Shenguang-II Facility Research Articles

Detection and analysis of the signal-to-noise ratio in the injection laser system of the Shenguang-II facility.

The temporal contrast levels of laser pulses in the injection laser system of the Shenguang-II facility are accurately measured and analyzed in detail. These signal-to-noise ratio levels are determined by the extinction ratios of an acousto-optic modulator, an electro-optic intensity modulator, and a combination of a Pockels cell and thin film polarizers. In addition, a drifting direct current bias voltage of an electro-optic intensity modulator and phase modulation are also demonstrated to affect the signal-to-noise ratio of the injection laser system. With a precise control of these factors, the injection laser system can output a nanosecond laser pulse with a signal-to-noise ratio of 47.37dB. Moreover, a nanosecond laser pulse signal-to-noise ratio measuring device with a dynamic range and accuracy of 50 and 1dB, respectively, is developed to detect the signal-to-noise ratio of a nanosecond laser pulse, which is 47.06dB within an accuracy range of 1dB. The signal-to-noise ratio measuring device can detect the high contrast laser pulse for the seed source unit in the high power laser facility accurately and conveniently.

Multi-keV X-ray area source intensity at SGII laser facility

Experiments for investigating the feasibility of multi-keV backlighters for several different metallic foil targets were performed at the Shenguang II (SGII) laser facility in China. Emission spectra in the energy range of 1.65–7.0 keV were measured with an elliptically bent crystal spectrometer, and the X-ray source size was measured with a pinhole camera. The X-ray intensity near 4.75 keV and the X-ray source size for titanium targets at different laser intensity irradiances were studied. By adjusting the total laser energy at a fixed focal spot size, laser intensity in the range of 1.5–5.0 × 1015 W/cm2, was achieved. The results show that the line emission intensity near 4.75 keV and the X-ray source size are dependent on the laser intensity and increase as the laser intensity increases. However, an observed “peak” in the X-ray intensity near 4.75 keV occurs at an irradiance of 4.0 × 1015 W/cm2. For the employed experimental conditions, it was confirmed that the laser intensity could play a significant role in the development of an efficient multi-keV X-ray source. The experimental results for titanium indicate that the production of a large (∼350 μm in diameter) intense backlighter source of multi-keV X-rays is feasible at the SGII facility.

Experimental investigation on dynamic lattice response by in-situ Xray diffraction method

Structure evolution under dynamic compression condition (high temperature, high pressure and high strain rate) is one of the most important problems in engineering and applied physics, which is vital for understanding the kinetic mechanism of shock-induced phase transition. In this work, an in-situ dynamic X-ray diffraction (DXRD) diagnostic method is established to probe the lattice response driven by shock waves. The geometry is suitable for the study of laser-shocked crystals. In order to eliminate the measurement error arising from the difference in experimental setup, the static and dynamic lattice diffraction signals are measured simultaneously in one shot by using a nanosecond burst of X-ray emitted from a laser-produced plasma. Experimental details in our investigation are as follows. 1) The laser driven shock wave transit time △ tShock and the shock pressure in sample are accurately determined from the shock-wave profile measurement by dual laser heterodyne velocimetry. 2) A laser pump-and-probe technique for adjusting the time-delay of DXRD diagnosis during △ tShock, with a series of repeated shock loadings is then employed to generate and measure the dynamic structure evolution. Using this method, the dynamic lattice response of[111] single-crystal iron is studied on Shenguang-Ⅱ facility. Single-shot diffraction patterns from both unshocked and shocked crystal are successfully obtained. An elastic-plastic transition process –elastic wave followed by a plastic wave– is observed in shocked[111] single-crystal iron on a lattice scale. The lattice compressibility values of the elastic wave and plastic wave are in agreement with those derived from the wave profiles. It is found that the Hugoniot elastic limit is measured to be about 6 GPa under nanosecond-pulsed laser shock compression. Such a high yield strength is consistent with recent laser ramp compression experimental results in polycrystalline Fe[Smith et al. 2011 J. Appl. Phys. 110 123515], suggesting that the peak pressure of elastic wave is dependent on the loading rate and the thickness of sample. Based on the analysis of diffraction patterns, the BCC phase is determined to be stable till 23.9 GPa, the highest pressure explored in this work, which might indicate that the phase transition strongly couples with the crystal orientation and loading rate. Some possible physical mechanisms remain to be further studied:whether the transition time hysteresis occurs or the metastable FCC phase exists in shocked[111] single crystal Fe, or the phase transition onset pressure increases under high strain-rate compression. Our DXRD results provide a primary experimental reference for the follow-up study on the phase kinetics.

Freestanding titanium metallic aerogel

Nanoporous titanium was prepared by an in-situ template confined synthetic strategy involving the magnesiothermic reduction on TiO2 aerogel template at relative low temperature and hydrochloric acid etching process. The resulting self-supporting monolith with α titanium as the major phase retained the similar morphology of initial template and exhibited a high specific surface area about 99.1m2/g. In addition, as the ideal laser-irradiated target material for production of multi-keV X-rays, the titanium aerogel has successfully realized the voluminal heating when irradiated by the high-energy laser pulse on the Shenguang-II facility, and the Ti K-shell emitted X-ray spectrum ranging from 3 to 9keV was detected using X-ray elliptical crystal spectrometer. More importantly, the result could provide significant reference for further research about the application of metal aerogel target in the field of the multi-keV X-ray sources, which was essential for radiography of inertial-confinement-fusion (ICF) experiments.

Formation of super-Alfvénic electron jets during laser-driven magnetic reconnection at the Shenguang-II facility: particle-in-cell simulations

Magnetic reconnection experiments in high-energy-density (HED) laser-produced plasmas have recently been conducted at the Shenguang-II (SG-II) facility. Two plasma bubbles and a ‘frozen-in’ magnetic field are generated by irradiating an Al foil using two laser beams. As the two bubbles with opposing magnetic fields expand and squeeze each other, magnetic reconnection occurs. In the experiments, three well-collimated high-speed electron jets are observed in the fanlike outflow region of the laser-driven magnetic reconnection. Based on two-dimensional (2D) particle-in-cell (PIC) simulations, we demonstrate that the three electron jets in the outflow region of laser-driven magnetic reconnection are super-Alfvénic, and their formation mechanism is also revealed in this paper. The two super-Alfvénic jets at the edge are formed by the outflow electrons, which move along magnetic field lines after they are accelerated in the vicinity of the X-line by the reconnection electric field. The super-Alfvénic jet at the center is formed by the electrons that come from the outside of the plasma bubbles. These electrons are reflected by the magnetic field in the pileup region and are meanwhile accelerated by the resulting electric field.

Particle-in-cell simulations of magnetic reconnection in laser-plasma experiments on Shenguang-II facility

Recently, magnetic reconnection has been realized in high-energy-density laser-produced plasmas. Plasma bubbles with self-generated magnetic fields are created by focusing laser beams to small-scale spots on a foil. The bubbles expand into each other, which may then drive magnetic reconnection. The reconnection experiment in laser-produced plasmas has also been conducted at Shenguang-II (SG-II) laser facility, and the existence of a plasmoid was identified in the experiment [Dong et al., Phys. Rev. Lett. 108, 215001 (2012)]. In this paper, by performing two-dimensional (2-D) particle-in-cell simulations, we investigate such a process of magnetic reconnection based on the experiment on SG-II facility, and a possible explanation for the formation of the plasmoid is proposed. The results show that before magnetic reconnection occurs, the bubbles squeeze strongly each other and a very thin current sheet is formed. The current sheet is unstable to the tearing mode instability, and we can then observe the formation of plasmoid(s) in such a multiple X-lines reconnection.

The multi-point X-ray source and phase contrast imaging used on implosion experiment

Traditional implosion backlight imaging experiment has disadvantages of nonuniform X-ray source, low contrast ablator interface and high requirement for diagnostic device tuning precision. A novel design of multi-point X-ray source combined with phase contrast imaging developed and optimized based on experimental research performed on Shenguang-II facility is presented. The novel design can obtain high-quality experimental result with uniform X-ray source, clear interface between ablator and inner DD gas and large image view. At the same time, the new design using diagnostic silt instead of diagnostic hole improves tuning precision. The experimental result proposes that novel design of laser driven plasma point x-ray source combined with phase contrast imaging has advantage of area X-ray source combined with absorb imaging and can be widely used in inertial confinement fusion and high energy density physics.

神光Ⅱ激光装置集中控制系统设计

In order to improve the level of the Shenguang-Ⅱs computer control, a centralized control system is designed and developed, by using the software development tools for control system of EPICS as the software framework. The overall structure and hardware of the control system are described in detail, and the development and application of the EPICS software in Linux are explored with the database and driver for instance. The experimental results show that the control system based on EPICS can meet the functional requirement of Shenguang-Ⅱ facility, which provides a new thought of using the EPICS in control system of high power laser facility.

2D hydrodynamic simulation of a line-focused plasma in Ni-like Ag x-ray laser research

In most collisional schemes of x-ray laser (XRL) experiments, a bow-like intensity distribution of XRL is often observed, and it is generally ascribed to the two-dimensional hydrodynamic behaviour of expanding plasma. In order to better understand its essence in physics, a newly developed two-dimensional non-equilibrium radiation hydrodynamic code XRL2D is used to simulate a quasi-steady state Ni-like Ag XRL experiment on ShenGuang-II facility. The simulation results show that the bow-like distribution of Ni-like ions caused by over-ionization in the central area of plasma is responsible for the bow-like shape of the XRL intensity distribution observed.