Min Shui

Measurement of the chirp characteristics of linearly chirped pulses by a frequency domain interference method

A linear optical technique for chirp characteristics measurement based on frequency domain interference is developed. This technique can be applied to measure the temporal structure of linearly chirped pulses which have become increasingly important in ultrafast optics. To confirm this technique, an experiment is carried out to measure the chirp rate and duration of a picosecond chirped pulse with an imaging spectrometer.

Hypervelocity launching of flyers at the SG-III prototype laser facility

Experiments of laser-driven hypervelocity flyers have been conducted at the SG-III prototype laser facility. Using the continuum phase plate technique, four laser beams each with a 3-ns quadratic profile are configured to produce relatively uniform irradiated spots of diameter size either 500 μm or 2000 μm. With the former, specifically designed multi-layered flyers (polyimide/copper) were accelerated by shock impedance and reverberation techniques via direct laser ablation to a super-high averaged velocity of 55 km/s, much faster than recently reported results. Light-emission signals of shock breakout and flyer impact on flat or stepped windows were obtained that indicated good planarity and integrity for the flyer. In the latter, single-layered aluminum flyers were gradually accelerated to a terminal velocity of 11 km/s, as measured by optical velocimetry, without melting and vaporization. The results suggest that the SG-III prototype laser facility has the capability to launch high-speed flyers to create extreme conditions for investigating the science of shock compression and its equation of state.

Investigation of ultrafast excited state dynamics of 2,2′,4,4′,6,6′-hexanitrostilbene using femtosecond transient absorption spectroscopy

A study on the dynamics and structures of the excited states of 2,2′,4,4′,6,6′-hexanitrostilbene shows equilibrium between vibrationally hot S1 (S*1) and S1 states with lifetimes of 0.8 and 6 ps, respectively. The T1 state has a lifetime of about 4 ns. It advances understanding of energetic materials ignition.

Blind deconvolution for spatial distribution of Kα emission from ultraintense laser-plasma interaction

The spatial distributions of the Kα emission from foil targets irradiated with ultra-intensity laser pulses have been studied using the x-ray coded imaging technique. Due to the effect of hard x-ray background contamination, noise as well as imperfection of imaging system, it is hard to determine the PSF analytically or measure it experimentally. Therefore, we propose a blind deconvolution method to restore both the spatial distributions of the Kα emission and the systems PSF from the coded images based on the maximum-likelihood scheme. Experimental restoration results from penumbral imaging and ring coded imaging demonstrated that both the structure integrity and the rich detail information can be well preserved.

Fragment size distribution statistics in dynamic fragmentation of laser shock-loaded tin

This work investigates the geometric statistics method to characterize the size distribution of tin fragments produced in the laser shock-loaded dynamic fragmentation process. In the shock experiments, the ejection of the tin sample with etched V-shape groove in the free surface are collected by the soft recovery technique. Subsequently, the produced fragments are automatically detected with the fine post-shot analysis techniques including the X-ray micro-tomography and the improved watershed method. To characterize the size distributions of the fragments, a theoretical random geometric statistics model based on Poisson mixtures is derived for dynamic heterogeneous fragmentation problem, which reveals linear combinational exponential distribution. The experimental data related to fragment size distributions of the laser shock-loaded tin sample are examined with the proposed theoretical model, and its fitting performance is compared with that of other state-of-the-art fragment size distribution models. The...

Excited-state dynamics and electron transfer process of 1,3,5-triamino-2,4,6-trinitrobenzene

Insights into the excited-state dynamics and electron transfer processes of nitro explosives offer an efficient tool for unravelling ultrafast and complex detonation physics. In this work, the excited state dynamics and electron transfer processes of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) were studied using femtosecond transient absorption spectroscopy and time-dependent density functional theory. The de-excitation of TATB after excitation at 400 nm involves an equilibrium between the vibrationally hot S1 (S*1) and S1 states, with lifetimes of 0.64 and 6 ps, respectively. After vertical excitation, the electron density is transferred from the C-ring and NH2 group to NO2 groups. The excitation energy activates the nitro groups, and energy is redistributed via their nuclear motion. The relaxation of the initial S*1 state shows an apparent structural change occurring at one activated nitro group, which becomes further twisted from the planar benzene ring and relaxes to the S1–T1 conical intersection. The T1 state is populated via the minimal intersystem crossing through the S1–T1 intersection. The nitro group charge transfer process is then explored following the ultrafast structural change. This study advances our understanding of photo-initiated reactions as well as the ignition of energetic materials.

A simple method to prevent hard X-ray-induced preheating effects inside the cone tip in indirect-drive fast ignition implosions

During fast-ignition implosions, preheating of inside the cone tip caused by hard X-rays can strongly affect the generation and transport of hot electrons in the cone. Although indirect-drive implosions have a higher implosion symmetry, they cause stronger preheating effects than direct-drive implosions. To control the preheating of the cone tip, we propose the use of indirect-drive fast-ignition targets with thicker tips. Experiments carried out at the ShenGuang-III prototype laser facility confirmed that thicker tips are effective for controlling preheating. Moreover, these results were consistent with those of 1D radiation hydrodynamic simulations.

Investigation of fragment sizes in laser-driven shock-loaded tin with improved watershed segmentation method

Studying dynamic fragmentation in shock-loaded metals and evaluating the geometrical and kinematical properties of the resulting fragments are of significant importance in shock physics, material science as well as microstructural modeling. In this paper, we performed the laser-driven shock-loaded experiment on the Shenguang-Ш (SGШ) prototype laser facility, and employed X-ray micro-tomography technique to give a whole insight into the actual fragmentation process. To investigate the size distribution of the soft recovered fragments from Poly 4-methyl-1-pentene (PMP) foam sample, we further developed an automatic analysis approach based on the improved watershed segmentation. Comparison results of segmenting fragments in slices with different methods demonstrated that our proposed segmentation method can overcome the drawbacks of under-segmentation and over-segmentation, and has the best performance in both segmentation accuracy and robustness. With the proposed automatic analysis approach, other parameters such as the position distribution and penetration depth are also obtained, which are very helpful for understanding the dynamic failure mechanisms.