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High-energy gamma-ray radiography has exceptional penetration ability and has become an indispensable nondestructive testing (NDT) tool in various fields. For high-energy photons, point projection radiography is almost the only feasible imaging method, and its spatial resolution is primarily constrained by the size of the gamma-ray source. In conventional industrial applications, gamma-ray sources are commonly based on electron beams driven by accelerators, utilizing the process of bremsstrahlung radiation. The size of the gamma-ray source is dependent on the dimensional characteristics of the electron beam. Extensive research has been conducted on various advanced accelerator technologies that have the potential to greatly improve spatial resolution in NDT. In our investigation of laser-driven gamma-ray sources, a spatial resolution of about 90 µm is achieved when the areal density of the penetrated object is 120 g/cm2. A virtual source approach is proposed to optimize the size of the gamma-ray source used for imaging, with the aim of maximizing spatial resolution. In this virtual source approach, the gamma ray can be considered as being emitted from a virtual source within the convertor, where the equivalent gamma-ray source size in imaging is much smaller than the actual emission area. On the basis of Monte Carlo simulations, we derive a set of evaluation formulas for virtual source scale and gamma-ray emission angle. Under optimal conditions, the virtual source size can be as small as 15 µm, which can significantly improve the spatial resolution of high-penetration imaging to less than 50 µm. [ABSTRACT FROM AUTHOR]

We propose a positron acceleration scheme in which a laser-driven positron beam is injected into a solid target hit by a laser beam and accelerated in the sheath field on the target back side. The positron beam injection and acceleration in the target have been investigated with numerical simulation. The feasibility of such an acceleration scheme was proved according to the simulation results, which show that a 10 MeV positron beam can be accelerated up to 30 MeV. The dependency of the positron beam properties on the positron injection location, injection time, and target thickness was studied. Related acceleration details were obtained and analyzed. The acceleration scheme provides a method in positron energy controlling and its related applications. [ABSTRACT FROM AUTHOR]

We present an application of short-pulse laser-generated hard x rays for the diagnosis of indirectly driven double shell targets. Cone-inserted double shell targets were imploded through an indirect drive approach on the upgraded SG-II laser facility. Then, based on the point-projection hard x-ray radiography technique, time-resolved radiography of the double shell targets, including that of their near-peak compression, were obtained. The backlighter source was created by the interactions of a high-intensity short pulsed laser with a metal microwire target. Images of the target near peak compression were obtained with an Au microwire. In addition, radiation hydrodynamic simulations were performed, and the target evolution obtained agrees well with the experimental results. Using the radiographic images, areal densities of the targets were evaluated. [ABSTRACT FROM AUTHOR]

The physics of shock-induced ejection is a crucial phenomenon in the field of shock compression science and technology. Limited by loading methods, the previous research primarily focused on the physics of ejecta induced by single shockwave, with few data available on multiple shockwave loading conditions. To solve this problem, we proposed a double shockwave production method based on the high-intensity laser facility, which allows the interval time between the shock waves to be adjusted in the nanosecond to microsecond timescale. Meanwhile, we applied loading techniques to study the ejection behavior of metal tin and integrated photonic doppler velocimetry and high-energy x-ray radiography technology to observe the ejection process. By comparing the experimental results for single and double shockwave, the multiple shock-induced ejection features have been clearly confirmed. Our experimental results provide valuable insight into the behavior of ejecta under multiple shockwave loading conditions, which is of great significance for deepening our understanding of the ejection mechanism. [ABSTRACT FROM AUTHOR]

X-ray source based on the Thomson scattering of laser interacting with energetic electron beams features high photon energy, small spot size, and good collimation. However, the photon number is insufficient for practical application because of the small cross section of the Thomson scattering. To solve this problem, here, we replace a traditional Gaussian laser pulse with a flying focus laser pulse to extend interaction length and restrain nonlinear effects. Simulation results show that the scattered photon number can be increased by about 25 and 2 times for high and low energy lasers, respectively. In particular, a 1010 photon number can be generated with a 10 J flying focus laser pulse, and the energy spread can also be greatly reduced for high energy laser, from a broad spectrum to a monoenergetic peak. Combining these two advantages, the peak spectrum brightness of x ray is 3 × 108 photons/keV at 240 keV, which is about three orders of magnitude more than the traditional case. [ABSTRACT FROM AUTHOR]

The origin of He + ions in picosecond intense laser–gas interaction was experimentally and numerically investigated. A nanosecond laser pulse was used to modulate the plasma condition, which helps to distinguish the origin of the helium ions with different charge states. By comparing the energy spectra of He 2 + and He + ions in the experiments with/without plasma shaping, we found neither ion–atom nor electron–ion recombination is responsible for the He + generation, which is supported by two-dimensional hydrodynamic simulations and theoretical calculations. [ABSTRACT FROM AUTHOR]

Fusion reactions in a plasma environment are fundamental issues with general interest in high energy density sciences. The reaction rate in an astro-system, which may differ from cold matter, is an important subject in the ambiguous problems of elemental abundance. In addition, the stopping of charged particle in plasma has a considerable impact on the design of nuclear fusion reactors as it is related to the α-particle heating process and ion-driven fast ignition, but still needs better understanding. In this research, an experiment on laser-driven D–D fusion reactions (D + D → 3He + n) has been carried out to investigate the effects of ion stopping power in plasma on fusion reactivities. The neutron yields, plasma density, and deuteron energy loss in the plasma have been measured simultaneously, and the plasma temperature has been analyzed from simulations. It is experimentally demonstrated that the fusion reaction yield is closely correlated with ion beam transportation in the plasma. As a cold target heated to plasma, the reaction probabilities from a deuteron beam and deuterated target interactions can be enhanced or suppressed, which is ascribed to the deuteron stopping power variation in the plasma. The results show the importance of considering the temperature adjusted ion stopping power to correctly model the fusion reaction yields. This work has an impact on understanding the fusion reactions in plasma environment, which is also likely to help achieve higher neutron yields. [ABSTRACT FROM AUTHOR]

The radiography of gamma-ray is one of the most important non-destructive testing in many fields. However, the spot size is always in millimeter scale for the generation of gamma-ray by conventional way. As the development of laser wakefield acceleration, the electron beam with small divergence and spot size can be generated easily in the experiment by tens of terawatt ultra-short laser pulse. Based on this electron beam, gamma-ray with micro spot size is generated and the properties are measured and tested in detail experimentally. The experiment demonstrates that the spot size of this gamma-ray is always smaller than 200 μm, no matter the conversion target thickness, and can be as small as about 40 μm when the conversion target thickness of 0.2 mm is used. The spatial resolution of this gamma-ray is much better than 2.5 LP/mm, the fitting temperature (which is relative to the average energy of gamma-ray) is between 5 MeV and 8 MeV, and the maximum yield per shot of the gamma-ray can be up to 9.1 × 109 photons (energy higher than 1 MeV). High-resolution radiography shows that the areal density of the gamma-ray radiography can be up to 51.3 g/cm2 (stainless steel thickness equivalent is about 6.5 cm). Such micro-spot gamma-ray can play an important role in the high-resolution radiography of high areal density objects. [ABSTRACT FROM AUTHOR]

Single shot gamma-induced positron annihilation spectroscopy (GiPS) based on a laser wakefield accelerator is proposed. In this spectroscopy, a large number of gamma rays generated by laser-wakefield accelerated electrons are injected into a sample in a very short time (<2 ps), and the positron lifetime is detected by measuring the time profile of the annihilation radiation (511 keV gamma ray) from the sample. The processes including gamma-ray production, shielding, annihilation generation, and annihilation detection were simulated by using the Monte Carlo method. The results show that up to 1010 gamma rays with a duration of 1.4 ps can be produced per shot with a 45 TW laser system. A Cherenkov radiator PbF2 was used to detect the annihilation radiation, and its optimal thickness of 15 mm was obtained. A whole process simulation with optimized parameters was conducted. A low statistical fluctuation lifetime spectrum can be produced, which confirms the feasibility of single shot GiPS with a typical 45 TW laser system. [ABSTRACT FROM AUTHOR]

The results of a commissioning experiment on the SILEX-Ⅱ laser facility (formerly known as CAEP-PW) are reported. SILEX-Ⅱ is a complete optical parametric chirped-pulse amplification laser facility. The peak power reached about 1 PW in a 30 fs pulse duration during the experiment. The laser contrast was better than 1010 at 20 ps ahead of the main pulse. In the basic laser foil target interaction, a set of experimental data were collected, including spatially resolved x-ray emission, the image of the coherent transition radiation, the harmonic spectra in the direction of reflection, the energy spectra and beam profile of accelerated protons, hot-electron spectra, and transmitted laser energy fraction and spatial distribution. The experimental results show that the laser intensity reached 5 × 1020 W/cm2 within a 5.8 µm focus (FWHM). Significant laser transmission did not occur when the thickness of the CH foil was equal to or greater than 50 nm. The maximum energy of the accelerated protons in the target normal direction was roughly unchanged when the target thickness varied between 50 nm and 15 µm. The maximum proton energy via the target normal sheath field acceleration mechanism was about 21 MeV. We expect the on-target laser intensity to reach 1022 W/cm2 in the near future, after optimization of the laser focus and upgrade of the laser power to 3 PW. [ABSTRACT FROM AUTHOR]

The metal–nonmetal transition is of fundamental interest in contemporary physics. We present a combination of the optical reflectivity measurement of an expanded gold fluid produced by gold L-band x-rays from a Hohlraum and the corresponding ab initio calculations to study the metal–nonmetal transition. The averaged density and temperature of the reflective layer are derived from 1D radiation hydrodynamic simulations constrained by matching the expansion velocity obtained from the fringe shift. A sharp change in time-resolved reflectivity occurs at one third of the initial density, near where the metal–nonmetal transition is predicted to occur. The reflectivity calculated based on the Helmholtz equations with the dielectric function from the density functional theory calculations is compared with the experimental measurement. Disagreement of reflectivity between calculation and measurement probably originates from the energy shift of the dielectric function due to the inaccurate Kohn–Sham eigenvalue. More accurate methods on calculating the dielectric function are still needed to improve the calculated reflectivity. [ABSTRACT FROM AUTHOR]

This study is devoted to the high-energy X-ray radiography investigation on the ejecta physics of laser shock-loaded tin. The ejecta were generated via laser shock loaded tin under sequential shock-breakout pressures by high-power nanosecond lasers. A high-energy X-ray (50∼200keV) source was created to radiograph the high dense ejecta. Due to its strong penetration, high-quality radiograph images were obtained with detailed inner information and topology structure of ejecta. The areal density distribution and total mass of ejecta were further inferred. It was found that the ejecta from laser shock-loaded tin under sequential pressures show obvious difference in density distribution between the samples in a solid state and in a melt-on-release state. In addition, the total mass of ejecta was demonstrated to increase sharply when the breakout pressure is larger than the onset of melt-on-release for tin. Such increase inferred a solid-liquid phase transition of ejecta production mechanism. [ABSTRACT FROM AUTHOR]

Ultraintense laser driven fast electrons play an increasingly important role in many applications. To predict and optimize the fast electron transport efficiency, we introduce a one-dimensional analytical model including resistive effects to estimate the transport efficiency as a function of transport distance and a key parameter named the penetration path. Based on the model, the transport efficiency of fast electrons with the same penetration coefficient can be calculated for different characteristic parameters including the target thickness and laser intensity. A double-layer Kα fluorescence measurement of fast electron transport efficiency is proposed to eliminate the influence of in-target electrons refluxing from the relative Kα photon yield of the rear and front sides of the target. By fixing the transport distance, we have experimentally measured the penetration path and the efficiency of planar Al2O3 targets, in good accordance with Monte Carlo simulations. The results show that the beam energy can be reduced to 25% in a penetration path of tens of microns. This measurement method provides a feasible route to characterize and compare the fast electron transport in various targets and laser conditions, making it possible to modulate and optimize the transport efficiency in actual research studies, which is of great significance in fast ignition, X-ray emission, positron–electron pair production, and many other applications. [ABSTRACT FROM AUTHOR]

The injecting time of the picosecond laser in an indirect-drive integrated fast ignition experiment was measured by using an x-ray streak camera. Despite overlapping spatially and temporally in experiments, the soft x-ray signal from the nanosecond laser ablating the inner wall of an Au hohlraum and the hard x-ray signal from the bremsstrahlung radiation of hot electrons generated by a picosecond laser were separated by different image processes by filtering and collimating the two signals differently. The time sequence between the two x-ray signals was analyzed to extract the injection time of the picosecond laser relative to the hohlraum emission. By tracking the neutron yield as a function of the injection time of the picosecond laser, a clear positive correlation between the neutron yield enhancement and the derived injection times was exhibited. The heating effect of the picosecond laser was confirmed. It is concluded that this method could be used to measure the injecting time and validate the picosecond laser injection. [ABSTRACT FROM AUTHOR]

The dynamic fragmentation of shock-loaded high-Z metal is of considerable importance for both basic and applied science. The areal density and mass-velocity distribution of dynamic fragmentation are crucial factors in understanding this issue. Experimental methods, such as pulsed X-ray radiography and proton radiography, have been utilized to obtain information on such factors; however, they are restricted to a complex device, and the spatial resolution is in the order of 100 μm. In this work, we present the high-quality radiography of the dynamic fragmentation of laser shock-loaded tin, with good two-dimensional (2D) spatial resolution. Dynamic fragmentation is generated via high-intensity ns-laser shock-loaded tin. A high-energy X-ray source in the 50–200 keV range is realized by the interaction of a high-intensity ps-pulse with an Au microwire target, attached to a low-Z substrate material. A high 2D resolution of 12 μm is achieved by point-projection radiography. The dynamic-fragmentation radiography is clear, and the signal-to-noise ratio is sufficiently high for a single-shot experiment. This unique technique has potential application in high-energy density experiments. [ABSTRACT FROM AUTHOR]

A multi-channel Thomson parabola spectrometer was designed and employed to diagnose ion beams driven by intense laser pulses. Angular-resolved energy spectra for different ion species can be measured in a single shot. It contains parallel dipole magnets and wedged electrodes to fit ion dispersion of different charge-to-mass ratios. The diameter and separation of the entrance pinhole channels were designed properly to provide sufficient resolution and avoid overlapping of dispersed ion beams. To obtain a precise energy spectral resolving, three-dimensional distributions of the electric and magnetic fields were simulated. Experimental measurement of energy-dependent angular distributions of target normal sheath accelerated protons and deuterons was demonstrated. This novel compact design provides a comprehensive characterization for ion beams. [ABSTRACT FROM AUTHOR]

An ultraintense femtosecond laser pulse was used, for the first time, to produce a strong magnetic field with controlled shapes by interactions with a capacitor-coil target with high efficiency. The temporal evolution of the strong magnetic field was obtained by the time-gated proton radiography method. A comparison of high-resolution radiographic images of proton deflection and particle-track simulations indicates a peak magnetic field of ∼20 T. The energy conversion efficiency from the ultraintense laser pulse to the magnetic field is as high as ∼10%. A simple model of the ultraintense laser-driven capacitor-coil target gives a relationship between the magnetic field strength and the electron temperature produced by the laser. Our results indicate that magnetic fields of tens of tesla could be stably produced by most of the existing ultraintense laser facilities. It potentially opens new frontiers in basic physics which require strong magnetic field environments. [ABSTRACT FROM AUTHOR]

Energetic electron bunches with more than 20 nC charge are generated from 100 TW level laser pulse interaction with 2% critical density plasma. Three-dimensional particle-in-cell simulations show that the unexpected high bunch-charge can be attributed to the multiple intensity peaks of the laser pulse and the resulting multiple-bubble wake structure. This charge is one of the highest among experiments on electron-bunch generation by laser-plasma interaction. Such highly charged ultra-short electron bunches are crucial for producing sufficiently bright Bremsstrahlung x-rays required in high-resolution flash radiography of large samples. [ABSTRACT FROM AUTHOR]

The Monte Carlo transport code Geant4 has been used to study positron production in the transport of laser-produced hot electrons in solid targets. The dependence of the positron yield on target parameters and the hot-electron temperature has been investigated in thick targets (mm-scale), where only the Bethe-Heitler process is considered. The results show that Au is the best target material, and an optimal target thickness exists for generating abundant positrons at a given hot-electron temperature. The positron angular distributions and energy spectra for different hot electron temperatures were studied without considering the sheath field on the back of the target. The effect of the target rear sheath field for positron acceleration was studied by numerical simulation while including an electrostatic field in the Monte Carlo model. It shows that the positron energy can be enhanced and quasi-monoenergetic positrons are observed owing to the effect of the sheath field. [ABSTRACT FROM AUTHOR]

Laser interaction with a nanobrush target plasma is investigated at the SILEX-I laser facility [X. F. Wei et al., J. Phys. Conf. Ser. 112, 032010 (2008)] with a laser of intensity 7.9×1018 W/cm2. Highly collimated fast electron beams with yields of more than three times higher than that from the planar target can be produced. Two-dimensional particle-in-cell simulation confirms that a layered surface structure can increase the efficiency of laser energy absorption, and the resulting fast electrons are tightly collimated and guided by the plasma layers to a cross section of about the laser spot size. [ABSTRACT FROM AUTHOR]

Positron production by ultra-intense laser-accelerated electrons has been studied with two-dimensional particle-in-cell and Monte Carlo simulations. The dependence of the positron yield on plasma density, plasma length, and converter thickness was investigated in detail with fixed parameters of a typical 100 TW laser system. The results show that with the optimal plasma and converter parameters a positron beam containing up to 1.9 × 1010 positrons can be generated, which has a small divergence angle (10°), a high temperature (67.2 MeV), and a short pulse duration (1.7 ps). [ABSTRACT FROM AUTHOR]

An extensive comparison of the properties of positron beams produced by an ultra-intense femtosecond laser in direct and indirect schemes has been performed with two-dimensional particle-in-cell and Monte Carlo simulations. It is shown that the positron beam generated in the indirect scheme has a higher yield (1010), a higher temperature (28.8 MeV), a shorter pulse duration (5 ps), and a smaller divergence (8°) than in the direct case (109 yield, 4.4 MeV temperature, 40 ps pulse duration, and 60° divergence). In addition, it was found that the positron/gamma ratio in the indirect scheme is one order of magnitude higher than that in the direct one, which represents a higher signal/noise ratio in positron detection. Nevertheless, the direct generation method still has its own unique advantage, the so-called target normal sheath acceleration, which can result in quasi-monoenergetic positron beams that may serve in some specialized applications. [ABSTRACT FROM AUTHOR]

The integrating current transformer (ICT) is a magnetic sensor widely used to precisely measure the charge of an ultra-short-pulse charged particle beam generated by traditional accelerators and new laser-plasma particle accelerators. In this paper, we present a new method to calculate the beam charge in an ICT based on circuit analysis. The output transfer function shows an invariable signal profile for an ultra-short electron bunch, so the function can be used to evaluate the signal quality and calculate the beam charge through signal fitting. We obtain a set of parameters in the output function from a standard signal generated by an ultra-short electron bunch (about 1 ps in duration) at a radio frequency linear electron accelerator at Tsinghua University. These parameters can be used to obtain the beam charge by signal fitting with excellent accuracy. [ABSTRACT FROM AUTHOR]