The gas mixture SF6-H2 with a non-chain chemical reaction was used as the active media. This mixture was studied previously at VNIIEF with excitation by a relativistic electron beam at a 40 ns pulse duration [19, 20].

One of these experiments [17] used a 480-cm long cylindrical laser cell 11 cm in diameter without a laser cavity. The cell was filled with a SF6-H2 (9:1) mixture at 2.1 atm. The angle between the propagation direction of the у-radiation and the optical axis of the cell was about 10°. Therefore, longitudinal excitation of the laser media was achieved in practice. Under these conditions, the divergence of the laser beam was determined by the geometric dimensions of the cell. To reduce the divergence, the section of the cell near the у-source was comprised of closely placed channels, 60-cm long and 3-mm in diameter, which formed beam directivity in the remaining part of the cell.

The distribution of the specific energy deposition along the length of the cell is shown in Fig. 12.1. The average energy deposition by volume was about 1 J/cm3 at a y-pulse duration of 18 ns at the base. This experiment obtained the following parameters for the laser radiation: the energy density at the output of the cell was 4.4 J/cm2; the half-amplitude pulse duration was 8 ns (Fig. 12.2); and the beam angular divergence was 7 x 10~3 rad.

The results of the experiment were analyzed based on a theoretical model developed by the authors [17]. This model is a one-dimensional transport equation for direct and reverse light waves at the vibrational-rotational transitions of the HF molecule (A = 2.7-3.0 ^m). It follows from the calculations that approximately 90 % of the radiation is in the forward direction corresponding to the direction of the initialization wave. Figure 12.2 shows the calculated shape of the laser pulse when energy is removed from the main volume of the cell as a result of the amplification of the narrow-band radiation from the former. Calculations also show that under experimental conditions, the energy of the narrow-band radiation from the cell is 20-25 % of the potential laser energy.

A significantly higher laser output, about 70 kJ, was obtained in an experiment with about 6 m3 of active medium [18]. The active medium and the conditions for pumping it (average specific energy contribution and y-pulse length) were the same

as in the previous work [17]. The experiments whose schematics are shown in Fig. 12.3 used a cylindrical laser cell, 130 cm in diameter and 480 cm long, without a laser cavity. Laser radiation distribution in the beam cross-section was measured using bolometric wire-netting calorimeters.

As in the work [17], to reduce the angular divergence of the beam, a 60-cm long former of radiation directivity was used that consisted of elongated channels 3-mm in diameter. The inner volume of the main part of the cell was divided into long partitions to reduce energy loss due to amplification of the spontaneous emission in the transverse direction. The average energy density in the cross­
section of the laser beam at the cell output was 5.1 J/cm2, and the shape and duration of the laser pulse were about the same as in Fig. 12.2.