These experiments used three various laser devices, which consisted of two vol­umes connected optically to each other by a small coupling aperture [22]. The first cylindrical volume, which was located near the y-radiation source, formed the field of the radiation that was synchronized with the pumping wave. Subsequent radia­tion proceeded through the coupling aperture and entered the second (conical) volume and was amplified to the necessary energy. As a result, a radiation wave front, which was almost spherical, formed at the output of the conical amplifier, and the equivalent beam divergence was determined by the ratio of the diameter of the coupling aperture to the cone length.

The cylindrical part of these three laser devices was 100-cm long with a diameter of 10 cm. The conical portion of two from these devices was 2-m long, and the third device which had target chamber, was 14-m long and 0.8-m base diameter. As the active medium, a Ne-Xe-NF3 (1,000:26.2:6.2) mixture was used at 5 atm. The dependence of the specific power deposition into the active medium on the distance to a coupling aperture 3 mm in diameter is shown in Fig. 12.4. The scheme of the laser device with a 30 l vacuum target chamber which was placed 12.5 m from the top of the cone is shown in Fig. 12.5. A spherical mirror 80-cm in diameter with a 2.1 m focal distance was situated at a distance of 15 m from the top of the cone. The mirror focused the laser radiation inside the chamber on the target through a quartz window 36-cm in diameter. The target was shielded from the direct influence of the y-radiation by a 50-cm thick steel screen. The experiments used a target with a reversed corona [24], which is a thin-walled polyethylene cone 30-mm high with an open base and an apex angle of 30°.

The experiments measured laser parameters and the characteristics of the X-ray emitted by the plasma that formed when the target was irradiated with the laser radiation. The transmittance of the target chamber window at the laser wavelength

Fig. 12.4 The dependence of the specific power deposition of the active medium on the distance to the coupling aperture placed at the top of the cone [23]

can, in principle, be reduced by y-radiation. Therefore, this parameter was controlled.

The laser device with a target chamber had the following laser characteristics [13, 21—23]: the energy (power) of the laser radiation was 700 J (1.4 x 1011 W), half-amplitude pulse duration of the laser radiation was 7 ns (Fig. 12.6), laser energy efficiency was 3 %, and the contrast of the output radiation was >104. The experiments showed that the transparency of the quartz window did not change during a pump pulse. The characteristics of the recorded X-ray radiation agree with the calculated results for the laser energy at the target—about 1 kJ. This testifies that a spherical light wave formed in the conical amplifier since only in this case is it possible to transfer the laser energy to the target without losses using a focusing system.

The reported energy parameters were not the maximum. In subsequent experi­ments, a cylindrical XeF-laser with a larger active medium volume showed a laser energy of 6 kJ at an intrinsic efficiency of about 4 % [13].