Experimental progress of quasi-isentropic compression under drive condition of Shen Guang-Ⅲ prototype laser facility

The high energy density pulse input into brittle structural materials will propagate as a shock wave. It induces compression fracture and function failure. In this work, voids are introduced to significantly enhance the shock plastic deformability of brittle structural materials, so that brittle structural materials can effectively absorb the shock wave energy, and restrain the propagation of shock-induced cracks. A lattice-spring model is established to investigate the mechanism of shock plastic, and the processes of energy absorbing and crack expanding in porous brittle materials. The shock wave inside porous brittle material splits into an elastic wave and a deformation wave. The deformation wave is similar to the plastic wave in ductile metal, however, its deformation mechanism is of volume shrinkage induced by voids collapse, and slippage and rotation deformation of scattered tiny scraps comminuted by shear cracks. We calculate the shock wave energy based on particle velocities and longitudinal stresses on nine interfaces of the modeled brittle sample, and further obtain the absorbed energy density. The absorbed energy density curve is composed of two stages: the absorbing stage and the saturation stage. The absorbing stage corresponds to the deformation wave, and the saturation stage corresponds to the shock equilibrium state (Hugoniot state). The energy absorb abilities of the dense sample and porous samples with 5% and 10% porosities are compared based on calculation results. It shows that the ability of the porous brittle material to absorb high energy density pulse is much higher than that of the dense brittle material. The ability of porous brittle materials to restrain the propagation of the shock fracture is also explored. The goal of this design is to freeze the propagation of the shock fracture in the middle of the brittle sample, so that the other parts of the sample keep nearly intact during the shock. Inside the protected area, the designed functions of brittle materials can be accomplished without the disturbance of the shock fracture. This design is used under the short pulse loading condition: the rarefaction wave on the rear of the short pulse will catch up and unload the deformation wave if it moves slowly; the deformation wave and the shock fracture propagate synchronously; when the deformation wave is unloaded, the shock fracture will be frozen in the middle of the porous sample. Under the short pulse loading condition, compared with the dense brittle material, whose entire regions are destructed, the porous brittle material can restrain the propagation and impenetration of the shock fracture, with the cost of increasing the damage extent in part of the sample. This is helpful to avoid the entirely function failure of the brittle structural material.

The ability of porous brittle materials to absorb and withstand high energy density pulse

Yin¹,

He²,

Wang³

et al. 2015

Optical transparency of transparent window LiF in laser-driven quasi-isentropic compression experiment

Zhang¹,

Zhao²,

Quan-Xi³

et al. 2015

LiF is often used as a window in laser-driven shock experiments, which can transmit and reflect visible probe laser. Researches of LiF transparency almost focus on its optical reflectivity compressed by strong shock, but there is almost no research on its optical transmissivity compressed by weak shock. In order to study the optical transmissivity of LiF, the quasi-isentropic compression experiment is carried out on the ShenGuang-III prototype laser facility, in which the velocity interferometer system for any reflector is used to diagnose the optical reflectivity of the quasi-isentropic compression sample CH/Al/LiF. The experimental results indicate that the velocity interferometer fringes are missing in the late stage of this experiment. The probe laser should penetrate LiF before it hits the rear surface of aluminum and the laser reflected by aluminum should penetrate LiF before it is collected by the velocity interferometer system for any reflector. Therefore, the reflectivity diagnosed by the velocity interferometer system for any reflector is the product of the optical reflectivity of aluminum and the optical transmissivity of LiF under the experimental condition. However, there is no research about the optical transmissivity model of thick LiF compressed by laser-driven shock. In this paper, we develop a transmissivity model for transparent window LiF and simulate the optical reflectivity of sample CH/Al/LiF. Firstly, we simulate the temperature and density of the sample by the code for one-dimensional multigroup radiation hydrodynamics (MULTI-1D). Then, based on the resulting temperature and density, we simulate the optical reflectivity of the sample by using the optical reflectivity model of aluminum and the optical transmissivity model of LiF. Without considering the transparency of LiF, the simulated result indicates that there is no signal missing in the late stage, which is different from the experimental result. By considering the transparency of LiF, the simulated result is in good agreement with the experimental result. The simulated result indicates that the formation of the strong shock, because of the later shock's catching up with the early one, obviously reduces the optical transparency of LiF and finally causes the velocity interferometer fringes to disappear. The simulated result also indicates that the energy gap of LiF calculated from density-functional theory is 1-2 eV. In this experiment, when LiF becomes opaque, its temperature is 1 eV and its pressure is 2-3 Mbar.

Research of time fiducial laser and probe laser of velocity interferometer system for any reflector for Shenguang-III laser facility

Zhang¹,

Xiao-Cheng²,

Dan-Dan³

et al. 2016

Time fiducial laser is an important timing marker for different diagnostic instruments in high energy density physics experiments. The probe laser in velocity interferometer system for any reflector (VISAR) is also vital for precise shock wave diagnosis in inertial confinement fusion (ICF) research. Here, time fiducial laser and VISAR probe laser are generated from one source in SG-III laser facility. After generated from a 1064 nm DFB laser, the laser is modulated by an amplitude modulator driven by a 10 GS/s arbitrary waveform generator. Using time division multiplexing technology, the ten-pulse time fiducial laser and the 20 ns pulse width VISAR probe laser are split by a 12 multiplexer and then the time fiducial and VISAR pulses will be selected individually by acoustic-optic modulators. Using this technology, the cost for the system can be reduced. The technologies adopted in the system also include pulse polarization stabilization, high stable Nd: YAG amplification, high precision thermally controlled frequency conversion, fiber coupling, and energy transmission. The fiber laser system is connected to the Nd: YAG rod amplifier stage with polarizing (PZ) fibers to maintain the polarization state. The output laser of Nd: YAG amplification stage is coupled with different kinds of energy transfer fibers to propagate enough energy and maintain the pulse shape for the time fiducial and VISAR probe laser. The input and output fibers are all coupled to the rod amplifiers with high precision and being easy to plug and play for users. Since the time fiducial and imaging VISAR laser system is far from the front end room and located in the target area, the system also uses an arbitrary waveform generator (AWG) to generate the shaped ten-pulse time fiducial laser and 20 ns VISAR laser. This AWG and the other three AWGs used for the main laser pulse of SG-III laser facility will be all synchronized by 10 GHz clock inputs, realizing the smaller than 7 ps (RMS) jitter between the main laser pulse, time fiducial laser and VISAR pulse. After amplification and frequency conversion, the time fiducial laser finally generates 12 beam 2 and 4-beam 3 laserbeams, providing important reference marks for different detectors in the ICF experiments and making it convenient for the analysis of multiple diagnostic data. The VISAR laser pulse is also amplified by the Nd: YAG amplifiers and frequency-converted to 532 nm green light by a thermally controlled LBO crystal, with output energy larger than 20 mJ. Finally, the 532 nm VISAR probe laser beam is coupled with a 1-mm core diameter fused silica optical fiber, and then propagates 30 meters to the imaging VISAR system. The VISAR probe laser has been used in many high energy density physics experiments. The shock wave loading and slowdown processes are measured. Function for measuring velocity history of shock wave front movement in different kinds of materials can be also added to the SG-III laser facility.