SG-III Prototype Laser Facility Research Articles

Extended x-ray absorption fine structure measurement of ramp compressed Ti using laser-irradiated metallic foil as x-ray source on SGIII prototype laser facility

Laser-irradiated metallic foils were considered as x-ray sources for extended x-ray absorption fine structure (EXAFS) measurements and confirmed by experiments on the SGIII prototype facility. The Au foils were irradiated by laser beams with a total energy of 2.77 kJ and full width at half maximum (FWHM) of 1 ns to create an x-ray source. The x-ray emission was spectrally smooth in the energy range of Ti EXAFS, the FWHM of Au foil x-ray radiation pulse in the energy range of 0.1–4000 eV was 0.99 ns, and the FWHM of x-ray pulse in the energy range of 5000–6000 eV was deduced to be 0.55 ns according to simulation results. A shaped laser pulse was designed to achieve the Ti sample’s laser-direct-driven ramp compression process. By creating a quasi-stable state lasting longer than 1 ns as the probing window during the compression process, the demand for temporal resolution was reduced. EXAFS spectra of compressed Ti in α and ω-phase were obtained and compared, and structural phase transition was verified by EXAFS pattern changes. The velocity of the back interface of the Ti sample was measured by the velocity interferometer system for any reflector, and the maximum of the deduced pressure in the middle of the Ti sample was 8.2 GPa, which is consistent with the α-ω phase transition.

High-resolution elliptical Kirkpatrick-Baez microscope for implosion higher-mode instability diagnosis.

High-resolution X-ray imaging diagnosis is a critical method for measuring Rayleigh-Taylor instability growth and hot spot interface morphology in inertial confinement fusion experiments. In this study, we develop a quasi-monochromatic elliptical Kirkpatrick-Baez microscope based on aberration theory, breaking the aberration limit of conventional Kirkpatrick-Baez microscopes. The microscope was characterized in the laboratory for spatial resolution performance and modulation transfer function before being implemented in cavity experiments at the SG-III prototype laser facility. The results demonstrate that the edge-based method achieves a spatial resolution of <2 µm in the central field of view and modulation of 800 lp/mm spatial frequency of >20%.

Simultaneous high-resolution x-ray backlighting and self-emission imaging for laser-produced plasma diagnostics using a two-energy multilayer Kirkpatrick–Baez microscope

A simultaneous high-resolution x-ray backlighting and self-emission imaging method for laser-produced plasma diagnostics is developed in which two Kirkpatrick–Baez imaging channels for high-energy and low-energy diagnostics are constructed using a combination of multilayer mirrors in near-coaxial form. By using a streak or framing camera placed on the image plane, both backlit and self-emission images of a laser-produced plasma with high spatial and temporal resolution can be obtained simultaneously in a single shot. This paper describes the details of the method with regard to its optical and multilayer design, assembly, and alignment method. In addition, x-ray imaging results with a spatial resolution better than 5 µm in the laboratory and experimental results with imploding capsules in the SG-III prototype laser facility are presented.

X-ray source improvements for EXAFS measurement on SGIII prototype facility

A proper x-ray source is crucial to extended x-ray absorption fine structure (EXAFS) measurement as parameter diagnosis for compressed materials on large laser facilities. In this article, modification of the CH capsule as an x-ray source has been done through simulations and experiments for the SGIII prototype laser facility. On the one hand, simulation results show that the radiation intensity of the CH capsule increases largely while wall thickness decreases. On the other hand, chlorine atoms whose atomic number is higher than that of carbon were brought into the capsule by doping on the inner side wall to enhance x-ray radiation intensity. CH capsules were designed, and experiments were carried out on the SGIII prototype laser facility. The x-ray source spectrum, size, radiation intensity, radiation pulse, and Ti K-edge EXAFS spectrum under ambient conditions were compared and discussed. Experimental results show that, with the doping of chlorine atoms and decreasing the wall thickness from 9 µm to 6 µm, the x-ray radiation intensity increases by about 63.5 times, energy efficiency is improved a lot, and other properties are also improved. The chlorine atom doped CH capsule with 6 µm wall thickness is proved to be the best x-ray source in the experiments in this article. Its size is 57 ± 0.5 µm, pulse width is 240 ps, and radiation intensity is about 9.1 times the undoped one with the same wall thickness. The methods can be extended to other laser facilities for the EXAFS measurement of other materials.

X-ray fluorescence imaging of jet flow in laser driven high-energy-density experiments

The technique of x-ray fluorescence imaging (XRFI) has been applied to the high-energy-density (HED) jet experiments on the SG-III prototype laser facility. The jet was produced by the interaction of the laser driven shock with the perturbed Ti disk. The fluorescence emission of the Ti ejecta was pumped by the V He-α emission and recorded using a Ross Pair Imager with an optimized configuration. We achieved a spatial resolution of 35 μm and a temporal resolution of 1 ns. We also developed a method to infer the density distribution of the jet from the fluorescence images and discussed the conditions under which this method was accurate. These experiments provided insight into the characteristics of a HED jet and implied that XRFI is a promising way for studying HED hydrodynamics.

Radiation flux study of hohlraum used to create uniform and strongly coupled warm dense matter

A hohlraum used to create uniform and strongly coupled warm dense matter and its radiation flux study on the SG-III prototype laser facility are described. Time evolution of the radiation temperature from laser entrance hole (LEH) and the incident radiation temperature at the target surface are obtained using flat response X-ray diode detectors. Then, a calculation of the radiation flux evolution for this hohlraum is carried out using a view-factor method, and the calculated results agree well with the experimental data within the error bar. Using the incident radiation at the target surface as a source, the validity to create uniform and strongly coupled warm dense matter by this hohlraum is verified.

Study of M-band X-ray preheating effect on shock propagation via streaked optical pyrometer system at SG-III prototype lasers

The M-band X-ray (1.6–4.4 keV) preheating and shock temperature of aluminum (Al) foil coated on a thick polystyrene (CH) layer is experimentally measured using a streaked optical pyrometer system (SOP) in the SG-III prototype laser facility for the first time. Multi-group hydrodynamic simulation captures the main characteristics of rear surface emission. Compared with the unpreheated target with bromine (Br) doped, M-band X-rays have a large impact on shock propagation inside the undoped target. Preheating changes target the initial state and generate a broader shock front, resulting in drastic variation in rear surface emission features. By comparing sensitivity analysis in the simulation and experimental uncertainty range, it is found that the SOP measurement could achieve valuable information on the shock transition time and unique emission profile, offering reliable boundary and benchmarks for simulation and analysis. Proper evaluation on the radiation source, including both the flux and the spectrum, and material opacities is essential for data interpretation. This study could promote our understanding and predictive capability required for inertial confinement fusion studies.

Laser-driven shock compression of gold foam in the terapascal pressure range

Shock compression experiments are carried out on gold foam with an initial density of 3.2 g/cm3 through indirectly laser-driven shock waves at the SG-III prototype laser facility. The impedance-matching technique is applied to determine the equation-of-state (EOS) data of the shocked gold foam. A passive shock breakout diagnostic system is employed to obtain the shock velocities in both the standard material and gold foam. The gold foams are compressed to a maximum density of 20 g/cm3 under a shock pressure of about 2 TPa. The effects of the unsteadiness of shock waves on the EOS measurement are quantitatively analyzed and corrected. The correction of unsteady waves, as well as the good planarity of the shock waves and the low preheating of the gold foam, contributes high-confidence EOS data for the gold foam. The corrected experimental data are compared with the Hugoniot states from the SESAME library. The comparison suggests that the database is suitable for describing the states of gold foam with an initial density of 3.2 g/cm3 under a pressure of about 2 TPa.

Measuring impact of M-band and soft X-rays on radiation-driven ablation performance

M-band and soft X-rays play different roles in radiation-driven ablation. In the SGIII-prototype laser facility, experiments were performed to deduce the absorbed M-band and soft X-rays at different angles. Both the absorbed energy and the absorption rate of the M-band and soft X-rays were studied. The results showed that the M-band (1.6–4.4 keV) and soft (0.1–1.6 keV) X-rays performed differently in radiation-driven ablation. The difference was determined by the radiation source, the state of the ablator, and the field of view. A time-varying radiation source, the opacity change of the ablator, and fields of view at different angles were studied by using the one-dimensional multi-group radiation hydrodynamic code Multi-1D and the three-dimensional view-factor code IRAD3D. Radiation-driven ablation is the interaction between X-rays and the ablator. These studies are useful for understanding the ablation process and provide guidance for increasing the ablation rate and reducing the preheat effect.

Comparison of the laser spot movement inside cylindrical and spherical hohlraums

Compared with cylindrical hohlraums, the octahedral spherical hohlraums have natural superiority in maintaining high radiation symmetry during the whole capsule implosion process in indirect drive inertial confinement fusion. However, the narrow space between laser beams and the hohlraum wall may disturb laser propagation inside the spherical hohlraum. In this work, the laser propagation inside the spherical hohlraum and cylindrical hohlraum is investigated experimentally by measuring laser spot movement at the SGIII-prototype laser facility. The experimental results show that the laser propagations inside the spherical hohlraum and the cylindrical hohlraum are totally different from each other due to different hohlraum structures. For the spherical hohlraum, although the laser energy is mainly deposited in the initial position of the laser spot during the whole laser pulse, some laser energies are absorbed by the ablated plasmas from the hohlraum wall. Because the laser beam is refracted by the thin plasmas near the laser entrance hole (LEH) region, the laser spot in the spherical hohlraum moves toward the opposite LEH. In contrast, the laser spot in the cylindrical hohlraum moves toward the LEH along the laser path due to the plasma expansion. When the laser is to be turned off, the accumulated plasmas near the LEH region in the cylindrical hohlraum absorb a majority of laser energy and hinder the laser arriving at the appointed position on the hohlraum wall.

Experimental progress of laser-driven flyers at the SG-III prototype laser facility

Laser-driven flyers have unique advantages of high flyer velocity, low cost, simple facility compared with the flyers driven by other conventional dynamic high-pressure loading techniques. With the fast development of laser technique, launching hypervelocity flyers with high-intensity laser pulse has become more and more prevalent. In this paper, we introduce the recent experiments of laser-driven flyers at the SG-III prototype laser facility. Three ways of launching hypervelocity flyers are developed and introduced, respectively. In the first way, multilayered aluminum flyers are gradually accelerated to a terminal velocity of 8 km/s, which is measured by optical velocimetry, without melting and vaporization. The pressure distribution within the flyer shows that the temporally ramped pulse ablation generates a compression wave, and the flyer is accelerated by this wave and its reverberation within the flyer. In the second way, a strong laser ablates the low-density reservoir foil and generates strong shock in the foil. The shock wave is strong enough, and when the shock breaks out from the free surface, the foil will unload as plasma towards the flyer with a density profile. The plasma decelerates upon colliding the flyer, and the single-layered flyer is gradually accelerated by the momentum transition. In our experiments, single-layered aluminum foil and single-layered tantalum foil are accelerated to 11.5 km/s and 6.5 km/s, respectively. According to the pressure distribution within the flyer, the flyer is also accelerated by the compression wave produced by the plasma collision, which is similar to the case of direct ablation by temporally ramped pulse. However, the way of plasma collision could better reduce X-ray and electron preheat and obtain cleaner flyers. In the last way, the flyers are launched by direct strong short-laser ablation. The multi-layered aluminum foil is accelerated to a high average velocity of 21.3 km/s by using a 3-ns quadrate laser pulse at 351 nm after spatial homogenization. A line-velocity interferometer system for any reflect (VISAR) is employed to monitor the processes of flyer launch and flight in a vacuum gap and the shock velocity associated with phase change in fused silica target after flyer impact is inferred. The reflectivity variations of the VISAR fringe pattern and the shock velocity in the fused silica suggest that the flyer owns a density gradient characteristic. Furthermore, specifically designed multi-layered flyers (polyimide/copper) are accelerated by shock impedance and reverberation techniques to a super high averaged velocity of 55 km/s, which is much faster than recently reported results. Light-emission signals of shock breakout and flyer impact on flat or stepped windows are obtained, which indicates the good planarity and integrity for the flyer. Compared with single-layer flyers, multi-layered flyers have a good planarity, and a high energy conversion efficiency from laser to flyers. In this paper, we give a comprehensive analysis and comparison of the experimental designs, technique means and data results about laser-driven flyers. This would provide a reference for further experimental study of laser-driven flyers and also verify that the SG-III prototype laser facility is a very promising facility for studying the hypervelocity flyers launching field.

First Investigation on the Radiation Field of the Spherical Hohlraum.

The first spherical hohlraum energetics experiment is accomplished on the SGIII-prototype laser facility. In the experiment, the radiation temperature is measured by using an array of flat-response x-ray detectors (FXRDs) through a laser entrance hole at four different angles. The radiation temperature and M-band fraction inside the hohlraum are determined by the shock wave technique. The experimental observations indicate that the radiation temperatures measured by the FXRDs depend on the observation angles and are related to the view field. According to the experimental results, the conversion efficiency of the vacuum spherical hohlraum is in the range from 60% to 80%. Although this conversion efficiency is less than the conversion efficiency of the near vacuum hohlraum on the National Ignition Facility, it is consistent with that of the cylindrical hohlraums used on the NOVA and the SGIII-prototype at the same energy scale.

Demonstration of enhancement of x-ray flux with foam gold compared to solid gold

Experiments have been conducted to compare the re-emission from foam gold with a 0.3 g cc−1 density and solid gold in a SGIII prototype laser facility. Measurements of the re-emission x-ray flux demonstrate that emission is enhanced by the low density foam gold compared to the solid gold under the same conditions. The emission fraction increases with time and is concentrated on soft x-ray flux between 0.1–1 keV. The simulation results with Multi 1D agree with the experimental results. There are potential advantages to using foam walls for improving the emission and soft x-ray flux in hohlraums.

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.

First demonstration of improving laser propagation inside the spherical hohlraums by using the cylindrical laser entrance hole

The octahedral spherical hohlraums have natural superiority in maintaining high radiation symmetry during the entire capsule implosion process in indirect drive inertial confinement fusion. While, in contrast to the cylindrical hohlraums, the narrow space between the laser beams and the spherical hohlraum wall is usually commented. In this Letter, we address this crucial issue and report our experimental work conducted on the SGIII-prototype laser facility which unambiguously demonstrates that a simple design of cylindrical laser entrance hole (LEH) can dramatically improve the laser propagation inside the spherical hohlraums. In addition, the laser beam deflection in the hohlraum is observed for the first time in the experiments. Our 2-dimensional simulation results also verify qualitatively the advantages of the spherical hohlraums with cylindrical LEHs. Our results imply the prospect of adopting the cylindrical LEHs in future spherical ignition hohlraum design.

Diagnosing x-ray power and energy of tungsten wire array z-pinch with a flat spectral response x-ray diode.

Fast z-pinch is a very efficient way of converting electromagnetic energy to radiation. With an 8-10 MA current on primary test stand facility, about 1 MJ electromagnetic energy is delivered to vacuum chamber, which heats z-pinch plasma to radiate soft x-ray. To develop a pulsed high power x-ray source, we studied the applicability of diagnosing x-ray power from tungsten wire array z-pinch with a flat spectral response x-ray diode (FSR-XRD). The detector was originally developed to diagnose radiation of a hohlraum in SG-III prototype laser facility. It utilized a gold cathode XRD and a specially configured compound gold filter to yield a nearly flat spectral response in photon energy range of 0.1-4 keV. In practice, it was critical to avoid surface contamination of gold cathode. It is illustrated that an exposure of an XRD to multiple shots caused a significant change of response. Thus, in diagnosing x-ray power and energy, we used each XRD in only one shot after calibration. In a shot serial, output of FSR-XRD was compared with output of a nickel bolometer. In these shots, the outputs agreed with each other within their uncertainties which were about 12% for FSR-XRD and about 15% for bolometer. Moreover, the ratios between the FSR-XRD and the bolometer among different shots were explored. In 8 shots, the standard deviation of the ratio was 6%. It is comparable to XRD response change of 7%.

Uranium hohlraum with an ultrathin uranium–nitride coating layer for low hard x-ray emission and high radiation temperature

An ultrathin layer of uranium nitrides (UN) has been coated on the inner surface of depleted uranium hohlraum (DUH), which has been proven by our experiment to prevent the oxidization of uranium (U) effectively. Comparative experiments between the novel depleted uranium hohlraum and pure golden (Au) hohlraum are implemented on an SGIII-prototype laser facility. Under a laser intensity of 6 × 1014 W cm−2, we observe that the hard x-ray (hν keV) fraction of the uranium hohlraum decreases by 61% and the peak intensity of the total x-ray flux (0.1 keV∼5.0 keV) increases by 5%. Radiation hydrodynamic code LARED is used to interpret the above observations. Our result for the first time indicates the advantages of the UN-coated DUH in generating a uniform x-ray source with a quasi-Planckian spectrum, which should have important applications in high energy density physics.

Detective technology with temporal resolution for the spatial distribution of near backscatter energy

The laser inertial confinement fusion experiment can provide very important data for the research of laser-plasma interaction to detect the spatial and temporal property of the backscatter light. The backscatter diagnostic system we already have dont have temporal resolution for the spatial distribution of backscatter energy. Therefore, based on the near backscatter diagnostic system on SG-Ⅲ prototype laser facility, we proposed and designed a kind of recording method which could detect the temporal process for the spatial distribution of backscatter energy. We verified the feasibility of this method through static experiment, whose result demonstrated that it was possible to record the messages transmitted by one-dimensional to two-dimensional imaging fiber bundles and retrieve these messages to two-dimensional image by data processing.

Direct measurement of x-ray flux for a pre-specified highly-resolved region in hohlraum.

A space-resolving flux detector (SRFD) is developed to measure the X-ray flux emitted from a specified region in hohlraum with a high resolution up to 0.11mm for the first time. This novel detector has been used successfully to measure the distinct X-ray fluxes emitted from hot laser spot and cooler re-emitting region simultaneously, in the hohlraum experiments on SGIII prototype laser facility. According to our experiments, the ratio of laser spot flux to re-emitted flux shows a strong time-dependent behavior, and the area-weighted flux post-processed from the measured laser spot flux and re-emitting wall flux agrees with that measured from Laser Entrance Hole by using flat-response X-ray detector (F-XRD). The experimental observations is reestablished by our two-dimensional hydrodynamic simulations and is well understood with the power balance relationship.

Laser-driven flier impact experiments at the SG-III prototype laser facility

Laser-driven flier impact experiments have been designed and performed at the SG-III prototype laser facility. The continuum phase plate (CPP) technique is used for the 3 ns quadrate laser pulse to produce a relatively uniform irradiated spot of 2 mm. The peak laser intensity is 2.7×1013 W/cm2 and it accelerates the aluminum flier with a density gradient configuration to a high average speed of 21.3 km/s, as determined by the flight-of-time method with line VISAR. The flier decelerates on impact with a transparent silica window, providing a measure of the flatness of the flier after one hundred microns of flight. The subsequent shock wave acceleration, pursuing, and decay in the silica window are interpreted by hydrodynamic simulation. This method provides a promising method to create unique conditions for the study of a material’s properties.