Development of a monochromatic crystal backlight imager for the recent double-cone ignition experiments

We developed a monochromatic crystal backlight imaging system for the double-cone ignition(DCI) scheme, employing a spherically bent quartz crystal. This system was used to measure the spatial distribution and temporal evolution of the head-on colliding plasma from the two compressing cones in the DCI experiments. The influence of laser parameters on the x-ray backlighter intensity and spatial resolution of the imaging system was investigated. The imaging system had a spatial resolution of 10 μm when employing a CCD detector. Experiments demonstrated that the system can obtain time-resolved radiographic images with high quality, enabling the precise measurement of the shape, size, and density distribution of the plasma.

Ultrafast two-dimensional x-ray imager with temporal fiducial pulses for laser-produced plasmas

It is challenging to make an ultrafast diagnosis of the temporal evolution of small and short-lived plasma in two dimensions. To overcome this difficulty, we have developed a well-timed diagnostic utilizing an x-ray streak camera equipped with a row of multi-pinhole arrays. By processing multiple sets of one-dimensional streaked image data acquired from various pinholes, we are capable of reconstructing high-resolution two-dimensional images with a temporal resolution of 38 ps and a spatial resolution of 18 μm. The temporal fiducial pulses accessed from external sources can advance the precise timing and accurately determine the arrival time of the laser. Moreover, it can correct the nonlinear sweeping speed of the streak camera. The effectiveness of this diagnostic has been successfully verified at the Shenguang-II laser facility,providing an indispensable tool for observing complex physical phenomena, such as the implosion process of laser-fusion experiments.

Acceleration and radiation of externally injected electrons in laser plasma wakefield driven by a Laguerre-Gaussian pulse

By using three-dimensional particle-in-cell simulations, externally injected electron beam acceleration and radiation in donut-like wake fields driven by a Laguerre-Gaussian pulse are investigated. Studies show that in the acceleration process the total charge and azimuthal momenta of electrons can be stably maintained at a distance of a few hundreds of micrometers. Electrons experience low-frequency spiral rotation and high-frequency betatron oscillation, which leads to a synchrotron-like radiation. The radiation spectrum is mainly determined by the betatron motion of electrons. The far field distribution of radiation intensity shows axial symmetry due to the uniform transverse injection and spiral rotation of electrons. Our studies suggest a new way to simultaneously generate hollow electron beam and radiation source from a compact laser plasma accelerator.

Characterization and application of plasma mirror for ultra-intense femtosecond lasers

The femtosecond laser pulses reflected from the self-induced plasma mirror（PM） surface are characterized. More than two orders of magnitude improvement on intensity contrast both in nanosecond and picosecond temporal scales are measured. The far-field distribution, i.e., focusability, is measured to degrade in comparison with that without using a PM. Experiments on proton accelerations are performed to test the effect of the balance between degraded focusability and increased reflectivity. Our results show that PM is an effective and robust device to improve laser contrast for applications.

Design, performance and application of a line-imaging velocity interferometer system for any reflector coupled with a streaked optical pyrometer system at the Shenguang-Ⅱ upgrade laser facility

The velocity interferometer system for any reflector(VISAR) coupled with a streaked optical pyrometer(SOP) system is used as a diagnostic tool in inertial confinement fusion(ICF) experiments involving equations of state and shock timing.To validate the process of adiabatically compressing the fuel shell through precise tuning of shocks in experimental campaigns for the double-cone ignition(DCI) scheme of ICF, a compact line-imaging VISAR with an SOP system is designed and implemented at the Shenguang-II upgrade laser facility. The temporal and spatial resolutions of the system are better than 30 ps and 7 μm, respectively. An illumination lens is used to adjust the lighting spot size matching with the target size. A polarization beam splitter and λ/4 waveplate are used to increase the transmission efficiency of our system. The VISAR and SOP work at 660 and 450 nm, respectively, to differentiate the signals from the scattered lights of the drive lasers. The VISAR can measure the shock velocity. At the same time, the SOP system can give the shock timing and relative strength. This system has been used in different DCI campaigns, where the generation and propagation processes of multi-shock are carefully diagnosed.

Machine-learning guided optimization of laser pulses for direct-drive implosions

The optimization of laser pulse shapes is of great importance and a major challenge for laser direct-drive implosions.In this paper,we propose an efficient intelligent method to perform laser pulse optimization via hydrodynamic simulations guided by the genetic algorithm and random forest algorithm.Compared to manual optimizations,the machine-learning guided method is able to efficiently improve the areal density by a factor of 63%and reduce the in-fiight-aspect ratio by a factor of 30%at the same time.A relationship between the maximum areal density and ion temperature is also achieved by the analysis of the big simulation dataset.This design method has been successfully demonstrated by the2021 summer double-cone ignition experiments conducted at the SG-II upgrade laser facility and has great prospects for the design of other inertial fusion experiments.

Study of backward terahertz radiation from intense picosecond laser–solid interactions using a multichannel calorimeter system

A multichannel calorimeter system is designed and constructed which is capable of delivering single-shot and broadband spectral measurement of terahertz(THz) radiation generated in intense laser–plasma interactions. The generation mechanism of backward THz radiation(BTR) is studied by using the multichannel calorimeter system in an intense picosecond laser–solid interaction experiment. The dependence of the BTR energy and spectrum on laser energy, target thickness and pre-plasma scale length is obtained. These results indicate that coherent transition radiation is responsible for the low-frequency component(<1 THz) of BTR. It is also observed that a large-scale pre-plasma primarily enhances the high-frequency component(>3 THz) of BTR.

Pulse shape of ultrashort intense laser reflected from a plasma mirror

The temporal profiles of high-power short-pulse lasers reflected from self-induced plasma mirrors（PMs） were measured with high temporal resolution in the sub-picosecond window. The leading front shape of the laser pulse is found to depend sensitively on the laser fluence on the PM surface. Spectral modulation plays a key role in pulse profile shaping. Our findings will extend our knowledge on properly using PMs.

Laser chirp controlled relativistic few-cycle mid-infrared pulse generation

Relativistic few-cycle mid-infrared(mid-IR)pulses are unique tools for strong-field physics and ultrafast science,but are difficult to generate with traditional nonlinear optical methods.Here,we propose a scheme to generate such pulses with high efficiency via plasma-based frequency modulation with a negatively chirped laser pulse(NCLP).The NCLP is rapidly compressed longitudinally due to dispersion and plasma etching,and its central frequency is downshifted via photon deceleration due to the enhanced laser intensity and plasma density modulations.Simulation results show that few-cycle mid-IR pulses with the maximum center wavelength of 7.9µm and pulse intensity of a_(MIR)=2.9 can be generated under a proper chirp parameter.Further,the maximum energy conversion efficiency can approach 5.0%.Such a relativistic mid-IR source is promising for a wide range of applications.

Efficient bright γ-ray vortex emission from a laser-illuminated light-fan-in-channel target

X/γ-rays have many potential applications in laboratory astrophysics and particle physics.Although several methods have been proposed for generating electron,positron,and X/γ-photon beams with angular momentum(AM),the generation of ultra-intense brilliant γ-rays is still challenging.Here,we present an all-optical scheme to generate a high-energy γ-photon beam with large beam angular momentum(BAM),small divergence,and high brilliance.In the first stage,a circularly polarized laser pulse with intensity of 10^(22) W/cm^(2) irradiates a micro-channel target,drags out electrons from the channel wall,and accelerates them to high energies via the longitudinal electric fields.During the process,the laser transfers its spin angular momentum(SAM)to the electrons’orbital angular momentum(OAM).In the second stage,the drive pulse is reflected by the attached fan-foil and a vortex laser pulse is thus formed.In the third stage,the energetic electrons collide head-on with the reflected vortex pulse and transfer their AM to the γ-photons via nonlinear Compton scattering.Three-dimensional particle-in-cell simulations show that the peak brilliance of the γ-ray beam is∼10^(22) photons·s^(-1)·mm^(-2)·mrad^(-2) per 0.1% bandwidth at 1 MeV with a peak instantaneous power of 25 TW and averaged BAM of 10^(6)h/photon.The AM conversion efficiency from laser to the γ-photons is unprecedentedly 0.67%.

Multichannel high extinction ratio polarized beam splitters based on metasurfaces

Separating lights into different, paths according to the polarization states while keeping their respective path's polarizations with high purification is keen for polarization multiplex in optical communications. Metallic nanowire gratings with multi-slits in a period are proposed to achieve polarized beam splitters (PBSs) in reflection and diffraction. The setting of multi-slits largely reduces the reflection of photons with a transverse magnetific field via the plasmonic waveguiding effect, which leads to highly polarized output lights with extinction ratio larger than 20 dB in each channel. The proposed reflection/diffraction PBSs enrich the approaches to control the polarization states with the advantages of wide incident angles and flexible beam splitting angles.