Investigations of optical inhomogeneities in sealed NPLs have revealed that flowing of the gas medium through the laser cavity is a necessary condition for achieving CW lasing in the stationary and quasi-stationary excitation modes. Gas heating is a no less important factor that requires the implementation of gas flowing. So, for example, at a thermal neutron flux density of Ф «1014 cm~2 x s_1, the average specific power deposition for helium with an average density of p0 = 3.2 x 10~4 g/cm3 (at initial pressure and temperature of P0 = 2 atm and T0 = 300°K) that fills a cylindrical laser cell with an internal radius of r1 = 1 cm at a metallic uranium active layer thickness of <5u = 2 .8 x 10 4 cm comes to

2 , qc = dqc/dt = OfN^E^e ~ 10 W/cm.

During stationary pumping in the absence of gas replacement, this specific deposition will result in a gas temperature increase to T > 104°K over a period of 1 s.

One method for implementing continuous gas replacement is longitudinal gas flowing [1] (in the direction of a channel’s optical axis), similar to the one used in high-temperature gas-cooled reactors.