 1.7 m and length of 2.5 m, with a longitudinal axial cavity for placement of two BARS-6 reactor cores. It consists of up to 800 laser elements, their simulators, and elements of the neutron reflector, made in the form of tubes 49-mm in diameter and 2.5-m long, filled with polyethylene. The space between the tubes of the laser block contains around 760 neutron moderator elements made from shaped polyethylene. The laser block, in neutron-physical relations, constitutes a deep subcritical core with a neutron multiplication coefficient kef < 0.9. Its neutron — physical characteristics were optimized in study [30].

Fig. 6.13 Arrangement of the BARS-6 reactor and laser block at the “second workplace” [23, 24]: (1) two-core BARS-6 reactor; (2) laser block; (3) laser elements and its simulators; (4) outside neutron reflector; (5) inside neutron reflector

The laser element (Fig. 6.15a) is a stainless-steel tube with a diameter of 50 mm, wall thickness of 0.5 mm, and length of 2.5 m, which is coated on the inside with a 5 ^m thick layer of metallic 235U. The laser element simulator is in the form of two annular aluminum tubes, with the small space between them filled with 235U3O8 (Fig. 6.15b). It is a copy of a laser element in neutron-physical respects and contains the same amount of 235U (32 g).

The specific features of neutron pulse generation in the reactor with the laser block are discussed in studies [23, 28]. The half-width neutron pulse duration for the BARS-6 reactor without the laser block is around 100 ^s, while in the reactor with the laser block it is much more, 2 ms when there is an internal reflector present, and 20 ms in its absence. At the same time, the energy release in each reactor core of the BARS-6 is equal to 2 x 1017 fissions, and in the laser block, 2.2 x 1017 fissions.

For the configuration “reactor + laser block” with an internal reflector, experi­ments were carried out with the use of laser elements as part of a laser block [23, 31]. The main goal of the experiments is to study the operating modes of the “master oscillator-single-pass amplifier” configuration. In contrast to the amplifier
element, the master oscillator has windows at the ends arranged at a Brewster angle, and a stable laser cavity consisting of spherical and flat dielectric mirrors. A telescope was used to allow passage of the beam of the master oscillator through the amplifier without losses. The laser medium was a He-Ar-Xe mixture (A = 2.03 pm) at a pressure of 1.05 atm. The specific power deposition, averaged over the active volume, at the maximum of the neutron pulse for the amplifier element and the master oscillator were 40 W/cm3 and 80 W/cm3, respectively, at a half-width neuron pulse duration of 1.8 ms.

From the energy parameters of the laser radiation measured at the amplifier input and output, the small-signal gain (a0) and saturation intensity (Is) of the laser medium were determined: a0 = 8.1 x 10~3 cm-1; Is = 92 W/cm2. Using these parameters, it is possible to determine the laser power efficiency qI = «о Is/ q ~ 2 %, which agrees with the results obtained when NPLs were studied (see Table 3.5, and Chap. 3, Sect. 3.1). The results of these investigations allow one to hope for high energy parameters when there is full-scale operation of the entire OKUYaN laser block.