ICF-PR-Net: a deep phase retrieval neural network for X-ray phase contrast imaging of inertial confinement fusion capsules.

X-ray phase contrast imaging (XPCI) has demonstrated capability to characterize inertial confinement fusion (ICF) capsules, and phase retrieval can reconstruct phase information from intensity images. This study introduces ICF-PR-Net, a novel deep learning-based phase retrieval method for ICF-XPCI. We numerically constructed datasets based on ICF capsule shape features, and proposed an object-image loss function to add image formation physics to network training. ICF-PR-Net outperformed traditional methods as it exhibited satisfactory robustness against strong noise and nonuniform background and was well-suited for ICF-XPCI's constrained experimental conditions and single exposure limit. Numerical and experimental results showed that ICF-PR-Net accurately retrieved the phase and absorption while maintaining retrieval quality in different situations. Overall, the ICF-PR-Net enables the diagnosis of the inner interface and electron density of capsules to address ignition-preventing problems, such as hydrodynamic instability growth.

Compact multichannel spectrometer employed for soft x-ray spectrum diagnostics at the Shenguang-III Laser Facility.

The quantitative measurement of plasma soft x-ray spectra is an important diagnostic problem in indirect-drive laser inertial confinement fusion (ICF). We designed, built, and tested a compact multichannel soft x-ray spectrometer with both spatial and temporal resolution capabilities for the detection of the spatiotemporal distribution of soft x-ray spectra. The spectrometer occupies a small solid angle, and the close measurement angle used for each channel enables the measurement of the angular distribution of emitting soft x-rays in ICF experiments. The spectrometer comprises pinhole, filter, and multilayer flat mirror arrays, and an x-ray streak camera. Its energy range is 0.1 - 3 keV. The dispersive elements of the spectrometer were calibrated at the Beijing Synchrotron Radiation Facility. The accuracy of the calibration was ≤ 5%, and the combined energy resolution (E/ΔE) of the calibrated dispersive elements of each channel was higher than 10. Finally, the instrument was tested at the Shenguang-III Laser Facility. The measurement results of x-ray radiation flux are agreed well with the experimental results of the M-band flat-response x-ray diode, demonstrating the feasibility of the proposed spectrometer configuration.

First Indirect Drive Experiment Using a Six-Cylinder-Port Hohlraum.

The new hohlraum experimental platform and the quasi-3D simulation model are developed to enable the study of the indirect drive experiment using the six-cylinder-port hohlraum for the first time. It is also the first implosion experiment for the six laser-entrance-hole hohlraum to effectively use all the laser beams of the laser facility that is primarily designed for the cylindrical hohlraum. The experiments performed at the 100 kJ Laser Facility produce a peak hohlraum radiation temperature of ∼222 eV for ∼80 kJ and 2 ns square laser pulse. The inferred x-ray conversion efficiency η∼87% is similar to the cylindrical hohlraum and higher than the octahedral spherical hohlraum at the same laser facility, while the low laser backscatter is similar to the outer cone of the cylindrical hohlraum. The hohlraum radiation temperature and M-band (>1.6 keV) flux can be well reproduced by the quasi-3D simulation. The variations of the yield-over-clean and the hot spot shape can also be semiquantitatively explained by the calculated major radiation asymmetry of the quasi-3D simulation. Our work demonstrates the capability for the study of the indirect drive with the six-cylinder-port hohlraum at the cylindrically configured laser facility, which is essential for numerically assessing the laser energy required by the ignition-scale six-cylinder-port hohlraum.

Measurement of Time-Dependent Drive Flux on the Capsule for Indirectly Driven Inertial Confinement Fusion Experiments.

A new method for measuring the time-dependent drive flux at the hohlraum center is proposed as a better alternative to conventional wall-based techniques. The drive flux here is obtained by simultaneous measurement of the reemitted flux and shock velocity from a three-layered "cakelike" sample. With these two independent observables, the influence induced by the uncertainty of the material parameters of the sample can be effectively decreased. The influence from the closure of the laser entrance hole, which was the main challenge in conventional wall-based techniques, was avoided through localized reemitted flux measurement, facilitating drive flux measurement throughout the entire time history. These studies pave a new way for probing the time-dependent drive flux, for both cylindrical hohlraums and novel hohlraums with six laser entrance holes.

Four-channel toroidal crystal x-ray imager for laser-produced plasmas.

The motion law of complex fluids under extreme conditions is an important aspect of high energy density physics research. It has been demonstrated that using multi-channel curved crystals and a framing camera to observe the laser-produced target pellets doped with tracer elements is an appropriate method for investigating this law. This paper presents a feasible design scheme for a multi-channel toroidal imager, with the ray trace model used to verify the rationality of the evaluation method and the aberration of single toroidal crystal imaging. We demonstrate that the field of view (FOV) consistency of the four-channel Ge(400) toroidal crystal imager is less than 50 µm, while the best spatial resolution is ∼4 µm and the FOV of each channel is >2.2 mm.

Development of a polar-view Kirkpatrick-Baez X-ray microscope for implosion asymmetry studies.

The development of a polar-view Kirkpatrick-Baez microscope, fielded in the upper polar zone of the Shenguang-III laser fusion facility, is presented. With this microscope, the resolving power of polar-direction X-ray imaging diagnostics is improved, to the 3 ~5 µm scale. The microscope is designed for implosion asymmetry studies, with response energy points at 1.2 keV, 3.5 keV, and 8 keV. A biperiodic multilayer scheme is adopted to accommodate multiple implosion stages. We present the overall optical system design, target aiming scheme, characteristic composite imaging diagnostic experiments and initial results. The inertial-driven quasi-one-dimensional spherical implosions were observed from orthogonal directions with a convergence ratio of ~14.4. Fine features of the stagnating hot spot core are also resolved.

First Octahedral Spherical Hohlraum Energetics Experiment at the SGIII Laser Facility.

The first octahedral spherical hohlraum energetics experiment is accomplished at the SGIII laser facility. For the first time, the 32 laser beams are injected into the octahedral spherical hohlraum through six laser entrance holes. Two techniques are used to diagnose the radiation field of the octahedral spherical hohlraum in order to obtain comprehensive experimental data. The radiation flux streaming out of laser entrance holes is measured by six flat-response x-ray detectors (FXRDs) and four M-band x-ray detectors, which are placed at different locations of the SGIII target chamber. The radiation temperature is derived from the measured flux of FXRD by using the blackbody assumption. The peak radiation temperature inside hohlraum is determined by the shock wave technique. The experimental results show that the octahedral spherical hohlraum radiation temperature is in the range of 170-182 eV with drive laser energies of 71 kJ to 84 kJ. The radiation temperature inside the hohlraum determined by the shock wave technique is about 175 eV at 71 kJ. For the flat-top laser pulse of 3 ns, the conversion efficiency of gas-filled octahedral spherical hohlraum from laser into soft x rays is about 80% according to the two-dimensional numerical simulation.

Development of high resolution dual-energy KBA microscope with large field of view for RT-instability diagnostics at SG-III facility.

High resolution X-ray diagnosis is a significant method for obtaining ablation-front and trajectory measurements targeting Rayleigh-Taylor (RT)-instability growth in initial confinement fusion (ICF) experiments. In this paper, a novel Kirkpatrick-Baez-type structure, as a kind of essential X-ray micro-imaging apparatus, has been developed that realizes a large field of view (FOV) and images with high resolution and energy response. Zoned multilayer coating technology is applied to the Kirkpatrick-Baez mirrors to transmit two specific quasi-monochromatic light through the same mirror and enables a compact dual-channel structure. This microscope has been assembled in the laboratory and later implemented at the Chinese SG-III laser facility. The characterization results show that this imaging system can achieve a good spatial resolution of 5 µm in a large FOV of 500 µm, while maintaining a strong monochromatic performance with bandwidth of 0.5 keV at 2.5 keV and 4.3 keV respectively.

Development of an x-ray eight-image Kirkpatrick-Baez diagnostic system for China's laser fusion facility.

This article presents the development of an x-ray eight-image Kirkpatrick-Baez diagnostic system to be used at China's Shenguang-III (SG-III) laser facility in aspects of the optical design, multilayers, and online/offline tests. Six pieces of concave spherical substrates are used for constituting a special optical structure. Dual-periodic tungsten/carbon (W/C) multilayers are used for high reflectivity and large angular bandwidth of ∼0.1°. The global spatial resolution is ∼5 µm in the ±100 µm range. The schemes of system installation, transport, collimation, and image acquisition at China's SG-III facility are also discussed.

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.

Realization of a flat-response photocathode for x-ray streak cameras.

We present a novel photocathode which can make x-ray streak cameras to be of a flat spectral response in the x-ray energy range of 0.1-5 keV. The photocathode consists of two layers of gold foils with optimized thickness ratio and structures. The photocathode was calibrated, and it is shown that a flat spectral response has been achieved.

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.

Mode-coupled perturbation growth on the interfaces of cylindrical implosion: A comparison between theory and experiment.

We present a mode-coupled weakly nonlinear model for the evolution of perturbations on cylindrical multilayered shells in a decelerating implosion. We show that nonlinear mode-mode interactions among large wave-number fundamental modes are able to induce the growth of small wave number harmonic modes, i.e., forming inverse cascade channels in the wave-number space. When uniform compression and interfacial coupling are taken into consideration, the amplitude of some perturbation modes exhibits an oscillatory growth pattern, which is beyond the intuition that perturbation amplitudes usually have a fast growth tendency in an implosion dominated by the Bell-Plesset effect. Our model accounts well for the previous experiments of Hsing et al. [Hsing et al., Phys. Rev. Lett. 78, 3876 (1997)0031-900710.1103/PhysRevLett.78.3876 and Phys. Plasmas 4, 1832 (1997)1070-664X10.1063/1.872326], which is among the few experiments of multimode multiinterface perturbation development in a cylindrical implosion. In particular, we find that the inverse cascade of modes is the origin of the excitation and growth of the wave number k=2 harmonic mode on the inner interface. The observed decrease of the fundamental modes on the inner interface is mainly attributed to the decreasing period of the oscillatory growth process. These results may afford further insight into the distortion of hot spots in inertial confined fusion implosion near the final stage, and also help to design multimode perturbation experiments in converging geometry in the coming future.

Soft x-ray low-pass filter with a square-pore microchannel plate.

A type of low-pass filter devices for soft x rays is investigated by using a microchannel plate (MCP) of small channels with square cross section. The measured transmission spectra on the Beijing Synchrotron Radiation Facility showed that the MCP has excellent bandpass effects below 1.5 keV by grazing incidence and internal multireflections. Combined with filters, the MCP energy bandwidth can be narrowed to 100 eV. In contrast to bandpass made of planar mirrors, the MCP has a much smaller size and better bandpass effects, and can be easily extended to high energy ranges. For low-resolution spectrometer applications of soft x rays, this method allows the monochromator to be replaced by a simple MCP filter and therefore significantly reduces alignment complexity in experiments.

K-shell photoabsorption edge of strongly coupled matter driven by laser-converted radiation.

The first observation of the K-shell photoabsorption edge of strongly coupled matter with an ion-ion coupling parameter of about 65 generated by intense x-ray radiation-driven shocks is reported. The soft x-ray radiation generated by laser interaction with a "dog bone" high-Z hohlraum is used to ablate two thick CH layers, which cover a KCl sample, to create symmetrical inward shocks. While the two shocks impact at the central KCl sample, a highly compressed KCl is obtained with a density of 3-5 times solid density and a temperature of about 2-4 eV. The photoabsorption spectra of chlorine near the K-shell edge are measured with a crystal spectrometer using a short x-ray backlighter. The redshift of the K edge up to 11.7 eV and broadening of 15.2 eV are obtained for the maximum compression. A comparison of the measured redshifts and broadenings with dense plasma calculations are made, and it indicates potential improvements in the theoretical description.

Determination of the hohlraum M-band fraction by a shock-wave technique on the SGIII-prototype laser facility.

The proposal of simultaneously determining the hohlraum peak radiation temperature T(R) and M-band fraction f(M) by shock velocity measurement technique [Y. S. Li et al. Phys. Plasmas 18, 022701 (2011)] is demonstrated for the first time in recent experiments conducted on SGIII-prototype laser facility. In the experiments, T(R) and f(M) are determined by using the observed shock velocities in Al and Ti. For the Au hohlraum used in the experiments, T(R) is about 160 eV and f(M) is around 4.3% under a 1 ns laser pulse of 2 kJ. The results from this method are complementary to those from the broadband x-ray spectrometer, and the technique can be further used to determine T(R) and f(M) inside an ignition hohlraum.

Plasmoid ejection and secondary current sheet generation from magnetic reconnection in laser-plasma interaction.

Reconnection of the self-generated magnetic fields in laser-plasma interaction was first investigated experimentally by Nilson et al. [Phys. Rev. Lett. 97, 255001 (2006)] by shining two laser pulses a distance apart on a solid target layer. An elongated current sheet (CS) was observed in the plasma between the two laser spots. In order to more closely model magnetotail reconnection, here two side-by-side thin target layers, instead of a single one, are used. It is found that at one end of the elongated CS a fanlike electron outflow region including three well-collimated electron jets appears. The (>1 MeV) tail of the jet energy distribution exhibits a power-law scaling. The enhanced electron acceleration is attributed to the intense inductive electric field in the narrow electron dominated reconnection region, as well as additional acceleration as they are trapped inside the rapidly moving plasmoid formed in and ejected from the CS. The ejection also induces a secondary CS.

Quasi-sinusoidal single-order diffraction transmission grating used in x-ray spectroscopy.

A novel design of quasi-sinusoidal single-order diffraction transmission grating (QSTG) is proposed, which can achieve a line density up to thousands line/millimeter as that of traditional transmission gratings with the current level of nanofabrication technique. We fabricate a 1000 line/mm QSTG using the new design approach, and display the calibration results of such QSTG on the soft x-ray beam of synchrotron radiation.

Quantitative observation of monochromatic X-rays emitted from implosion hotspot in high spatial resolution in inertial confinement fusion.

In inertial confinement fusion, quantitative and high-spatial resolution ([Formula: see text]m) measurements of the X-rays self-emitted by the hotspot are critical for studying the physical processes of the implosion stagnation stage. Herein, the 8 ± 0.39-keV monochromatic X-ray distribution from the entire hotspot is quantitatively observed in 5-[Formula: see text]m spatial resolution using a Kirkpatrick-Baez microscope, with impacts from the responses of the diagnosis system removed, for the first time, in implosion experiments at the 100 kJ laser facility in China. Two-dimensional calculations along with 2.5% P2 drive asymmetry and 0.3 ablator self-emission are congruent with the experimental results, especially for the photon number distribution, hotspot profile, and neutron yield. Theoretical calculations enabled a better understanding of the experimental results. Furthermore, the origins of the 17.81% contour profile of the deuterium-deuterium hotspot and the accurate Gaussian source approximation of the core emission area in the implosion capsule are clarified in detail. This work is significant for quantitatively exploring the physical conditions of the hotspot and updating the theoretical model of capsule implosion.

Spin depolarization induced by self-generated magnetic fields during cylindrical implosions.

Spin-polarized fuels are promising for inertial confinement fusion due to the enhanced fusion cross section. One significant concern of spin-polarized inertial confinement fusion is whether the nuclei polarization could survive in the implosions and contribute to ignitions. Here we present numerical simulation methods and results of spin dynamics of polarized deuterium-tritium fuels in strong self-generated magnetic fields during the implosions of dense cylindrical shells. The magnetic field generation and evolution is modeled with generalized Ohm's laws combined with hydrodynamic equations. The spin dynamics is investigated with a particle-tracking method, by solving the spin precession equations of tracked particles. Rayleigh-Taylor instabilities and Richtmyer-Meshkov instabilities are found to be the main cause of depolarization. Hydrodynamic instabilities lead to depolarization of nuclei near the hot-spot shell interface, and an asymmetric shock front leads to depolarization of nuclei inside a hot spot. Deuterium polarization is more stable than tritium polarization due to its smaller gyromagnetic ratio. Low-mode perturbations can lead to higher depolarization inside a hot spot than high-mode perturbations. In the multimode simulations, the modes around 16-32 are significant for hot-spot depolarization.