Target Density Effects on Charge Transfer of Laser-Accelerated Carbon Ions in Dense Plasma.

We report on charge state measurements of laser-accelerated carbon ions in the energy range of several MeV penetrating a dense partially ionized plasma. The plasma was generated by irradiation of a foam target with laser-induced hohlraum radiation in the soft x-ray regime. We use the tricellulose acetate (C_{9}H_{16}O_{8}) foam of 2 mg/cm^{3} density and 1 mm interaction length as target material. This kind of plasma is advantageous for high-precision measurements, due to good uniformity and long lifetime compared to the ion pulse length and the interaction duration. We diagnose the plasma parameters to be T_{e}=17 eV and n_{e}=4×10^{20} cm^{-3}. We observe the average charge states passing through the plasma to be higher than those predicted by the commonly used semiempirical formula. Through solving the rate equations, we attribute the enhancement to the target density effects, which will increase the ionization rates on one hand and reduce the electron capture rates on the other hand. The underlying physics is actually the balancing of the lifetime of excited states versus the collisional frequency. In previous measurement with partially ionized plasma from gas discharge and z pinch to laser direct irradiation, no target density effects were ever demonstrated. For the first time, we are able to experimentally prove that target density effects start to play a significant role in plasma near the critical density of Nd-glass laser radiation. The finding is important for heavy ion beam driven high-energy-density physics and fast ignitions. The method provides a new approach to precisely address the beam-plasma interaction issues with high-intensity short-pulse lasers in dense plasma regimes.

Two-dimensional time- and space-resolved diagnostic method for an integrated implosion process.

To precisely measure and evaluate X-ray generation and evolution in a hohlraum during an implosion process, we present a two-dimensional (2D) time- and space-resolved diagnostic method by combining a compressed ultrafast photography (CUP) system and a simplified version of space-resolving flux detector (SSRFD). Numerical experiment results showed that the reconstruction quality of the conventional CUP significantly improved owing to the addition of the external SSRFD, especially when a coded mask with a large pixel size was used in the CUP. Further, the performance of the CUP cooperation with the SSRFD was better than that of adding an external charge-coupled device or streak camera. Compared with existing ultrafast imaging techniques in laser fusion, the proposed method has a prominent advantage of measuring the 2D evolution of implosion by combining high temporal resolution of streak camera and high spatial resolution of SSRFD; moreover, it can provide guidance for designing diagnostic experiments in laser fusion research.

First Inertial Confinement Fusion Implosion Experiment in Octahedral Spherical Hohlraum.

In inertial confinement approaches to fusion, the asymmetry of target implosion is a major obstacle to achieving high gain in the laboratory. A recently proposed octahedral spherical hohlraum makes it possible to naturally create spherical target irradiation without supplementary symmetry control. Before any decision is made to pursue an ignition-scale laser system based on the octahedral hohlraum, one needs to test the concept with the existing facilities. Here, we report a proof-of-concept experiment for the novel octahedral hohlraum geometry on the cylindrically configured SGIII laser facility without a symmetry control. All polar and equatorial self-emission images of the compressed target show a near round shape of convergence ratio 15 under both square and shaped laser pulses. The observed implosion performances agree well with the ideal spherical implosion simulation. It also shows limitations with using the existing facilities and adds further weight to the need to move to a spherical port geometry for future ignition laser facilities.

Three-dimensional reconstruction of integrated implosion targets from simulated small-angle pinhole images.

To indirectly evaluate the asymmetry of the radiation drive under limited measurement conditions in inertial confinement fusion research, we have proposed an integral method to approximate the three-dimensional self-radiation distribution of the compressed plasma core using only four pinhole images from a single laser entrance hole at a maximum projection angle of 10°. The simultaneous algebraic reconstruction technique (SART) that uses spatial constraints provided by the prior structural information and the central pinhole image is utilized in the simulation. The simulation results showed that the normalized mean square deviation between the original distribution and reconstruction results of the central radiation area of the simulated cavity was 0.4401, and the structural similarity of the cavity radiation distribution was 0.5566. Meanwhile, using more diagnostic holes could achieve better structural similarity and lower reconstruction error. In addition, the results indicated that our new proposed method could reconstruct the distribution of a compressed plasma core in a vacuum hohlraum with high accuracy.

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.

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.

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.

Numerical study and improvement of the dynamic performance of dilation x-ray imager.

We present in this Note a numerical study on the dynamic performance of a Dilation X-ray Imager (DIXI). The DIXI including a photoelectron tube (PT) and a magnetic solenoid is modeled in 3D space. The initial parameters of the photoelectrons are sampled with a Monte Carlo code. The trajectories of the photoelectrons are calculated by using the particle-in-cell method, and the transit time spread (TTS) and temporal magnification are analyzed in detail. We have designed a PT with a double-microstrip structure and compared the performance of the double-microstrip PT with the traditional single-microstrip PT. The results show that the sensitivity of the TTS and the temporal magnification to the emission time of the photoelectrons can be significantly reduced by using the double-microstrip PT, resulting in an improvement of the time window. Therefore, the dynamic performance of the DIXI is improved.

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.

Theoretical study on temporal and spatial performance of magnetic solenoid used in dilation x-ray imager.

In the Dilation X-ray Imager (DIXI), which is characterized by an ultra-short gating time, a magnetic solenoid is used to keep the photoelectrons from defocusing during the drift process. The performance of the magnetic solenoid has an important influence on the performance of the DIXI. We present in this paper the efforts on studying the spatial and temporal performance of the magnetic solenoid used in the DIXI by tracking the photoelectrons with the particle-in-cell method. The initial parameters of the photoelectrons of the Au cathode were sampled with a Monte Carlo code. A novel magnetic solenoid with a shielding shell made of soft iron was proposed. We compared the performance of this solenoid with a normal solenoid. The simulation results of magnetic field distribution, spatial resolution, transit time spread, and temporal distortion were presented in detail to demonstrate that the performance of the DIXI can be significantly improved by using the magnetic solenoid with the proposed iron shell.

Observation of a high degree of stopping for laser-accelerated intense proton beams in dense ionized matter.

Intense particle beams generated from the interaction of ultrahigh intensity lasers with sample foils provide options in radiography, high-yield neutron sources, high-energy-density-matter generation, and ion fast ignition. An accurate understanding of beam transportation behavior in dense matter is crucial for all these applications. Here we report the experimental evidence on one order of magnitude enhancement of intense laser-accelerated proton beam stopping in dense ionized matter, in comparison with the current-widely used models describing individual ion stopping in matter. Supported by particle-in-cell (PIC) simulations, we attribute the enhancement to the strong decelerating electric field approaching 1 GV/m that can be created by the beam-driven return current. This collective effect plays the dominant role in the stopping of laser-accelerated intense proton beams in dense ionized matter. This finding is essential for the optimum design of ion driven fast ignition and inertial confinement fusion.

Development of a gated X-ray imager with multiple views and spectral selectivity for observing plasma evolution in hohlraums.

In the study of indirect drive Inertial Confinement Fusion (ICF), X-ray images through the laser entrance hole of the hohlraums can provide key information. In this paper, we present the advances on the development of a gated X-ray imager with multiple views and spectral selectivity. A diagnostic snout which contains four X-ray planar mirrors has been designed and coupled to a gated X-ray detector which includes an air box and has a spatial resolution better than 35 µm and a temporal resolution better than 70 ps. The two inner mirrors in the diagnostic snout operate at M band (1.6-3.5 keV), while the two outer mirrors operate at N band (0.5-1 keV). The four mirrors define four observation angles. Each mirror can reflect 4 X-ray images to the four microstrips of the gated X-ray detector with almost the same observation angle. The imager enables us to observe the spatial and temporal evolution of plasma in the hohlraums from four different observation angles based on both N band and M band images. The imager has been used in the ICF experiments conducted at Shenguang laser facility in China, and the obtained X-ray images of Au cylindrical hohlraum are presented and discussed in this paper.

First exploration of radiation temperatures of the laser spot, re-emitting wall and entire hohlraum drive source.

This study explores the radiation field temperatures introduced by the laser spot, the re-emitting wall in a hohlraum and the entire hohlraum drive source. This investigation, which is the first of its kind, is based on the radiation fluxes from the laser spot and the re-emitting wall, which have been accurately measured using time- and space-resolving flux detectors in a recent work, and additional flux data. The temperature difference between the laser spot and the entire hohlraum drive source was 6.08-35.35% of the temperature of the latter throughout the entire laser pulse, whilst that for the re-emitting wall was 3.90-12.81%. The radiation temperature of the cooler re-emitting wall had more influence on the temperature increase of the entire hohlraum drive source than the hot laser-spot temperature, which has been quantitatively discussed. Experimentally, we established the average distributions of the temperature fields of all the emitting sources, namely laser spot and re-emitting wall, of the irradiating fluxes on the capsule region in the hohlraum radiation field. This important progress in the exploration of radiation temperature distributions within a hohlraum will provide a foundation for determination of the irradiating radiation on the capsule and evaluation of capsule symmetry.

Note: New method for high-space-resolving hotspot electron temperature measurements on Shenguang-III prototype.

High-space-resolving information of hotspot electron temperature is a foundation for further research on physical processes of implosion in inertial confinement fusion. This work proposed a novel high-space-resolving electron temperature detector, which is based on the bremsstrahlung radiation mechanism of the implosion hotspot and uses two-channel Kirkpatrick-Baez microscopes. In this novel detector, an optical quasi-coaxis method was used to eliminate the strong impact of the view field difference on the high space resolution and correctness of the electron temperature diagnosis, and a compound KB microscope method was proposed to reduce the number of spherical reflectors and save space.

Calibration of the linear response range of x-ray imaging plates and their reader based on image grayscale values.

X-ray imaging plates are one of the most important X-ray imaging detectors and are widely used in inertial-confinement fusion experiments. However, their linear response range, which is the foundation of their quantitative data analysis, has not been sufficiently deeply investigated. In this work, we develop an X-ray fluorescer calibration system and carefully explore the linear response range of X-ray imaging plates. For the first time, nearly the entire grayscale range of the X-ray imaging plate linear response-7819-64 879 in the range of 0-65 535-has been observed. Further, we discuss the uncertainties involved in the calibration process. This work demonstrates the excellent linear response qualities of X-ray imaging plates and provides a significant foundation for expanding their quantitative applied range.

Three-dimensional particle-in-cell simulation on gain saturation effect of microchannel plate.

We present here the results of the simulation work, using the three-dimensional particle-in-cell method, on the performance of the lead glass microchannel plate under saturated state. We calculated the electron cascade process with different DC bias voltages under both self-consistent condition and non-self-consistent condition. The comparative results have demonstrated that the strong self-consistent field can suppress the cascade process and make the microchannel plate saturated. The simulation results were also compared to the experimental data and good agreement was obtained. The simulation results also show that the electron multiplication process in the channel is accompanied by the buildup process of positive charges in the channel wall. Though the interactions among the secondary electron cloud in the channel, the positive charges in the channel wall, and the external acceleration field can make the electron-surface collision more frequent, the collision energy will be inevitably reduced, thus the electron gain will also be reduced.