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.
The influence of organic contamination (rubber outgassing) on the transmittance of the SiO2 sol‐gel antireflection (AR) film and laser‐induced damage threshold (LIDT) at 355 nm for 3ω AR film and at 1064 nm for 1ω AR film is studied. The correlation between the contamination time and the transmittance loss/LIDT of 1ω/3ω AR film is also investigated both in atmospheric and vacuum environments. The results show that the transmittance loss increases with increasing contamination time, and the LIDT decreases with increasing contamination time for both in atmospheric and vacuum environments. In addition, the resistance against contamination of the 1ω film is stronger than 3ω film, and the contamination is more serious in vacuum than in an atmosphere environment for the same contamination time. Meanwhile, the damage mechanism is also discussed. It indicated that both the porous structure and photo‐thermal absorption contribute to the decreasing LIDT of the sol‐gel AR film.
The residual stress field of fused silica induced by continuous wave CO2 laser irradiation is investigated with specific photoelastic methods. Both hoop stress and axial stress in the irradiated zone are measured quantitatively. For the hoop stress along the radial direction, the maximum phase retardance of 30 nm appears at the boundary of the laser distorted zone (680-μm distance to center), and the phase retardance decreases rapidly and linearly inward, and decreases slowly and exponentially outward. For the axial stress, tensile stress lies in a thin surface layer (<280 μm) and compressive stress lies just below the tensile region. Both tensile and compressive stresses increase first and then decrease along the depth direction. The maximum phase retardance induced by axial tensile stress is 150 nm, and the maximum phase retardance caused by axial compression stress is about 75 nm. In addition, the relationship between the maximum axial stress and the deformation height of the laser irradiated zone is also discussed.
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.
Potassium Dihydrogen Phosphate (KDP) is a non-linear optical material for the Pockelscells and laser frequency
conversion used in inertial confinement fusion (ICF).But the performances are decreased by the presence of defects or
impurities from the growth process. Furthermore, it is difficult to identify them due to their small size and sparse
distribution. Laser conditioning provides an increase for the material damage resistance due to an unknown physical
mechanism resulting from the interaction of sub-damage laser pulses with KDP defects.
In this paper, the crystals first were laser conditioned by raster scanning using Nd:YAG laser at 355nm with pulse
durations of approximately 7 ns. Then we investigated the KDP crystals with different non-destructive optical
diagnostics. We measured in the same time the fluorescence, the photothermal absorption, Raman and FTIR
spectroscopy of the material. We concentrated on the differences of conventionally grown KDP crystals before and after
the laser conditioning. We obtained the different spectra information of KDP crystals.
Damage sites as large as 600 μm in fused silica surface were successfully mitigated with a new protocol by hydrofluoric acid (HF) etching combined with carbon dioxide laser treatment. The damage sites were first etched in 40% HF solution to blunt the fractures, and then the etched damage sites were smoothed with a CO2 laser. It has been found that the etching rate of damaged material in the lateral direction is larger than in the longitudinal direction; thus, an optimized etching time was chosen to etch the damage sites based on the etching ratio. Three types of damage test methods were used to confirm the mitigation efficiency of the protocol. The results indicate that the damage resistance capability of mitigated sites can recover to the level of pristine substrate.
The dynamics of 355-nm laser ablation on fused silica were studied by instantaneous scattering pulse
measurement and a time-resolved shadowgraph imaging. The sharp increase of scattered light of pumped
pulse is assumed to be the damage precursor, therefore, the damage start nearly at the peak of the
pumped pulse. The plasmas flash due to ion-electron recombination occurred about 21ns after the
peak of pumped pulses. The propagating shock wave and ejected material to the air were imaged by
shadowgraphic technology. The damage process of fused silica under UV laser ablation was also
In industrial applications, shape memory alloys (SMAs) are very often used as two-way actuators, in the form of coil
springs, which capitalize on the capacity of small shear to produce large displacement. One way to achieve the two-way
actuators is using a one-way shape memory effect SMAs spring with a bias spring, the other way is two-way shape
memory effect (TWSME) SMAs spring. However, the TWSME is not inherent property of SMAs, suitable thermomechanical
training is required to develop this effect. In this work, methods of developing TWSME are designed for
SMAs and factors which affect the TWSME are investigated. Two types of TWSME spring are obtained using the
designed methods. One type is extension spring, which can extend upon heating and contract upon cooling and this
other type is contraction spring, which can contract upon heating and extend upon cooling. The actuation behaviors of
these two types TWSME are investigated. And the stability of TWSME was also studied with working cycles and