Lightning in a Forest (Wild) Fire: Mechanism at the Molecular Level

The mechanism of lightning that ignites a forest fire and the lightning that occurs above a forest fire are explained at the molecular level. It is based on two phenomena, namely, internal charge separation inside the atmospheric cloud particles and the existence of a layer of positively charged hydrogen atoms sticking out of the surface of the liquid layer of water on the surface of rimers. Strong turbulence-driven collisions of the ice particles and water droplets with the rimers give rise to breakups of the ice particles and water droplets into positively and negatively charged fragments leading to charge separation. Hot weather in a forest contributes to the updraft of hot and humid air, which follows the same physical/chemical processes of normal lightning proposed and explained recently[1]. Lightning would have a high probability of lighting up and burning the dry biological materials in the ground of the forest, leading to a forest (wild) fire. The burning of trees and other plants would release a lot of heat and moisture together with a lot of smoke particles (aerosols) becoming a strong updraft. The condition for creating lightning is again satisfied which would result in further lightning high above the forest wild fire.

Polaron mobility modulation by bandgap engineering in black phaseα-FAPbI_(3)

Lead halide hybrid perovskites(LHP)have emerged as one of the most promising photovoltaic materials for their remarkable solar energy conversion ability.The transportation of the photoinduced carriers in LHP could screen the defect recombination with the help of the large polaron formation.However,the physical insight of the relationship between the superior optical-electronic performance of perovskite and its polaron dynamics related to the electron-lattice strong coupling induced by the substitution engineering is still lack of investigation.Here,the bandgap modulated thin films ofα-FAPbI_(3)with different element substitution is investigated by the time resolved Terahertz spectroscopy.We find the polaron recombination dynamics could be prolonged in LHP with a relatively smaller bandgap,even though the formation of polaron will not be affected apparently.Intuitively,the large polaron mobility in(FAPb I_(3))0.95(MAPbI_(3))0.05thin film is~30%larger than that in(FAPb I_(3))0.85(MAPbBr_(3))0.15.The larger mobility in(FAPb I_(3))0.95(MAPb I_(3))0.05could be assigned to the slowing down of the carrier scattering time.Therefore,the physical origin of the higher carrier mobility in the(FAPb I_(3))0.95(MAPbI_(3))0.05should be related with the lattice distortion and enhanced electron–phonon coupling induced by the substitution.In addition,(FAPbI_(3))0.95(MAPbI_(3))0.05will lose fewer active carriers during the polaron cooling process than that in(FAPb I_(3))0.85(MAPbBr_(3)),indicating lower thermal dissipation in(FAPbI_(3))0.95(MAPbI_(3))0.05.Our results suggest that besides the smaller bandgap,the higher polaron mobility improved by the substitution engineering inα-FAPbI_(3)can also be an important factor for the high PCE of the black phaseα-FAPbI_(3)based solar cell devices.

Charging Mechanism of Lightning at the Molecular Level

Cloud electrification is one of the oldest unresolved puzzles in the atmospheric sciences. Though many mechanisms for charge separation in clouds have been proposed, a quantitative understanding of their respective contribution in a given meteorological situation is lacking. Here we suggest and analyze a hitherto little discussed process. A qualitative picture at the molecular level of the charge separation mechanism of lightning in a thundercloud is proposed. It is based on two key physical/chemical natural phenomena, namely, internal charge separation of the atmospheric impurities/aerosols inside an atmospheric water cluster/droplet/ice particle and the existence of liquid water layers on rimers (graupels and hailstones) forming a layer of dipoles with H+ pointing out from the air-water interface. Charge separation is achieved through strong collisions among ice particles and water droplets with the rimers in the turbulence of the thundercloud. This work would have significant contribution to cloud electrification and lightning formation.

Bunching enhancement for coherent harmonic generation by using phase merging effects

In this paper,promising but simple schemes are investigated to enhance the micro-bunching of relativistic electron beams for coherent harmonic generation(CHG)by using phase merging effects.In contrast to the standard CHG scheme,two specially designed dispersion sections(DSs)are adopted with the DS-modulator–DS configuration.The phase space of the e beam is appropriately coupled in the first DS,and the electrons within one seed wavelength can merge to the same phase with a matched second DS.Micro-bunching of the e beam can thus be enhanced by a large margin with much higher-harmonic components.Taking e beams from laser wakefield accelerators(LWFAs)as an example,start-to-end simulations are performed to show the effectiveness and robustness of the proposed schemes with several configurations.The beam current can be optimized to several tens to hundreds of kiloamperes,and the radiation power reaches hundreds of megawatts in the extreme ultraviolet regime within a 3.5 m-long beamline.The proposed schemes offer new opportunities for future compact free-electron lasers driven by LWFAs and provides prospects for truly compact and widely applicable systems.

Design and Test of the Semi-Automatic Test-Bed with Inclined Belt of Garlic Transplanting Machine

In order to verify the feasibility of semi-automatic garlic planter with inclined belt program and determine its reasonable operating parameters, the semi-automatic test-bed with inclined belt of garlic transplanting machine was designed, and the garlic box experiments were conducted. The angle of inclined belt on the test-bed and its running speed were adjustable. Single factor test results showed that the program of the semi-automatic garlic planter with inclined belt was feasible, and the angle of inclined belt and the test bed running speed affected the indicators. Orthogonal experiment results showed that the angle of inclined belt was the main factor affecting the test indicators. It is also found that the best angle was 30 degrees, while the most reasonable running speed was 0.75 Km/h.

Functional Studies of Castor(Ricinus communis L.)PLC Family Genes in Arabidopsis Inflorescence Development

Castor(Ricinus communis L.)is one of the top 10 oil crops in the world,and inflorescence is a trait that directly affects its yield.Phospholipase C(PLCs)is involved in many plant activities and metabolic processes.To study the functions of PLC family genes in the regulation of the inflorescence development of the female line of Lm-type castor aLmAB2,we determined the expression levels of six PLC family genes of three types of inflorescences of aLmAB2(isofemale line,female line,bisexual line)at different developmental stages.The results showed that the 6 genes of the castor PLC family had relative expression levels at different developmental stages of the three types of inflorescences.The subcellular location of all six protein products was the cell membrane.The six genes were heterologously overexpressed in Arabidopsis thaliana to obtain the T3 generation-resistant Arabidopsis thaliana plants.The results showed that the overexpression of six genes significantly promoted the maturation of Arabidopsis thaliana,the growth of lateral moss,and the development of flowers and pods,but the development of basal leaves and stem leaves of Arabidopsis thaliana was significantly inhibited.According to homology analysis,it is speculated that PLC2,PLC2M,PLC2N,PLC4,PLC4X2,and PLC6 genes have the same regulatory function.

Generating a tunable narrow electron beam comb via laser-driven plasma grating

We propose a novel approach for generating a high-density,spatially periodic narrow electron beam comb(EBC)from a plasma grating induced by the interference of two intense laser pulses in subcritical-density plasma.We employ particle-in-cell(PIC)simulations to investigate the effects of cross-propagating laser pulses with specific angles overlapping in a subcritical plasma.This overlap results in the formation of a transverse standing wave,leading to a spatially periodic high-density modulation known as a plasma grating.The electron density peak within the grating can reach several times the background plasma density.The charge imbalance between electrons and ions in the electron density peaks causes mutual repulsion among the electrons,resulting in Coulomb expansion and acceleration of the electrons.As a result,some electrons expand into vacuum,forming a periodic narrow EBC with an individual beam width in the nanoscale range.To further explore the formation of the nanoscale EBC,we conduct additional PIC simulations to study the dependence on various laser parameters.Overall,our proposed method offers a promising and controlled approach to generate tunable narrow EBCs with high density.

Stable,intense supercontinuum light generation at 1 kHz by electric field assisted femtosecond laser filamentation in air

Supercontinuum(SC)light source has advanced ultrafast laser spectroscopy in condensed matter science,biology,physics,and chemistry.Compared to the frequently used photonic crystal fibers and bulk materials,femtosecond laser filamentation in gases is damage-immune for supercontinuum generation.A bottleneck problem is the strong jitters from filament induced self-heating at kHz repetition rate level.We demonstrated stable kHz supercontinuum generation directly in air with multiple mJ level pulse energy.This was achieved by applying an external DC electric field to the air plasma filament.Beam pointing jitters of the 1 kHz air filament induced SC light were reduced by more than 2 fold.The stabilized high repetition rate laser filament offers the opportunity for stable intense SC generation and its applications in air.

Expansion Strategy-Driven Micron-Level Resolution Mass Spectrometry Imaging of Lipids in Mouse Brain Tissue

A novel method for enhanced resolution,termed expansion mass spectrometry imaging,has been developed for lipid mass spectrometry imaging,utilizing existing commercially available mass spectrometers without necessitating modifications.This approach involves embedding tissue sections in a swellable polyelectrolyte gel,with the target biomolecules indirectly anchored to the gel network.By employing matrix-assisted laser desorption ionization mass spectrometry imaging,the method has realized an enhanced spatial resolution that surpasses the conventional resolution limits of commercial instruments by approximately 4.5 fold.This enhancement permits the detailed visualization of intricate structures within the mouse brain at a subcellular level,with a lateral resolution nearing 1μm.As a physical technique for achieving resolution beyond standard capabilities,this readily adaptable approach presents a powerful tool for high-definition imaging in biological research.

Comprehensive multiphysics analysis of high-energy nanosecond lasers based on liquid-cooled Nd-doped rods pumped by flashlamps

Ultra-short pulse,ultra-intense(USUI)lasers have become indispensable tools for scientific research.High-energy pump lasers are crucial for ensuring adequate energy and beam quality of these USUI lasers.Pump lasers with rod amplifiers are a cost-effective and reliable option for meeting high-energy pump requirements.However,there is a notable dearth of comprehensive reports on the design of high-energy rod amplifiers.This study provides a detailed analysis of rod amplifiers,focusing primarily on the pump unit utilized at SULF-10 PW and SEL-100 PW prototypes.The analysis covers aspects such as gain management,thermal effects,and spatiotemporal evolution.

Ultra-broadband pulse generation via hollow-core fiber compression and frequency doubling for ultra-intense lasers

We demonstrate an ultra-broadband high temporal contrast infrared laser source based on cascaded optical parametric amplification,hollow-core fiber(HCF)and second harmonic generation processes.In this setup,the spectrum of an approximately 1.8μm laser pulse has near 1μm full bandwidth by employing an argon gas-filled HCF.Subsequently,after frequency doubling with cascaded crystals and dispersion compensation by a fused silica wedge pair,9.6 fs(~3cycles)and 150μJ pulses centered at 910 nm with full bandwidth of over 300 nm can be generated.The energy stability of the output laser pulse is excellent with 0.8%(root mean square)over 20 min,and the temporal contrast is>10^(12)at-10 ps before the main pulse.The excellent temporal and spatial characteristics and stability make this laser able to be used as a good seed source for ultra-intense and ultrafast laser systems.

Observation of Refractive Index Line Shape in Ultrafast XUV Transient Absorption Spectroscopy

Ultrafast extreme ultraviolet (XUV) transient absorption spectroscopy measures the time- and frequencydependent light losses after light–matter interactions. In the linear region, the matter response to an XUV light field is usually determined by the complex refractive index ̃n. The absorption signal is directly related to the imaginary part of ̃n, namely, the absorption index. The real part of ̃n refers to the real refractive index, which describes the chromatic dispersion of an optical material. However, the real refractive index information is usually not available in conventional absorption experiments. Here, we investigate the refractive index line shape in ultrafast XUV transient absorption spectroscopy by using a scheme that the XUV pulse traverses the target gas jet off-center. The jet has a density gradient in the direction perpendicular to the gas injection direction, which induces deflection on the XUV radiation. Our experimental and theoretical results show that the shape of the frequency-dependent XUV deflection spectra reproduces the refractive index line profile. A typical dispersive refractive index line shape is measured for a single-peak absorption;an additional shoulder structure appears for a doublet absorption.Moreover, the refractive index line shape is controlled by introducing a later-arrived near-infrared pulse to modify the phase of the XUV free induction decay, resulting in different XUV deflection spectra. The results promote our understanding of matter-induced absorption and deflection in ultrafast XUV spectroscopy.

Coherent free-electron light sources

Free-electron light sources feature extraordinary luminosity,directionality,and coherence,which has enabled significant scientific progress in fields including physics,chemistry,and biology.The next generation of light sources has aimed at compact radiation sources driven by free electrons,with the advantages of reduction in both space and cost.With the rapid development of ultra-intense and ultrashort lasers,great effort has been devoted to the quest for compact free-electron lasers(FELs).This review focuses on the current efforts and advancements in the development of compact FELs,with a particular emphasis on two notable paths:the development of compact accelerators and the construction of micro undulators based on innovative materials/structures or optical modulation of electrons.In addition,the physical essence of inverse Compton scattering is discussed,which offers remarkable capability to develop an optical undulator with a spatial period that matches the optical wavelength.Recent scientific developments and future directions for miniaturized and integrated free-electron coherent light sources are also reviewed.In the future,the prospect of generating ultrashort electron pulses will provide fascinating means of producing superradiant radiation,promising high brilliance and coherence even on a micro scale using optical micro undulators.

Measurement of electron beam transverse slice emittance using a focused beamline

A single-shot measurement of electron emittance was experimentally accomplished using a focused transfer line with a dipole.The betatron phase of electrons based on laser wakefield acceleration(LWFA)is energy dependent owing to the coupling of the longitudinal acceleration field and the transverse focusing(defocusing)field in the bubble.The phase space presents slice information after phase compensation relative to the center energy.Fitting the transverse size of the electron beam at different energy slices in the energy spectrum measured 0.27 mm mrad in the experiment.The diagnosis of slice emittance facilitates local electron quality manipulation,which is important for the development of LWFA-based free electron lasers.The quasi-3D particle-in-cell simulations matched the experimental results and analysis well.

Effect of pulse duration on generation of attosecond pulse with coherent wake emission

High-order harmonics and attosecond pulse generation with coherent wake emission are theoretically investigated for the effect of pulse duration and carrier envelope phase(CEP)of few-cycle laser pulse.We find that short pulse duration will cause the negative chirp for the high harmonics.When the laser pulse is shortened to a few cycles,the influence of the laser CEP on the chirp of the harmonics will also become more prominent.

Coherent beam combination far-field measuring method based on amplitude modulation and deep learning

A deep convolutional neural network is employed to simultaneously measure the beam-pointing and phase difference of sub-beams from a single far-field interference fringe for coherent beam combining systems.The amplitudes of sub-beams in the measurement path are modulated in order to prevent measuring mistakes caused by the symmetry of beam-pointing.This method is able to measure beam-pointing and phase difference with an RMS accuracy of about 0.2μrad andλ/250,respectively,in a two-beam coherent beam combining system.

Femtosecond mid-IR optical vortex laser based on optical parametric chirped pulse amplification

A femtosecond mid-infrared optical vortex laser can be used for high harmonic generation to extend cutoff energy to the kilo-electron-volt range with orbital angular momentum,as well as other secondary radiations.For these,we demonstrate a high-energy femtosecond 4μm optical vortex laser based on optical parametric chirped pulse amplification(OPCPA)for the first time.The optical vortex seed is generated from a femtosecond 4μm laser by a silicon spiral phase plate with the topological charge l of 1 before the stretcher.Through using a two-stage collinear OPCPA amplifier,the chirped vortex pulse is amplified to 12.4 m J with 200 nm full width at half-maximum bandwidth.After compression,the vortex laser pulse with 9.53 m J,119 fs can be obtained.Furthermore,the vortex characteristics of the laser beam are investigated and evaluated.This demonstration can scale to generate a higher-peak-power vortex mid-IR laser and pave a new way for high field physics.

The 1 PW/0.1 Hz laser beamline in SULF facility

In this paper,we report the recent progress on the 1 PW/0.1 Hz laser beamline of Shanghai Superintense Ultrafast Laser Facility(SULF).The SULF-1 PW laser beamline is based on the double chirped pulse amplification(CPA)scheme,which can generate laser pulses of 50.8 J at 0.1 Hz after the final amplifier;the shot-to-shot energy fluctuation of the amplified pulse is as low as 1.2%(std).After compression,the pulse duration of 29.6 fs is achieved,which can support a maximal peak power of 1 PW.The contrast ratio at-80 ps before main pulse is measured to be 2.5×10^-11.The focused peak intensity is improved by optimizing the angular dispersion in the grating compressor.The maximal focused peak intensity can reach 2.7×10^19W/cm2 even with an f/26.5 off-axis parabolic mirror.The horizontal and vertical angular pointing fluctuations in 1 h are measured to be 1.89 and 2.45μrad,respectively.The moderate repetition rate and the good stability are desirable characteristics for lasermatter interactions.The SULF-1 PW laser beamline is now in the phase of commissioning,and preliminary experiments of particle acceleration and secondary radiation under 300–400 TW/0.1 Hz laser condition have been implemented.The progress on the experiments and the daily stable operation of the laser demonstrate the availability of the SULF-1 PW beamline.

Even-order high-harmonic generation from solids in velocity gauge

We present a velocity-gauge model for the generation of even-order high harmonics, and reveal that the even-order harmonics originate from the multiple-step transitions among the energy bands in momentum space, while the odd-order harmonics are mainly from direct transitions. The lower valence band is found vital for the generation of even harmonics. Relative intensity of even-order harmonics versus the odd orders is calculated and shows a growing trend as the laser field amplitude increases.

Simultaneous generation of controllable double white light lasers by focusing an intense femtosecond laser pulse in air

We report on a simultaneous generation of double white light lasers through filamentation by focusing a femtosecond laser pulse. The appearance of the two white light lasers can be controlled by tilting the focusing lens. The spectral bandwidth and the pulse energy of the double white light lasers were controlled by tuning laser filamenting pulse energy and polarization. Two white light lasers with pulse energies of 1.54 m J and 1.84 m J,respectively, were generated with the pump laser energy of 7.43 m J. Besides being beneficial in understanding the multiple white light lasers generation process through multiple filamentation and its control, the results are also valuable for white light laser-based applications.