Shaobo He

Status of the SG-III solid-state laser facility

SG-III laser facility beam begins with a nanojoule energy laser pulse from the master oscillator and a fiber front-end system that can provide a variety of pulse shapes suitable for a wide range of experiments. The chirped pulse stacking method is used in the front-end system to generate arbitrarily shaped pulse with a rise time less than 50ps. The system stacks a set of 100-ps chirped pulses in fiber time-delay lines to obtain a 5-ns flat-top pulse with a spectral bandwidth of 1.2nm. The pulse is then transported to preamplifier modules under the middle of CSF for amplification and beam shaping. There is a total of 48 preamplifier modules on SG-III, each feeding a single laser beams. The main amplifier column of 4 high by 2 wide has been chosen as a module and the clear optical aperture is 40cm × 40cm. Small PEPC are chosen for system isolation and beam can be rotated by 90 degree in U-turn beam reverser located in the middle of TSF. After main amplifier, beams are subsequently redirected to final optics assembly in switchyard and are focused on the center of the target chamber with the diameter of 6m.

Status of prototype of SG-III high-power solid-state laser

We are currently developing a large aperture neodymium-glass based high-power solid state laser, Shenguang-III (SG-III), which will be used to provide extreme conditions for high-energy-density physical experiments in China. As a baseline design, SG-III will be composed of 48 beams arranged in 6 bundles with each beam aperture of 40cm×40cm. A prototype of SG-III (TIL-Technical Integration experimental Line) was developed from 2000, and completed in 2007. TIL is composed of 8 beams (four in vertical and two in horizontal), with each square aperture of 30cm×30cm. After frequency tripling, TIL has delivered about 10kJ in 0.351 μm at 1 ns pulsewidth. As an operational laser facility, TIL has a beam divergence of 70 μrad (focus length of 2.2m, i.e., 30DL) and pointing accuracy of 30 μm (RMS), and meets the requirements of physical experiments.

Laser-Induced Damage Initiation and Growth of Optical Materials

The lifetime of optical components is determined by the combination of laser-induced damage initiation probability and damage propagation rate during subsequent laser shots. This paper reviews both theoretical and experimental investigations on laser-induced damage initiation and growth at the surface of optics. The damage mechanism is generally considered as thermal absorption and electron avalanche, which play dominant roles for the different laser pulse durations. The typical damage morphology in the surface of components observed in experiments is also closely related to the damage mechanism. The damage crater in thermal absorption process, which can be estimated by thermal diffusion model, is typical distortion, melting, and ablation debris often with an elevated rim caused by melted material flow and resolidification. However, damage initiated by electron avalanche is often accompanied by generation of plasma, crush, and fracture, which can be explained by thermal explosion model. Damage growth at rear surface of components is extremely severe which can be explained by several models, such as fireball growth, impact crater, brittle fracture, and electric field enhancement. All the physical effects are not independent but mutually coupling. Developing theoretical models of multiphysics coupling are an important trend for future theoretical research. Meanwhile, more attention should be paid to integrated analysis both in theory and experiment.

Preliminary experimental results of Shenguang III Technical Integration Experiment Line

We are now constructing a technical integration experiment line (TIL) at CAEP, which is the prototype facility of Shenguang III laser fusion driver. Currently, many important results have been obtained on the first integrated beam line, which established a sound foundation for Shenguang III engineering design.

Thermal-recovery optimization of the SG-III prototype

Thermal recovery of the main amplifier of the SG-III prototype is numerically simulated. The calculations indicate that, with a shot period of 4 h, we can achieve an acceptable laser slab average temperature and thermal gradient across the slab aperture through optimized active cooling for both the flashlamp cassettes and the slab cavities. After 4 h of thermal recovery, the average temperature and the largest temperature drop across the aperture of the laser slab will be less than 0.13°C above ambient and 0.11°C respectively. The active cooling for all the flashlamp cassettes will need a total cleaned-air flow rate of 80 m3/min (10 ft3/min per lamp) and a total nitrogen flow rate of 16 m3/min for all the slab cavities. Additionally, the temperature of the cooling gas in the flashlamp cassette must be reduced to ?1.0°C (relative to ambient temperature) during the first 2.5 h of the thermal recovery cycle.

Characterization of 355?nm laser-induced damage of mitigated damage sites in fused silica

The laser-induced damage threshold (LIDT) is one of the important methods to determine the mitigation efficiency of mitigated damage sites in fused silica optics. In this work, two sizes of laser beam with a wavelength of 355 nm and a pulse width of 6.3 ns are used to test the LIDTs and properties of mitigated damage sites. Meanwhile, the R-on-1 average threshold and threshold distribution are used to analyze damage behavior of mitigated sites. It is found that the R-on-1 average threshold shows an S-shaped curve and the threshold distribution obeys a normal law for the mitigated damage sites, which is similar to the law of pristine substrates reported before. The LIDT discrepancy of mitigated sites tested with different sizes of laser beam is attributed to the spot-size effect of the test laser beam, i.e., the LIDT of the mitigated site increases with decreasing test laser spot size. It is shown that most damage occurs at the edge of the laser affected zone, which could be ascribed to the weak location between laser affected and unaffected zones. Finally, the correlations between damage size and the laser fluence which causes damage initiation or damage growth are investigated for the pristine substrate and mitigated sites, and can be fitted by an exponential law.

Bulk damage and stress behavior of fused silica irradiated by nanosecond laser

Abstract. The laser-induced bulk damage and stress behaviors of fused silica are studied by using a neodymium-doped yttrium aluminum garnet laser operated at 1064 nm with pulse width of 11.7 ns. Three zones of bulk damage are defined: columned cavity zone, compacted zone, and crack zone. The damage morphology and stress distribution are characterized by a three-dimensional digital microscope and a polarizer stress analyzer. The results show that the stress in the columned cavity zone and compacted zone is approximately zero. From the laser beam center to fringe, both tensile and compressive stresses in the crack zone increase abruptly and linearly and then decrease exponentially. Thermal annealing is used to prove the phase retardation caused by the residual stress. The formation mechanism of bulk damage is also discussed.

Mitigation of surface damage growth by hydrofluoric acid etching combined with carbon dioxide laser treatment

Yong JiangUniversity of Electronic Science and Technologyof ChinaDepartment of Applied PhysicsChengdu 610054, ChinaandChina Academy of Engineering PhysicsResearch Center of Laser FusionMianyang 621900, ChinaE-mail: yxd66my@163.comXiaodong YuanShaobo HeWanguo ZhengHaijun WangHaibing LuWei RenChina Academy of Engineering PhysicsResearch Center of Laser FusionMianyang 621900, ChinaChengsi LuoChunming LiuXia XiangXiaotao ZuUniversity of Electronic Science and Technologyof ChinaDepartment of Applied PhysicsChengdu 610054, ChinaAbstract. Damage sites as large as 600 μm in fused silica surface weresuccessfully mitigated with a new protocol by hydrofluoric acid (HF) etch-ing combined with carbon dioxide laser treatment. The damage sites werefirst etched in 40% HF solution to blunt the fractures, and then the etcheddamage sites were smoothed with a CO

Design and performances of prototype laser amplifiers for technical-integration-line facility

We present work on the design and physical performances for a large-aperture multisegment amplifier, which is an engineering prototype amplifier for our technical-integration-line facility. The amplifier consists of 4×2 apertures with each clear aperture about 300×300 mm. Each amplifier module consists of eight laser slabs, arranged four high, two wide, and one long, and twenty high-pulse power flashlamps with an input electric energy of about 23 kJ per lamp. These twenty flashlamps are arranged in a 6-8-6 scheme. The central eight flashlamps will irradiate their energy to the laser slabs in two directions to increase the pumping transfer efficiency. Experimental results indicated that the energy stored in the upper laser level is about 0.24 J/cm 3 , which corresponds to an energy storage efficiency of about 3.0%. Finally, with an 1.053-μm laser probe we obtained a small-signal gain coefficient of 5.0% cm –1 .

Influence of Ambient Temperature on Nanosecond and Picosecond Laser-Induced Bulk Damage of Fused Silica

The nanosecond (ns) and picosecond (ps) pulsed laser-induced damage behaviors of fused silica under cryogenic and room temperature have been investigated. The laser-induced damage threshold (LIDT) and damage probability are used to understand the damage behavior at different ambient temperatures. The results show that the LIDTs for both ns and ps slightly increased at cryogenic temperature compared to that at room temperature. Meanwhile, the damage probability has an inverse trend; that is, the damage probability at low temperature is smaller than that at room temperature. A theoretical model based on heated crystal lattice is well consistent with the experimental results.