Abstract: When a shock wave reflects from the free surface of a solid sample, fragments may be emitted from the surface. Understanding the process of the fragments mixing with gas is an important subject for current researches in inertial confinement fusion and high pressure science. Particularly, obtaining the fragments size and distribution is important for developing or validating the physical fragmentation model. At present, the reported quantitative data are less due to the great challenges in the time-resolved measurements of the fragments.#br#Recently, high-power laser has appeared as a promising shock loading means for fragment investigation. The advantages existing in such means mainly include small sample (~μm to mm-order), convenient dynamic diagnosis and soft recovery of fragments. Our group has performed the dynamic fragmentation experiments under laser shock loading metal. The ejected fragments under different loading pressures are softly recovered by low density medium of poly 4-methy1-1-pentene (PMP) foam. The sizes, shapes and penetration depths of the fragments are quantitatively analyzed by X-ray micro-tomography and the improved-watershed method.#br#This paper mainly reports the research advances in the process of the fragments mixing with gas. The laser-driven shock experiments of tin sample are performed at Shenguang-Ⅲ prototype laser facility. Under two typical loading pressures, the fragments mixed with gas (N2) are recovered by PMP foam with a density of 200 mg/cm3, and the pressure of gas is 1 atm. The high resolution reconstructed images of the recovered fragments provided by X-ray micro-tomography and computed tomography reconstruction show that the shapes of the fragments are almost homogeneous, and their sizes are in a range of about 1-20 micron. These images are very different from the images of the fragments recovered in vacuum under similar loading pressures. The observed fragments under loading pressure less than 10 GPa in vacuum are some thin layers, while the loading pressure is increased up to more than 30 GPa, a large number of small spherical particles are observed in the front of the recovery fragments, thin layers in the middle, and these spherical particles have diameters ranging from one dozen to several hundreds of micrometers. The sizes and number of fragments are analyzed by the improved watershed method. The resulting distribution of the fragments mixed with gas follows bilinear exponential distribution. Comprehensive analyses of former simulations and our experimental results show that the secondary fragmentation should occur in the process of the fragments mixing with gas.

Abstract: To accelerate the electrons efficiently in the laser wake-field accelerator, it is necessary to suppress the instability induced by the diffraction and the defocusing of the laser pulse. The gas-filled capillary discharge waveguide can generate an approximately parabolic density distribution, which can guide the laser pulse efficiently and suppress the instability. Using the Stark effect, this plasma density distribution is measured in this paper, and the relationship between plasma density and filled pressure is presented. By using the MHD code CRMHA, the formation of the capillary waveguide is simulated and researched in detail.

Abstract: Neutron source has broad application prospects in crystallography, neutron irradiation, neutron therapy for cancer, and so on. As a new scheme to produce bright pulsed neutron source, the laser-driven neutron has attracted wide interest. In recent years, laser driven neutron sources have been extensively studied and the great progress has been made. Short pulsed laser driven neutron sources could be a compact and relatively cheap way to produce quasi-monoenergetic neutrons. The yields and the angular distributions of the laser-driven neutron sources are important in the research of laser-driven neutron sources and relevant applications. We conduct experimental investigation of this respect by using the XingGuang-Ⅲ high intense laser facility, which delivers synchronized picosecond and nanosecond laser pulses. The picosecond laser energy is 100 J, the pulse width is 1 ps, and the focusing spot diameter is 20 μm. At this time, the corresponding laser power density reaches 3×1019 W/cm2. A high-energy deuterium ion beam is produced by focusing the picosecond laser on a deuterated polyethylene foil, and the deuterium ion beam is incident on a secondary deuterated polyethylene planar target to activate the D-D reaction to obtain the neutron beam. In the experiment, the neutron yield and its angular distribution are measured by the different-sensitivity BD-PND bubble detectors, which are placed in the target chamber around the target. The emission of the neutron beam is found to be non-uniform. A maximum intensity of 5.13×107 n/sr is observed in the forward direction. The angular distribution of the neutron beam is theoretically calculated by taking into account the energy-angle cross section, the angular and energy distribution of the incident deuterium ion beam. The probability of the neutron energy-angle distribution in the laboratory system is obtained by the coordinate transformation from the probability in the center of mass frame. The results show good agreement with the experimental measurements. This experiment has a certain reference value in the practical application of D-D reaction neutron source.

Abstract: The laser-driven proton acceleration experiment is carried out on the SGⅡ-U device based on charged particle activation method, and the target parameters are optimized. The charged particle method is used to measure the maximum cutoff energy of proton, angular profile, total yield and conversion efficiency of laser energy to proton energy for different copper film thickness under the same laser condition. It is found that the optimal copper film thickness for the SGⅡ-U picoseond laser-driven proton experiment is 10 μm, the highest proton energy obtained is about 40 MeV, and the total yield of protons (〉4 MeV) is about 4×1012, the conversion efficiency of laser energy to proton energy is about 2%. Thicker or thinner copper film can reduce the maximum cut-off energy of accelerated proton; when the target thickness is reduced to 1 μm, the pre-pulse of the laser begins to have a significant effect on the target normal sheath acceleration (TNSA) proton, proton energy drops sharply, the proton beam porfile exhibits a hollow structure; when the target thickness is increased to 35 μm, although the energy of the proton is reduced, the proton beam spot is more uniform. According to our experimental results, when using SGⅡ-U picosecond laser to generate protons as a backlight diagnostics, a thicker Cu film can be selected which can supply more uniform proton beams. When the target is too thin, the TNSA proton itself has a modulation structure which will cause interference to yield the photographic results; when the protons generated by the SGⅡ-U picosecond are used to generate neutron source, the higher proton energy and yield are required, and 10 μm Cu film is suitable. The further enhancing the TNSA accelerated proton energy and quantity of the SGⅡ-U picosecond laser requires the further improving of the laser contrast.

Abstract: The spatiotemporal coupling distortion of large aperture ultra-high peak power laser will degrade the pulsed beam in both near-field and far-field. To accurately predict the light field distribution at the focus and compensate for the spatiotemporal coupling distortion, a single-frame measurement of full three-dimensional spatiotemporal coupling distortion is proposed based on the frequency domain separate spatial-spectral interference. The setup requires only a slit array attached to the front of an Imaging spectroradiometer. The whole procedure of carrier frequency distinguished spectral interference measurement is simulated in this study. The simulation results prove that the presented measuring method is correct and effective. The effectiveness of this method will be further verified experimentally in next step.

Abstract: Fiber lasers show several advantages over other types of lasers. They are efficient, compact, and rugged since they require few bulk components and are virtually unaffected by the surrounding environment. Mode-locked mid-infrared (mid-IR) lasers are essential for a wide variety of applications. The promising applications of mode-locked fiber lasers at wavelengths near 3 m include combs generation (metrology), spectroscopic sensors, infrared countermeasures, laser surgery, high-efficient pump sources for longer-wavelength oscillators and mid-IR supercontinuum source pumping. Based on the nonlinear Schrdinger equation (NLSE), a theoretical model of passively mode-locked Er3+-doped fluoride fiber laser using a saturable absorber is set up. Some mechanisms for generating mid-IR ultrashort pulse in fiber lasers are investigated. When the dispersion of the cavity is managed properly, the numerical simulation mainly focuses on the evolution process of mid-IR ultrashort pulse in fluoride fiber oscillators. Influences of the intracavity net dispersion and the small-signal gain on the generation of mode-locked pulses are analyzed in detail. And the reasonable parameter windows are given. Just as the simulated results showed, for a case of 4 m Er3+-doped fluoride fiber, small-signal gain g0= 0.6 m-1 and unsaturated loss l0 = 0.7, the stable mode-locked pulses are achieved by tuning the net intracavity dispersion within a certain range from 0.72 ps2 to 0.83 ps2. As the net intracavity dispersion increases, the output pulse duration increases gradually, while the spectrum width (FWHM) and peak power decrease accordingly. In addition, for the case of 4 m Er3+-doped fluoride fiber, unsaturated loss l0 = 0.7 and net intracavity dispersion of 0.8 ps2, the stable mode-locked pulses can also be obtained by tuning the small-signal gain within a certain range from 0.55 to 0.70 m-1. As the small-signal gain increases, the output pulse duration, spectral width, and peak power increase gradually. This work may be beneficial to the design of experiments for achieving more narrow pulse duration, wide spectral width, and high peak power mid-infrared ultrashort pulse.

Abstract: Backward Raman amplification (BRA) in plasma can be used for generating ultra-powerful laser pulses. In this paper, the plasma density effect on backward Raman laser amplification is studied by using particle-in-cell method. It is found that using a low plasma density can lead to the premature Langmuir wave breaking and thus result in a small energy-transfer efficiency. On the other hand, using a high plasma density will enhance the developments of unwanted instabilities, which rapidly disturb the Raman amplification, thus limiting the interaction length and output power. Therefore, an optimal plasma density for BRA is near the threshold of Langmuir wave breaking in order to achieve both high efficiency and large energy flux. The space frequency spectrum analysis shows that the saturated intensity of amplified pulses is limited mainly by the self-phase modulation instability. By using a 1013 W·cm-2 pump pulse, our simulation results show that the initial 1013 W·cm-2 seed pulse can be well be well amplified into a pulse with an energy power of 1017 W·cm-2, a duration of 40 fs, and and an energy conversion efficiency of up to 58%.

Abstract: High intensity laser is an efficient method for shock generator to study the dynamic fragmentation of materials, in which the direct drive is widely utilized. The continuum phase plate is used for smoothing the focal spot of the laser, but the loading region is usually smaller than the designed value. In this work, we study an experimental technique for investigating the dynamic fragmentation of metal via indirectly driving a high-intensity laser. Firstly, the radiation distributions on the sample for four different hohlraums each with a diameter of 2 mm but different length are simulated via the IRAD software, in which the proper hohlraum with a diameter of 2 mm and a height of 2 mm is selected for the experiments. Secondly, the peak temperatures and radiation waves under different laser energy and pulse durations are measured. The peak temperature decreases simultaneously as the laser energy decreases. In addition, the loading shock waves under a peak temperature of 140 eV and different radiation waves are estimated via the hydrodynamic simulation. It is revealed that a peak pressure of several tens of gigapascals is acquired and the peak pressure is greatly increased when the 10 μm CH layer is placed on the sample. In the end, the dynamic fragmentation process via indirect drive is investigated by using the high energy X-ray radiography and photonic Doppler velocimetry. The radiograph is a snapshot at 600 ns and shows a typical result of the spall process. The first layer is measured to be 0.06 mm thick and 0.3 mm away from the unperturbed free surface. It is also exhibited that the hohlraum is expanded to a large extent but is not broken up. The jump-up velocity and time of spall are measured to be 0.65 km/s and 131 ns, respectively. The average velocity of the first layer is estimated to be (0.63 ± 0.1) km/s, obtained via the distance of 0.3 mm divided by the time difference of 469 ns (600 ns minus 131 ns). The one-dimensional loading region is 2 mm, and the flatness is better than 5 %. This work provides a reference for designing new hohlraum, shock wave loading technique and dynamic fragmentation process.

Abstract: When evaluating the plasma parameters in inertial confinement fusion, the flux-limited local Spitzer-Härm (S-H) model in radiation hydrodynamics simulations may be invalid when electron temperature gradient is too large. In other publications, the electron distribution function (EDF) could be explained by comparing the energy equipartition rate 〈inline-formula〉〈tex-math id="M7"〉\begin{document}$R_{\rm eq}=\dfrac{1}{2}m_{\rm e}v_{\rm te} ^2\nu_{\rm ee}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M7.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M7.png"/〉〈/alternatives〉〈/inline-formula〉 with the heating rate 〈inline-formula〉〈tex-math id="M8"〉\begin{document}$R_{\rm heat}=\dfrac{1}{2}m_{\rm e}v_{\rm os} ^2\nu_{\rm ei}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M8.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M8.png"/〉〈/alternatives〉〈/inline-formula〉. When the condition 〈inline-formula〉〈tex-math id="M9"〉\begin{document}$R_{\rm heat}\sim R_{\rm eq}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M9.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M9.png"/〉〈/alternatives〉〈/inline-formula〉 is satisfied, the EDF deviates from Maxwell equilibrium distribution, and is well fitted to the super-Gaussian distribution 〈inline-formula〉〈tex-math id="M10"〉\begin{document}$f({{ v}})=C_m{\rm e}^{-(v/v_m)^m}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M10.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M10.png"/〉〈/alternatives〉〈/inline-formula〉 with the index 〈i〉m 〈/i〉(〈inline-formula〉〈tex-math id="M11"〉\begin{document}$2〈m〈5$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M11.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M11.png"/〉〈/alternatives〉〈/inline-formula〉). The number of energetic electrons of the super-Gaussian distribution is less than that of the Maxwell distribution, which plays an important role in electron heat flux, especially for electrons of 3.7〈inline-formula〉〈tex-math id="M12"〉\begin{document}$v_{\rm te}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M12.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M12.png"/〉〈/alternatives〉〈/inline-formula〉. So electron heat flux of the super-Gaussian distribution is smaller than that of the Maxwell distribution. In this paper, EDF and electron heat flux in laser-produced Au plasma are simulated by using 1D3V PIC code (Ascent). It is found that in the coronal region, the laser intensity is larger, and the electron temperature is lower than the high-density region. So 〈inline-formula〉〈tex-math id="M13"〉\begin{document}$\alpha=Z(v_{\rm os}/v_{\rm te})^2〉1$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M13.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M13.png"/〉〈/alternatives〉〈/inline-formula〉, 〈inline-formula〉〈tex-math id="M14"〉\begin{document}$R_{\rm heat}〉R_{\rm eq}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M14.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M14.png"/〉〈/alternatives〉〈/inline-formula〉, the EDF is well fitted to super-Gaussian distribution, where the index 〈i〉m〈/i〉 is evaluated to be 3.34. In this region, the large electron temperature gradient leads to a small temperature scale length (〈inline-formula〉〈tex-math id="M15"〉\begin{document}$L_{\rm e}=T_{\rm e}/(\partial T_{\rm e}/\partial x)$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M15.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M15.png"/〉〈/alternatives〉〈/inline-formula〉), but the low e-e and e-i collision frequencies lead to a large electron mean-free-path (〈inline-formula〉〈tex-math id="M16"〉\begin{document}$\lambda_{\rm e}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M16.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M16.png"/〉〈/alternatives〉〈/inline-formula〉). So the Knudsen number 〈inline-formula〉〈tex-math id="M17"〉\begin{document}$\lambda_{\rm e}/L_{\rm e}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M17.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M17.png"/〉〈/alternatives〉〈/inline-formula〉 is evaluated to be 0.011, which is much larger than the critical value 〈inline-formula〉〈tex-math id="M18"〉\begin{document}$2\times10^{-3}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M18.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M18.png"/〉〈/alternatives〉〈/inline-formula〉 of the S-H model, flux-limited local S-H electron heat flux is invalid. As a result, the limited-flux S-H predicts too large an electron heat flux, which results in much higher electron temperature of radiation hydrodynamics simulation than that of SG experiments. This heat flux inhibition phenomenon in coronal region cannot be explained by the flux-limited local S-H model, and non-local electron heat flux should be considered. In the high density region, the laser intensity is weaker, and the electron temperature is higher, so 〈inline-formula〉〈tex-math id="M19"〉\begin{document}$\alpha=Z(v_{\rm os}/v_{\rm te})^2〈1$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M19.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M19.png"/〉〈/alternatives〉〈/inline-formula〉, 〈inline-formula〉〈tex-math id="M20"〉\begin{document}$R_{\rm heat}〈R_{\rm eq},$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M20.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M20.png"/〉〈/alternatives〉〈/inline-formula〉 but EDF is still well fitted to super-Gaussian distribution, where the index m is evaluated to be 2.93. In this region, 〈inline-formula〉〈tex-math id="M21"〉\begin{document}$L_{\rm e}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M21.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M21.png"/〉〈/alternatives〉〈/inline-formula〉 is larger, 〈inline-formula〉〈tex-math id="M22"〉\begin{document}$\lambda_{\rm e}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M22.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M22.png"/〉〈/alternatives〉〈/inline-formula〉 is smaller, so the Knudsen number is smaller, which is evaluated to be 〈inline-formula〉〈tex-math id="M23"〉\begin{document}$7.58\times10^{-4}〈2\times10^{-3}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M23.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M23.png"/〉〈/alternatives〉〈/inline-formula〉. As a result, The flux-limited local S-H electron heat flux is valid. However, the electron heat flux depends on the flux limiting factor (〈inline-formula〉〈tex-math id="M24"〉\begin{document}$f_{\rm e}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M24.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191423_M24.png"/〉〈/alternatives〉〈/inline-formula〉) that varies with laser intensity and electron temperature.

Abstract: Optical elements such as stretcher, compressor and thick lenses will lead to spatially-dependent temporal properties of a large aperture laser pulse, which is called spatiotemporal coupling (STC). Beyond pure temporal characterization measurement, a measure of spatiotemporal coupling distortion based on spatial-spectral interference is proposed in this study. Full one-dimensional spatiotemporal coupling characteristics can be obtained in a single-shot measurement, and the complete spatiotemporal coupling characteristics in the near field can be obtained by scanning along another spatial dimension. The spatiotemporal coupling characteristics introduced by the wedge glasses are measured, and the experimental results accord well with the theoretical results.