To support stationary operation of a NPL with transverse gas flow through the laser channels, the gas flow velocity must be >100 m/s. Chapter 9 shows that, at these circulation speeds, the gas flow is turbulent and gas density fluctuations emerge, which negatively affect the optical quality of the medium and, consequently, on the characteristics of the laser radiation. The calculations show that the gas velocity at which turbulent density fluctuations do not have a direct influence on the laser parameters should be <30 m/s for helium-based laser media and <10 m/s for argon — based.

Velocities ~10 m/s may be realized with transverse circulation (see Chap. 9, Sect. 9.3). In this case, the RL core consists of separate laser channels with transverse (with respect to the optical axis) circulation of the gas medium [13, 14]. A single laser channel with a rectangular profile contains a ~10-cm wide flat layer of uranium deposited to the internal surface of the side walls parallel to the direction of gas flow. A cross section of this channel is shown in Fig. 10.2b (see also Figs. 9.1 and 9.2). To cool the mixture heated in the channel, a radiator is placed at its output. The next laser channel can be placed beyond the radiator output, which serves as its input and so on. Thus, a chain of laser channels can be built that is combined in a single gas loop. The gasdynamic and thermophysical characteristics of the laser channel, their relationship with laser parameters, and also the optimal operation modes for this channel in the case of transverse gas flow are examined in detail in Chap. 9.

The RL core should contain a sizable (~1,000) number of laser cells. Due to design, manufacturing, and operational considerations, this system is conveniently implemented in sections consisting of functionally complete modules.

A possible schematic for the RL is given in Fig. 10.4. The device does not have a special enclosure. This function is fulfilled by the reactor matrix together with a hard support layer made of a solid neutron moderator (4) and biological shielding

(5) . The reactor matrix may be produced with graphite blocks and zirconium tubes containing nuclear reactivity regulators (2) of the reactor (boron carbide rods) and devices for monitoring nuclear-physical parameters (3). The matrix contains replaceable laser modules (1) forming, in total, the RL core.