The distribution of the radiation intensity, shape, and area of a light spot on the cavity mirror surface of a sealed laser with plane-parallel arranged uranium layers was studied in ref. [52]. At the moment in time that lasing originates, the radiative zone is concentrated near the laser cell axis, then it begins to enlarge and its shape becomes close to rectangular at the lasing pulse peak. Radiation intensity distribution is symmetrical relative to the planes of symmetry, y — const and x — const, that passes through the optical axis. The lasing volume reaches ~60 % of the laser cell’s volume.

This behavior is determined by the dynamics of the development of gas density inhomogeneities and the spatial redistribution of specific energy deposition in the laser’s sealed cell over the course of the pumping pulse. It is natural that the dynamics for a laser with transverse gas pumping have a different, more complex nature. Consequently, the pattern of radiation intensity distribution throughout the laser channel cross-section should change as well, which was also experimentally proven in ref. [53].

During experimentation, an Ne-Ar-Xe mixture (300:100:1) at a pressure of 1 atm was used, which was flowed through the laser channel of an LM-4 module

at a velocity of ~7.2 m/s. The average thickness of the metallic uranium layers (90 % enriched by 235U) was ^ = 2 .67 pm. The uranium layers were deposited to aluminum substrates with a thickness of 4 mm and were covered with a thin (0.5 pm) aluminum film for the purpose of preventing 235U atom ejection into the gas [54, 55]. Just as during the experiments described in ref. [52], which involved a sealed channel, the distance between the layers was 2 cm and the channel dimen­sion along the gas flow direction was 6 cm. The length of the laser channel along the optical axis direction was 1 m. A high-reflectivity spherical dielectric mirror with a radius of curvature of 20 m and a semi-transparent planar dielectric mirror were used as the cavity mirrors. The distance between the mirrors was 1.4 m.

Excitation was accomplished by means of neutrons from the core of a BIGR reactor. Diagrams of the gas flow direction, as well as the mutual position of the laser module and the BIGR reactor, are presented in Figs. 9.1 and 9.15. The duration of the reactor’s exciting quasi-pulse was ~1.5 s. The average specific power deposition in the gas over the length of the laser channel at the exiting pulse peak reached ~6.2 W/cm3. The lasing threshold came to ~35 % of the maximum neutron flux. The maximum laser output power reached 12 W.

A charge-coupled camera operating in the infrared region of the spectrum recorded an image of the laser spot. An image of the laser beam’s transverse cross-section obtained from a single laser module channel was formed on the sensing element of a camera with a pixel size of ~0.1 mm. During neutron pumping, the camera operated in the burst recording mode and captured images of the laser spot at an interval of 30 ms. Over the course of the pulse, 25 frames were produced that depicted the laser beam’s development in chronological order.

FSA-G1 sensors with a photoresistor based on lead sulfide and IMO-2N optical radiation power meters were used to determine laser output power.

The experimental dependences of neutron flux density and laser output power upon time are shown in Fig. 9.27. Lasing begins at the moment in time that the

Fig. 9.27 Time dependencies of neutron flux density (1) and laser output (2) threshold is reached and thereafter its dependence upon time almost fully duplicates the similar dependence of neutron flux density.

Radiation intensity distributions at the surface of output mirror are presented in Fig. 9.28 for a number of successive moments in time.

The relative distribution of laser radiation intensity in the perpendicular and parallel directions relative to the gas flow’s motion is shown in Fig. 9.29.

At a velocity of U ~7 m/s and a channel dimension along the gas flow direction of b = 6 cm, the typical time that a portion of the gas resides in the channel is t0~bU~ 10~2 s, which is considerably shorter than the neutron pulse’s duration. This fact makes it possible to apply the procedure described in ref. [46] for calculating the gasdynamic characteristics of the stationary excitation mode to the calculation of the spatial distributions of gas density and specific power deposition in the laser channel at each given moment in time.

The cumulative results of the experiments and the calculations (Fig. 9.30) performed using the procedure described in ref. [46] under conditions of an excitation and a geometry that fully corresponds to the experimental conditions categorically evidences that:

• In flowing nuclear-pumped lasers, lasing is “spawned” in two regions that are symmetrical relative to the longitudinal plane of symmetry and that closely border the laser channel gas inlet, within which the specific power deposition and gas density approach the maximum values;

• In direct proximity to the uranium layer surface where power deposition reaches the maximum value, two narrow regions exist that do not take part in lasing, the presence of which can be explained by the origination of passive zones; and

• As the neutron flux density and subsequently power deposition increases at each given point in the gas volume, both of the symmetrical lasing zones merge into one, which expands in the gas flow direction, reaching a maximum size (~60 % of the laser channel’s volume) at a moment in time that corresponds to the exciting pulse peak.