Shenguang-III Prototype Laser Facility Research Articles

High-resolution, highly monochromatic, time-framed, x-ray-backlit-imaging diagnostics with dual-channel toroidal crystals

Here, we developed a high-resolution, dual-channel toroidal-crystal x-ray imager for time-framed x-ray backlit imaging diagnostics using the 4.727 keV helium-like Ti line. We also presented a method for adjusting the dual-channel imager through the self-imaging of a two-dimensional periodic object. Offline x-ray experiments achieved a spatial resolution of ∼5.0 μm in the center and better than 8.0 μm within a field of view (FOV) of ∼2 mm. At the ShenGuang-III prototype laser facility, we obtained imaging results with a spatial resolution of better than 5 μm within an FOV of ±40 µm. This imager thus provides a way of observing with high spatial, temporal, and spectral resolutions to diagnose the behavior of laser-produced plasma.

Study of the spatial growth of stimulated Brillouin scattering in a gas-filled hohlraum via detecting the driven ion acoustic wave

In an experiment performed on the Shenguang-III prototype laser facility, collective Thomson scattering (TS) is used to study the spatial growth of stimulated Brillouin scattering (SBS) in a gas-filled hohlraum by detecting the SBS-driven ion acoustic wave. High-quality time-resolved SBS and TS spectra are obtained simultaneously in the experiment, and these are analyzed by a steady-state code based on the ray-tracing model. The analysis indicates that ion–ion collisions may play an important role in suppressing SBS growth in the Au plasma; as a result, the SBS excited in the filled gas region is dominant. In the early phase of the laser pulse, SBS originates primarily from the high-density plasma at the edges of the interaction beam channel, which is piled up by the heating of the interaction beam. Throughout the duration of the laser pulse, the presence of the TS probe beam might mitigate SBS by perturbing the density distribution around the region overlapping with the interaction beam.

The effect of the shroud window membrane on the x-ray drive characterization of cryogenic hohlraums

Both direct and indirect drive concepts of inertial confinement fusion rely on targets with cryogenic thermonuclear fuel shells for ignition. Experiments on the Shenguang-III prototype laser facility using laser-driven gas-filled hohlraums show distinct differences between cryogenic (20 K) and warm hohlraums. Although the measured x-ray flux in the photon range from 1.6 to 4.4 keV (Au M-band) is identical between cryogenic and warm hohlraums, the cryogenic hohlraum has a much slower rate of rise and is 20% lower in peak intensity of x-ray flux in the photon range from 0.1 to 4 keV. The reasons for this drive deficit between cryogenic and warm hohlraums are investigated using a similar series of hohlraum experiments. The experiments employ three types of hohlraums to distinguish the effect of a shroud window membrane and condensates. Warm hohlraums with a shroud window membrane replicate the slower rate of rise of radiation flux of cryogenic targets. When the shroud window is present, the measured x-ray flux in the hohlraum shows a drive deficit that decreases with time. However, the measured deficit increases as the viewing angle increases. All of these results indicate that the portion of the shroud not illuminated by the lasers absorbs the outgoing x-ray flux from the hohlraum.

Characterization of radiation drive by measuring the localized re-emitted flux from the capsule in inertial confinement fusion experiments

We present for the first time characterization of the time-dependent radiation drive on the capsule by measuring the localized re-emitted flux at Shenguang-III prototype laser facility. The drive flux was obtained with measured re-emitted flux from the capsule and radiation fluxes from the hohlraum wall, in combination with radiation hydrodynamic simulations. It revealed that the temporal behavior of the drive flux was quite distinguished from the radiation flux from the hohlraum wall, and the drive flux was approximately 6 eV (12 eV) lower than the measured flux at up 55° (up 30°). This technique presents a novel way for the assessment of the drive flux, both in cylindrical hohlraums and novel hohlraums with more than two laser entrance holes. Pre-processed radiation hydrodynamic simulations indicate that this technique can also be applied in integrated implosion experiments utilizing standard fusion capsule with carbon-hydrogen ablators.

Implementation of a large-aperture Thomson scattering system for diagnosing driven ion acoustic waves on Shenguang-III prototype laser facility

A large-aperture (f/3) Thomson scattering diagnostic system, for measuring the ion acoustic waves driven by stimulated Brillouin scattering, has been recently implemented at the bottom of the target chamber on Shenguang-III prototype laser facility. With a 4ω (263.3 nm) probe beam entering the target chamber in the horizontal plane, this system is capable of detecting the Thomson scattered light off ion acoustic waves driven by a 3ω (351 nm) interaction beam. The Thomson scattered light off a driven ion acoustic wave emits at a specific direction, for a given electron density. The f/3 aperture of the diagnostic system allows measurement of driven ion acoustic waves in plasmas with electron densities up to 4×1021 cm-3, which covers the typical density range of interest in experiments on Shenguang-III prototype. Since the driven Thomson scattered spectrum is usually orders of magnitude stronger than the accompanying thermal Thomson scattered spectrum, to record both signals, a half area filter is used in the system, which suppress the stronger signal to a comparable level with the weaker one. By reducing the system aperture, Thomson scattered light off driven ion acoustic waves can be blocked. Therefore, this system can be also used to detect the Thomson scattered light off thermal ion acoustic waves for measurement of plasma parameters. In recent experiments with gas-filled hohlraums, Thomson scattered spectra off both driven and thermal ion acoustic waves have been diagnosed.

Theoretical investigations on x-ray transport in radiation transport experiments on the Shenguang-III prototype laser facility

Experiments exploring the propagation of heat waves within cylindrical CH foams were performed on the Shenguang-III prototype laser facility in 2012. In this paper, the radiation fluxes out of CH foam cylinders at different angles are analyzed theoretically using the two-dimensional radiation hydrodynamics code LARED-R. Owing to the difficulty in validating opacity and equation of state (EOS) data for high-Z plasmas, and to uncertainties in the measured radiation temperature Tr and the original foam density ρ0, multipliers are introduced to adjust the Au material parameters, Tr, and ρ0 in our simulations to better explain the measurements. The dependences of the peak radiation flux Fmax and the breakout time of the heat wave thalf (defined as the time corresponding to the radiation flux at half-maximum) on the radiation source, opacity, EOS, and ρ0 scaling factors (ηsrc, ηop, ηeos, and ηρ) are investigated via numerical simulations combined with fitting. Then, with the uncertainties in the measured Tr and ρ0 fixed at 3.6% and 3.1%, respectively, experimental data are exploited as fiducial values to determine the ranges of ηop and ηeos. It is found that the ranges of ηop and ηeos fixed by this experiment overlap partially with those found in our previous work [Meng et al., Phys. Plasmas 20, 092704 (2013)]. Based on the scaled opacity and EOS parameters, the values of Fmax and thalf obtained via simulations are in good agreement with the measurements, with maximum errors ∼9.5% and within 100 ps, respectively.

A simple analytical model of laser direct-drive thin shell target implosion

A high-neutron yield platform imploded by a thin shell target is generally built to probe nuclear science problems, and it has the advantages of high neutron yield, ultrashort fusion time, micro fusion zone, isotropic and monoenergetic neutron. Some analytical models have been proposed to interpret exploding-pusher target implosion driven by a long wavelength laser, whereas they are imperfect for a 0.35 μm laser implosion experiment. When using the 0.35 μm laser, the shell is ablated and accelerated to high implosion velocity governed by Newton’s law, ablation acceleration and quasi-adiabatic compression models are suitable to explain the implosion of a laser direct-drive thin shell target. The new analytical model scales bang time, ion temperature and neutron yield for large variations in laser power, target radius, shell thickness, and fuel pressure. The predicted results of the analytical model are in agreement with experimental data on the Shenguang-III prototype laser facility, 100 kJ laser facility, Omega, and NIF, it demonstrates that the analytical model benefits the understanding of experiment performance and optimizing the target design of high neutron yield implosion.

Multi-keV x-ray radiator from titanium cylindrical cavity at the Shenguang-III prototype laser facility

Great efforts have been made to create a bright K-shell source using the thin metal cylindrical cavities in the past few decades. Several metal materials such as titanium (Ti), iron, and so on have been tested for high x-ray conversion efficiency mainly at the OMEGA and NIF laser facilities. Recently, x-ray sources in Ti K-shell transition energy range were investigated at the Shenguang-III prototype laser facility with ∼5 kJ laser energy. The experiments were aimed to reproduce the previous ones at OMEGA, but with an extraordinarily small volume of cylindrical cavity and detailed characterizations of the x-ray source. The cavities were 800 μm inner diameter, 800 μm length, and 30 μm thick plastic tubes supporting 1 μm thick Ti. Seven laser beams were focused to 200 μm diameter. The combination of the small cavity volume and the focused laser spots is intended to improve the electron temperature with limited laser energy since the electron temperature is a key issue for high x-ray conversion efficiency. Thomson scattering was adopted to experimentally probe the electron temperatures at special time and space zones as well as the average temperature obtained from the Ti K-shell spectrum. The evolutions of the electron temperature and density are predicted by the radiation hydrodynamic simulation. A top view and two photon energy bands of x-ray source images provide a way to directly observe the plasma movement toward the cavity axis and distinguish the different emission mechanisms between the Ti K-shell and lower energy x rays. Six Higher-energy x-ray detectors located at different angles were used to record the Ti K-shell x-ray emission and demonstrate its isotropic feature. The characteristics of the x-ray radiate intensity including the time evolution, the angular distribution, and the total yields for both the photon energy regions above and below 4 keV are compared between the cavity and planar targets. Obviously, different behaviors were found between the two photon energy regions and the two types of targets. The x-ray conversion efficiency of the Ti cylinder was determined to be ∼4% and ∼21% in 4π sr in the Ti K-shell (4–7 keV) and <4 keV range, respectively. The Ti K-shell conversion efficiency obtained in the present experiments is between the ones driven by 13.5 and 4.5 kJ laser energy at OMEGA.

Demonstration of bright x-ray sources from solid and foam TiO2 targets at the Shenguang-III prototype laser facility

Enhancement of x-ray sources from laser-produced plasmas is significant in wide-ranging applications. Solid and foam TiO2 targets were both used to generate bright x-ray sources at the Shenguang-III prototype laser facility, with a total laser power of 3.2 TW. The new foam targets were with an ultra-low initial density of 8.1 mg/cm3 and a high Ti fraction of 33 sat. %. By absolute measurements, the multi-keV x rays of the 4 keV–7 keV band and 1.6 keV–4.4 keV band and the total x rays above 0.1 keV of the foam target have simultaneously shown conversion enhancements of 1.4, 3.1, and 2.3 times, respectively, compared with the solid target. A much larger emission volume and an average electron temperature of 3.2 keV by moving the foci of laser beams inner were obtained for the foam target, providing a good condition for multi-keV x-ray production.

X-ray fluorescence imaging of laser-driven hydrodynamic instability systems with a Ross Pair Imager

An experiment has been conducted at the Shenguang-Ⅲ prototype laser facility, demonstrating the application of x-ray fluorescence imaging (XRFI) in diagnosing laser-driven hydrodynamic instability systems in planar geometry. An aluminum (Al)-foam target was used, including a modulated Al disk followed by a low density poly-4-methyl-1-pentene foam cylinder which was doped with titanium (Ti) dioxide nanoparticles to serve as the tracer layer. A blast wave was driven through the rippled interface, triggering the development of hydrodynamic instabilities and the growth of the perturbation. Vanadium plasma radiation dominated by He-α x-rays, was generated with a delay in time, inducing the emission of Ti K-shell fluorescent photons from the tracer layer. A Ross Pair Imager with matched scandium and Ti filters was employed to provide two-dimensional quasi-monochromatic fluorescent images, and the temporal evolution of the perturbation structures in the tracer layer was successfully observed. Furthermore, one-dimensional fluorescent intensity profiles across the perturbation structures were extracted and compared with the simulation results via a combination of the MULTI code, a buoyancy-drag model, a self-similar analysis and an imaging code. The boundaries of the measured profiles were shown to be connected together due to the small initial length of the tracer layer and the large boundary width caused by the curved interfaces and blurring effects, which made the absolute positions of the perturbation structures inaccessible. This research suggests that XRFI works well in qualitatively diagnosing the instability evolution and the experimental design has to be modified to further improve its performance in achieving quantitative information.

Study of the kinetic effects in indirect-drive inertial confinement fusion hohlraums

Kinetic physics has the potential to impact the performance of indirect-drive inertial confinement fusion (ICF) experiments. In near vacuum hohlraums (or vacuum hohlraums), the high-Z plasma expands from the hohlraum wall and collides with the blow-off from the capsule (or the low-density fill-gas). Such collision produces conditions in which kinetic effects may dominate since the ion-ion mean free paths are larger than the size of the interaction region. In the present work, we present the experimental evidence supported by simulations of kinetic effects launched in the interpenetration layer between the laser-driven hohlraum plasma bubbles and the corona plasma of the compressed pellet on the Shenguang-III prototype laser facility. Our study showed kinetic shocks arisen in the hohlraum wall/ablator (or the low-density fill-gas) interpenetration region, which result in efficient acceleration of high energy ions. When these high energy ions were deposited inside the capsule, it resulted in significant low-mode asymmetry of the implosion capsule since there are no high energy ions from the laser entrance holes. These studies provide novel insight into the interactions and dynamics of a vacuum hohlraum and near vacuum hohlraum.

Measurement of P2 M-band flux asymmetry in indirect-drive hohlraum on Shenguang-III prototype laser facility.

M-band flux asymmetry (MFA) may be another critical factor which can lead to low-mode distortion of implosion. We describe an experimental technique employing the x-ray fluorescence signature of material to investigate the MFA onto the capsule. Si was chosen to be the tracer layer since the inducing threshold for the K-shell fluorescence is much higher than soft x-ray but a little lower than M-band x-ray. The fluorescence images were recorded from a surrogate Si-coated ball, and thus, the P2 MFA was deduced from the fluorescence images.

Experimental Evidence of Kinetic Effects in Indirect-Drive Inertial Confinement Fusion Hohlraums.

We present the first experimental evidence supported by simulations of kinetic effects launched in the interpenetration layer between the laser-driven hohlraum plasma bubbles and the corona plasma of the compressed pellet at the Shenguang-III prototype laser facility. Solid plastic capsules were coated with carbon-deuterium layers; as the implosion neutron yield is quenched, DD fusion yield from the corona plasma provides a direct measure of the kinetic effects inside the hohlraum. An anomalous large energy spread of the DD neutron signal (∼282 keV) and anomalous scaling of the neutron yield with the thickness of the carbon-deuterium layers cannot be explained by the hydrodynamic mechanisms. Instead, these results can be attributed to kinetic shocks that arise in the hohlraum-wall-ablator interpenetration region, which result in efficient acceleration of the deuterons (∼28.8 J, 0.45% of the total input laser energy). These studies provide novel insight into the interactions and dynamics of a vacuum hohlraum and near-vacuum hohlraum.

Experimental and simulation studies on radiative properties of uranium planar target coated with an ultrathin aluminum layer

An ultrathin aluminum (Al) layer is proposed to be coated on the surface of the uranium (U) planar target for preventing the oxidization of U and optimizing the hard x-ray spectrum. Comparative experiments using several planar targets with different materials are implemented on the Shenguang-III prototype laser facility to study their radiative properties. Under the laser intensity of W cm−2, we observe that the U target coated with 30 nm thick Al produces almost the same peak intensity of the total x-ray flux and decreases the hard x-ray fraction by 38.5% compared to the U target coated with 100 nm thick gold. Simulations using XRL2D (2D radiation hydrodynamic code) interpret the measured results and indicate that the radiative property of Al-coated U approaches that of pure U, which theoretically has a high laser to x-ray conversion efficiency and a quasi-Planckian spectrum. Based on our analysis, Al-coated U is a promising material of the ignition hohlraum and should have many applications in high energy density physics.

First measurement of plasma stagnation radiation in a hohlraum in the Shenguang-III prototype

The measurement of plasma stagnation radiation in a hohlraum is of key importance when exploring the influence of plasma stagnation on hohlraum drive symmetry in indirectly driven inertial confinement fusion experiments. In two typical hohlraum experiments conducted on the Shenguang-III prototype laser facility, the x-ray flux and radiation temperature from a plasma stagnation area 0.2 mm in diameter inside the hohlraum were measured for the first time through the hohlraum laser entrance hole (LEH) using a time and space-resolving flux detector (SRFD). The different flux intensities and radiation temperatures of the plasma stagnation area in the LEH measured using the SRFD were compared with the results detected for the entire LEH using flat-response x-ray detectors. The stagnation area radiation detected using the SRFD was found to be transient and 8.57% lower than that of the entire LEH, which is obviously higher than the tolerable drive asymmetry of an implosion. This important improvement in x-ray radiation measurement of plasma stagnation will further the quantitative understanding of the drive symmetry inside the hohlraum and conduct a strict adjustment for that.

Ramp-Wave Compression Experiment with Direct Laser Illumination on ShenGuang-III Prototype Laser Facility**Supported by the National Natural Science Foundation of China under Grant No 11305160.

Ramp-wave compression experiment to balance the high compression pressure generation in aluminum and x-ray blanking effect in transparent window is demonstrated with an imaging velocity interferometer system for any reflector (VISAR) on ShenGuang-III prototype laser facility. The highest pressure is about 500 GPa after using the multilayer target design Al/Au/Al/LiF and 1013 W/cm2 laser pulse illuminated on the planar Al target, which generates the spatial uniformity to over 500 μm on the ablation layer. A 2-μm-thick Au layer is used to prevent the x-ray from preheating the planar ablation Al layer and window material LiF. The imaging VISAR system can be used to record the abrupt loss of the probe beam ( nm) caused by absorption and reflection of 20-μm, 30-μm and 40-μm-thick Al, i.e., the blanking effect. Although there are slight shocks in the target, the peak pressure 500 GPa, which is the highest data up to now, is obtained with ramp-wave compression.

Experimental investigation of tin fragments mixing with gas subjected to laser driven shock

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.

基于神光Ⅲ原型的双轴速度干涉仪

基于神光Ⅲ原型的双轴速度干涉仪

Radiation flux study of spherical hohlraums at the SGIII prototype facility

An octahedral spherical hohlraum is a promising candidate in target design for inertial confinement fusion study, because of its potential superiority in uniform radiation and efficient coupling [Lan et al., Phys. Plasmas 21, 010704 (2014)]. Before the experimental investigation for octahedral spherical hohlraum, an energetics experiment is accomplished on the Shenguang-III prototype laser facility by using spherical hohlraums with two cylindrical laser entrance holes. Time evolution of the radiation temperature is obtained with flat response X-ray diode detectors at four different viewing angles with demonstrated repeatability of the measurements. The experimental observations are successfully explained by using a phenomenological model which considers not only the radiation flux contributed from the laser ablated and radiation ablated plasma from hohlraum wall, but also that contributed from the filling plasma inside the hohlraum. This method proves to be a simple but effective way to interpret the time-dependent behaviour of the radiation temperatures at different viewing angles.

Experimental study on the incident-angle-dependent laser coupling features of polystyrene targets

Laser-produced plasmas have attracted great interest due to their potential utility in wide-ranging applications, especially in the field of inertial confinement fusion (ICF). For direct-driven ICF, laser coupling with polystyrene targets is a crucial and fundamental problem. In addition, oblique incidence is also a common phenomenon for laser facilities with multiple beams. It is necessary to evaluate the effects of oblique incidence on the laser coupling features relevant to the direct-driven ICF. Experiments using an intense nanosecond flat-top laser at around W cm–2 to irradiate polystyrene planar targets from three different incidence angles have been performed on the Shenguang-III prototype laser facility. The time-integrated absolute values of the full aperture backscatter (FABS), near backscatter scattering (NBS), and the x-ray conversion efficiency (CE) have been measured quantitatively. According to the experimental results, with the increase of the incidence angle, the percentage of the stimulated Brillouin backscatter and the overall x-ray CE decreased while the stimulated Raman backscatter fraction rose. Theoretical analyses based on hydrodynamic simulations and linear theory were qualitatively consistent with the experimental results. In addition, the specularly reflected light was also observed at 30° laser oblique incidence.