The laser pulse should be shaped to satisfy the ICF physical requirement and the profile should be flattened to increase the extraction efficiency of the disk amplifiers and to ensure system safety in ICF laser facility. The spatial-temporal distribution of the laser pulse is affected by the gain saturation, uniformity gain profile of the amplifiers, and the frequency conversion process. The pulse spatial-temporal distribution can’t be described by simply analytic expression, so new iteration algorithms are needed. We propose new inversion method and iteration algorithms in this paper. All of these algorithms have been integrated in SG99 software and the validity has been demonstrated. The result could guide the design of the ICF laser facility in the future.
Large aperture Nd:glass disk is often used as the amplifier medium in the inertial confinement fusion (ICF) facilities. The typical size of Nd:glass is up to 810mm×460mm×40mm and more than 3,000 Nd:glass components are needed in the ICF facility. At present, the 3ω fused silica glass and DKDP crystal are mainly responsible for the damage of driver used for ICF. However, with the enlargement of the facility and increase of laser shot number, the laser damage of Nd:glass at 1ω waveband is still an important problem to limit the stable operation of facility and improvement of laser beam quality. In this work, the influence of Nd:glass material itself, mechanical processing, service environment, and laser beam quality on its damage behavior is investigated experimentally and theoretically. The results and conclusions can be summarized as follows: (1) It is very important to control the concentration of platinum impurity particles during melting and the sputtering effect of the cladding materials. (2) The number and length of fractural and brittle scratches should be strictly suppressed during mechanical processing of Nd:glass. (3) The B-integral of high power laser beam should be rigorously controlled. Particularly, the top shape of pulses must be well controlled when operating at high peak laser power. (4) The service environment should be well managed to make sure the cleanness of the surface of Nd:glass better than 100/A level during mounting and running. (5) The service environment and beam quality should be monitored during operation.
Integration-Test-Bed(ITB) is China's first laser devices with single-beam ten-thousand joules output for Inertial Confinement Fusion (ICF) research. In this paper we describe the development of single-segment slab amplifiers for Integration-Test-Bed (ITB) with 400mm× 400mm aperture. The experiment results shows that the average small signal gain coefficient in 400mm×400mm aperture reach 5.28%/cm with the gain uniformity is about 1.09:1(maximum value/ average value), and up to 1.063:1 (maximum value/ average value) in 360mm×360mm beam-diameter clear aperture. The storage efficiency of system is about 1.47%. The pump-induced wave-front distortion is 5.3λ for the laser beams, which within the correction range of deformable mirror; the thermal recovery time was less than 4 hours. All of this guaranteed the output of 19.6kJ/5ns with wavelength of 1053nm from the Integration-Test-Bed (ITB) device.
The Integration Test Bed (ITB) is a large-aperture single-beam Nd:glass laser system, built to demonstrate the
key technology and performance of the laser drivers. It uses two multipass slab amplifiers. There are four
passes through the main amplifier and three passes through the booster amplifier. The output beam size is
360mm by 360mm, at the level of 1% of the top fluence. The designed output energy of ITB at 1053nm is
15kJ in a 5ns flat-in-time (FIT) pulse, the third harmonic conversion efficiency is higher than 70%. The first
phase of the ITB has been completed in July 2013. A series of experiments demonstrated that laser
performance meets or exceeds original design requirements. It has achieved maximum energies at 1053nm of
19.6kJ at 5ns and 21.5kJ at 10ns. Based on a pair of split third harmonic generation KDP crystals, the third
harmonic conversion efficiency of about 70% and 3ω mean fluences as high as 8.4 J/cm2 have been obtained
with 5ns FIT pulse.
optical propagation simulation by SG99 code and invert algorithm has been made for two typical laser architecture,
namely the National Ignition Facility (model A) and SG-III laser facility (model B) based on measured 400mm aperture
Nd:glass slab gain distribution data on ITB system. When the gain nonuniformity is about 5%, 7%, and 9% respectively
within 395x395mm2 aperture and output beam aperture is 360x360mm2, and output energy is about 16kJ/5ns(square)
with B-integral limited, 1ω(1053nm) nearfield modulation is about 1.10, 1.15, and 1.30 respectively for model A (11+7
slab configuration), and 1.07, 1.08, and 1.17 respectively for model B (9+9 slab configuration) without spatial gain
compensation. With the above three gain nonuniformity and slab configuration unchanged, to achieve flat-in-top output
near field, the compensation depth of the input near field is about 1.5:1, 2.0:1, and 6.0:1 respectively for model A, and
1.3:1, 1.4:1, and 3.5:1 respectively for model B. Compared with model A (the beam aperture unchanged in multi-pass
amplification), the influence of slab gain nonuniformity on model B (beam aperture changed) is smaller. All the above
simulation results deserve further experiment study in the future.