In the United States, high-power NPL-based laser setups were developed in the scope of the FALCON program (Fission-Activated Laser CONcept) with Depart­ment of Energy support [37]. The basic executors of this program were the Sandia Laboratories with the support of the Idaho National Engineering Laboratory. Other participants included the Air Force Philips Laboratory, Los Alamos National Laboratory (LANL), Oak Ridge National Laboratory (ORNL), Mission Research Corporation, Babcock & Wilcox, W. J. Schafer Associates, Lockheed, Kaman, Science Research Labs, University of New Mexico, University of Illinois, and Texas A&M University. As is noted in study [37], the program examined two versions of high-power nuclear laser setups: a stationary RL with a laser radiation power of several megawatts, and a laser setup with a power of about 500 kW, which was pumped using a pool-type reactor. It is proposed that high-power NPL-based setups can be placed not only on land, but on ships as well.

Experimental investigations [38, 39] directed at modeling the pumping condi­tions of laser media in high-power nuclear-laser setups (such as large volumes of medium, long pumping pulses, effect of the medium-flowing and — heating on the quality of the laser beam) were carried out at Sandia Laboratories on an ACRR pool-type pulsed reactor. The description and characteristics of the ACRR reactor are provided, for example, in monograph [7]. The cylindrical core of the ACRR reactor, with a diameter of 80 cm and height of about 51 cm, is made of a mixture of UO2-BeO oxides placed on the bottom of a tank of water at a depth of 7 m (Fig. 6.23). The duration of single pulses varied from 7 to 250 ms. At maximal energy release in the reactor core of 300 MJ, the neutron flux density in the central channel was 8.7 x 1017 cm-2 s-1 [38].

The first experiments with NPLs on the ACRR reactor were carried out in 1987, with irradiation of a 1.5 x 7 x 60 cm laser cell in the central channel. An Ar-Xe mixture was used as the active medium. Placement of a laser cell with large dimensions in the central channel was difficult, because the channel has a diameter of just 22.8 cm. Therefore, in subsequent experiments, an ALEC (Advanced Laser External Cavity) was placed close to the surface of the core in a rectangular cavity with a cross-section of 20 x 132 cm2 (Fig. 6.23).

Successful experiments with the ALEC device made it possible to obtain baseline data for study of stationary modes of NPL operation with flowing of the gas medium through the active volume. For this purpose, experiments were carried with FLE-1 and FLE-2 (Flowing Laser Experiments) devices, which were irradi­ated in the cavity of the FREC-II subcritical assembly [7], located close to the ACRR reactor core. Centrifugal air compressors were used to pump the gas. The active volume of the laser was ~5l, with specific power depositions of up to several tens of W/cm3. The next phase of the investigations at Sandia involved experiments with the VSE (Volume Scaling Experiment) device, with an active volume of 50l, specific pumping power of 5 W/cm3, and operating time of 0.2 s [39].

Fig. 6.23 ACRR reactor with ALEC laser device [38]: (1) ALEC device; (2) laser cell; (3) ACRR reactor core

Fig. 6.24 Oscillograms of laser radiation for Ar-Xe mixture (2 = 1.73 pm) at argon pressures of 0.44 atm (1), 0.88 atm (2), and 1.25 atm (3) (specific power deposition 7 W/cm3; scale division 2 ms) [40]

Unfortunately, studies [38, 39] provided only a schematic description of the conditions of experiments on the ACRR reactor, and there are virtually no data on the characteristics of the laser radiation. Only in study [40] is there some informa­tion on the results of the first experiments with the Ar-Xe mixture (2 = 1.73 pm) at various pressures and specific power depositions in the case of placement of the laser cell in the central channel of the reactor (Fig. 6.24).