Probing the peripheral self-generated magnetic field distribution in laser-plasma magnetic reconnection with Martin-Puplett interferometer polarimeter

Magnetic reconnection of the self-generated magnetic fields in laser-plasma interaction is an important laboratory method for modeling high-energy density astronomical and astrophysical phenomena.We use the Martin-Puplett interferometer(MPI)polarimeter to probe the peripheral magnetic fields generated in the common magnetic reconnection configuration,two separated coplanar plane targets,in laser-target interaction.We introduce a new method that can obtain polarization information from the interference pattern instead of the sinusoidal function fitting of the intensity.A bidirectional magnetic field is observed from the side view,which is consistent with the magneto-hydro-dynamical(MHD)simulation results of self-generated magnetic field reconnection.We find that the cancellation of reverse magnetic fields after averaging and integration along the observing direction could reduce the magnetic field strength by one to two orders of magnitude.It indicates that imaging resolution can significantly affect the accuracy of measured magnetic field strength.

First experimental comparisons of laser-plasma interactions between spherical and cylindrical hohlraums at SGⅢ laser facility

We present our recent laser-plasmas instability(LPI)comparison experiment at the SGIII laser facility between the spherical and cylindrical hohlraums.Three kinds of filling are considered:vacuum,gas-filling with or without a capsule inside.A spherical hohlraum of 3.6 mm in diameter,and a cylindrical hohlraum of 2.4 mm?4.3 mm are used.The capsule diameter is 0.96 mm.A flat-top laser pulse with 3 ns duration and up to 92.73 kJ energy is used.The experiment has shown that the LPI level in the spherical hohlraum is close to that of the outer beam in the cylindrical hohlraum,while much lower than that of the inner beam.The experiment is further simulated by using our 2-dimensional radiation hydrodynamic code LARED-Integration,and the laser back-scattering fraction and the stimulated Raman scatter(SRS)spectrum are post-processed by the high efficiency code of laser interaction with plasmas HLIP.According to the simulation,the plasma waves are strongly damped and the SRS is mainly developed at the plasma conditions of electron density from 0.08 n_(c) to 0.1 n_(c) and electron temperature from 1.5 keV to 2.0 keV inside the hohlraums.However,obvious differences between the simulation and experiment are found,such as that the SRS back-scattering is underestimated,and the numerical SRS spectrum peaks at a larger wavelength and at a later time than the data.These dif-ferences indicate that the development of a 3D radiation hydrodynamic code,with more accurate physics models,is mandatory for spherical hohlraum study.

Superthermal Electron Produced in Relativistic Laser-Plasma Interaction

The dynamics of a relativistic electron submitted to an intense, plane wave, linearly polarized laser field is reviewed. Based on the dynamics, the temperature of the electron in the laser field is delined and calculated. It is found that the calculated temperature fits the first temperature observed in the experiment by Malka et al. A model to evaluate the electron temperature by taking the electron-ion scattering into account is proposed. It is found that when I ≥ 4.0 × 101s W/cm2 the electron temperature by considering the scattering, T hs, is evidently larger than the electron temperature without considering the scattering, Th. This result is in favor of explaining the two-temperature distribution of the electron energy observed in the experiment by Malka et al.

High power terahertz pulses generated in intense laser-plasma interactions

Terahertz （THz） radiation has attracted much attention due to its wide potential applications. Though radiation can be generated with various ways, it is still a big challenge to obtain strong tabletop sources. Plasma, with the advantage of no damage limit, is a promising medium to generate strong THz radiation. This review reports recent advances on strong THz radiation generation from low-density gases and high-density solid targets at different laser intensities.

Laser-Plasma Deposition of Silicon Carbonitride Films by the HMDS Vapor Gas Flow Activation after a Laser Beam Focus

A new laser-plasma deposition method has been developed for the plasma chemical deposition of hard silicon carbonitride coatings on stainless steel substrates from the hexamethyldisilazane (HMDS) Si2NH(CH3)6 vapor in a high-speed Ar and Ar + 10 vol.% He gas stream at the HMDS gas flow activation after the laser beam focus. The method allows depositing silicon carbonitride coatings at the rate of 0.4 - 1.2 μm·min-1, i.e. ~2 times higher than that at introducing HMDS in the laser beam focus zone. The properties of the prepared coatings have been studied by the methods of IR and Raman spectroscopy, atomic force microscopy, nanoindentation and X-ray diffraction (XRD) analysis. Studying the film structure with the use of XRD showed that the prepared silicon carbonitride coatings are X-ray amorphous. It has been found that the coating deposition rate and the structure of coatings depend on the process parameters: HMDS flow rate and plasma-generating gas (argon or (Ar + He). The method allows depositing SiCN films at a high speed and a hardness of 20 - 22 GPa.

Electron Dynamics and Characteristics of Attosecond Electromagnetic Emissions in Relativistic Laser-Plasma Interactions

Generation of attosecond electromagnetic （EM） pulses and the associated electron dynamics are studied using particle-in-cell simulations of relativistic laser pulses interacting with over-dense plasma foil targets. The inter- action process is found to be so complicated even in the situation of utilizing driving laser pulses of only one cycle. Two electron bunches closely involved in the laser-driven wavebreaking process contribute to attosecond EM pulses through the coherent synchrotron emission process whose spectra are found to follow an exponential decay rule. Detailed investigations of electron dynamics indicate that the early part of the reflected EM emission is the high-harmonics produced through the relativistic oscillating mirror mechanism. High harmonics are also found to be generated through the Bremsstrahlung radiation by one electron bunch that participates in the wavebreaking process and decelerates when it experiences the local wavebreaking-generated high electrostatic field in the moving direction.

Intense Cherenkov-type terahertz electromagnetic radiation from ultrafast laser-plasma interaction

A Cherenkowtype terahertz electromagnetic radiation is revealed, which results efficiently from the collective effects in the time-domain of ultrafast pulsed electron current produced by ultrafast intense laser plasma interaction. The emitted pulse waveform and spectrum, and the dependence of laser pulse parameters on the structure of the radiation field are investigated numerically. The condition of THz radiation generation in this regime and Cherenkov geometry of the radiation field are studied analytically.

High-energy-density electron beam generation in ultra intense laser-plasma interaction

By using a two-dimensional particle-in-cell simulation,we demonstrate a scheme for highenergy-density electron beam generation by irradiating an ultra intense laser pulse onto an aluminum（Al） target.With the laser having a peak intensity of 4×10^23W cm^-2,a high quality electron beam with a maximum density of 117 nc and a kinetic energy density up to8.79×10^18J m^-3 is generated.The temperature of the electron beam can be 416 Me V,and the beam divergence is only 7.25°.As the laser peak intensity increases（e.g.,1024 W cm^-2）,both the beam energy density（3.56×10^19J m^-3） and the temperature（545 Me V） are increased,and the beam collimation is well controlled.The maximum density of the electron beam can even reach 180 nc.Such beams should have potential applications in the areas of antiparticle generation,laboratory astrophysics,etc.

Ion Acceleration in Laser-Plasma Interaction: Shock, Sheath, and Scaling

在激光血浆相互作用的离子加速上的一些笔记和评价被给，特别地为吃惊的含意，鞘并且可伸缩。一个简单模型被吃惊和鞘的联合为离子加速建议。在最大的离子精力和激光参数之间的获得的可伸缩的关系(力量，脉搏持续时间)也血浆参数(血浆密度)，例如 E 离子，最大 P 7/12 L , E 离子，最大τ 1/3 L ,和 E 离子，最大 n 2/3 e ,与以前的工作相比。模型和结果的一些缺乏和含意被讨论。

Electron trajectory evaluation in laser-plasma interaction for effective output beam

Using the ellipsoidal cavity model, the quasi-monoenergetic electron output beam in laser-plasma interaction is described. By the cavity regime the quality of electron beam is improved in comparison with those generated from other methods such as periodic plasma wave field, spheroidal cavity regime and plasma channel guided acceleration. Trajectory of electron motion is described as hyperbolic, parabolic or elliptic paths. We find that the self-generated electron bunch has a smaller energy width and more effective gain in energy spectrum. Initial condition for the ellipsoidal cavity is determined by laser-plasma parameters. The electron trajectory is influenced by its position, energy and cavity electrostatic potential.

Characteristics of Self-Focusing Dueto the Pondermotive Forcein Laser-Plasma Interaction

The Max’s equation of self-focusing in laser-plasma interaction due to the pondermotive force has been discussed and its physical meaning of the assumption has been predicted. The characteristics of selffocusing due to the pondermotive force with different initial situations have been analyzed through the numerical calculation.

Photon Acceleration Based On Laser-Plasma

One dimensional electron density perturbation is derived by using the cold fluid equation, Possion's equation and the continuity equation. The perturbation is generated by a driving laser pulse propagating through a plasma. The upshifting of the frequency of a trailing pulse induced by density perturbation is studied by using optical metric. The results show that it is possible that the photon will gain energy from the wakefield when assuming photon's number to be conserved, i.e. , the photon will be accelerated.

Influences of Uncaptured Electron on Energy Conversion of Photon Compton Scattering in High Power Laser-plasma

Using the single particle theory and the non-flexibility collision model of electron and photon, the influence of the uncaptured electrons on the energy conversion efficiency of multi-photon nonlinear Compton scattering in the extra stationary laser-plasma is investigated. It shows that in extra stationary laser-plasma,the uncaptured electrons make the Δω of the scattering frequency of the multi-photon Compton fall down with the increases of the incident radiation electron speed,the materials of the incident collision of electron and photon, and the number of the photons which work with the electrons at the same time. Under the modulation of the uncaptured electrons to the laser field, the energy conversion efficiency between electrons and photons will fall down with the increase of the electron incident radiation speed, using the low-power electrons for incident source, the loss can be efficiently reduced.

Data-driven science and machine learning methods in laser-plasma physics

Laser-plasma physics has developed rapidly over the past few decades as lasers have become both more powerful and more widely available.Early experimental and numerical research in this field was dominated by single-shot experiments with limited parameter exploration.However,recent technological improvements make it possible to gather data for hundreds or thousands of different settings in both experiments and simulations.This has sparked interest in using advanced techniques from mathematics,statistics and computer science to deal with,and benefit from,big data.At the same time,sophisticated modeling techniques also provide new ways for researchers to deal effectively with situation where still only sparse data are available.This paper aims to present an overview of relevant machine learning methods with focus on applicability to laser-plasma physics and its important sub-fields of laser-plasma acceleration and inertial confinement fusion.

LIBS结合图像筛选方法提高钢铁中Cu、Cr、Mn元素检测稳定性研究

优质特种钢材和低端粗钢之间的性能差异主要受其构成元素种类及其成分含量的影响,因此,如何快速准确地对物质成分进行定性及定量分析对钢铁产品的质量评估至关重要。针对传统方法难以实现对钢铁合金成分的快速准确检测的难题,采用激光诱导击穿光谱(LIBS)结合等离子体图像信息的方法,通过快速地对不同元素的特征光谱强度与激发生成的等离子体图像进行采集,分析两者之间的相关性,并通过提取的图像特征信息的异常值剔除了部分无效光谱数据,进而实现了对钢铁成分的高精度分析。通过分析延迟时间和激光能量等不同实验条件对元素特征光谱强度及其对应等离子体图像的影响规律,不仅证明了等离子体图像与光谱之间存在相关性,还利用等离子体图像特征信息的局部最优值确定了最优延迟时间、激光能量分别为1000 ns与50 mJ,并根据图像特征的平均阈值来筛选无效光谱数据。结果表明,图像筛选优化数据后,各元素谱线校准模型的决定系数(R^(2))分别从原始数据的0.978、0.986、0.957、0.935提升至0.995、0.997、0.968、0.957,且其定标曲线对未知样品元素的预测浓度相对标准偏差(RSD)下降为原始数据预测浓度RSD的50%左右。由此可知采用LIBS结合图像筛选方法可以减少定量分析的误差,提高预测结果精确度。

弱相对论涡旋光在等离子体中传播的波前畸变及补偿

涡旋光与等离子体相互作用近年来在激光等离子体领域引起了广泛的关注.深入研究涡旋光在等离子体中的传播对粒子加速和辐射源产生等工作具有重要意义.本文着重探讨了弱相对论涡旋光在等离子中传播时,传播过程对电磁波结构的影响.基于三维粒子模拟,发现弱相对论涡旋光在等离子体中传播时会产生波前畸变.在给定等离子体密度时,畸变程度与电磁波强度及传播距离密切相关.基于相位修正模型,通过考虑电子相对论质量修正,在理论上对该现象进行了解释.此外,研究还发现可以通过设置适当的初始密度调制对波前畸变进行补偿抑制,并通过三维粒子模拟进行了验证.本工作加强了对涡旋光在等离子体中传播过程的理解,并为设计应用于相对论涡旋光的等离子体器件提供了参考.

直出式绿激光膀胱肿瘤剜除术与经尿道膀胱肿瘤等离子电切在治疗非肌层浸润性膀胱癌的围手术期疗效的比较

目的比较直出式绿激光膀胱肿瘤剜除术与经尿道膀胱肿瘤等离子电切术治疗非肌层浸润性膀胱癌(NMIBC)的疗效。方法回顾性分析2019年3月至2020年3月于安庆市立医院泌尿外科治疗的80例NMIBC患者的临床资料,通过随机数表法将80例患者均分为两组,其中实验组采用直出式绿激光膀胱肿瘤剜除术,对照组采用经尿道膀胱肿瘤等离子电切术。对两组的术中出血量、术后膀胱冲洗时间、留置尿管时间、术后住院时间以及术中、术后并发症发生情况进行比较分析。结果两组患者手术均顺利完成,实验组在手术时间、复发率方面与对照组相比差异无统计学意义(P>0.05)。实验组术中出血量、术后膀胱冲洗时间、留置尿管时间、术后住院时间小于对照组;实验组在术后出血、闭孔神经反射和膀胱穿孔发生率明显低于对照组,差异有统计学意义(P<0.05)。结论直出式绿激光膀胱肿瘤剜除术治疗NMIBC较经尿道膀胱肿瘤等离子电切术更安全、有效,具有微创、出血少、并发症风险低等优点,值得临床推广使用。

激光轰击金属液面溅射过程的数值模拟

目前,极紫外光被认为是制备特征尺寸小于7 nm芯片所必备的光源,激光轰击液态金属锡靶是其产生的主要方式之一.采用体积分数(volurne of fluid,VOF)方法建立了激光轰击金属锡液面产生溅射的模型,并对液料溅射和雾化的演化过程进行了数值模拟,研究了冠状水花的产生机制以及雾化时的流场变化.在此基础上,进一步研究了金属液态锡在不同的能量、光斑直径和脉宽的激光轰击下溅射产生的冠的宽度和高度随时间演化情况.研究表明:在激光辐照产生的高压等离子体的高速冲击下,液膜经历快速运动、冠状射流和雾化3个阶段,液膜生长主要原因是惯性力作用;液膜上下两端存在较大的速度梯度是射流形状发生变化的主要原因;冠边缘处雾化现象的产生是由Rayleigh-Taylor和Plateau-Rayleigh不稳定性共同作用的结果.在激光轰击下,冠的高度和宽度随激光能量增加而增大,但随着时间推移,冠宽和冠高的增长速率逐渐减小;激光光斑和脉宽对冠宽及冠高的影响较为复杂,在前期影响较小,在后期冠宽及冠高随它们数值增加而减小.

激光硬化与渗氮复合处理H13钢组织与性能

通过激光淬火/离子渗氮复合方法对H13钢进行表面改性处理,从而提高其表面的性能.利用X射线衍射技术、光学显微观察、扫描电子显微观察、能谱分析技术、显微硬度和纳米测试系统以及高频往复磨损试验,分别研究了离子渗氮、激光淬火、以及激光淬火与离子渗氮复合处理对H13钢改性层组织结构、力学性能、摩擦磨损性能的影响过程和影响机理.结果表明:复合处理工艺可以显著改善改性层的综合性能.和单一渗氮处理相比,复合处理改性层硬度和有效硬化层深度分别从1180 HV和80μm提高至1360 HV和550μm,摩擦系数和磨损率分别从0.68和5.78×10^(-8)mm^(3)·N^(-1)·m^(-1)降低至0.59和1.35×10^(-8)mm^(3)·N^(-1)·m^(-1).试样的硬度和耐磨性显著增加,平均摩擦系数明显降低.

Multi-layer phenomena in petawatt laser-driven acceleration of heavy ions

Laser-accelerated high-flux-intensity heavy-ion beams are important for new types of accelerators.A particle-in-cell program(Smilei) is employed to simulate the entire process of Station of Extreme Light(SEL) 100 PW laser-accelerated heavy particles using different nanoscale short targets with a thickness of 100 nm Cr, Fe, Ag, Ta, Au, Pb, Th and U, as well as 200 nm thick Al and Ca. An obvious stratification is observed in the simulation. The layering phenomenon is a hybrid acceleration mechanism reflecting target normal sheath acceleration and radiation pressure acceleration, and this phenomenon is understood from the simulated energy spectrum,ionization and spatial electric field distribution. According to the stratification, it is suggested that high-quality heavy-ion beams could be expected for fusion reactions to synthesize superheavy nuclei. Two plasma clusters in the stratification are observed simultaneously, which suggest new techniques for plasma experiments as well as thinner metal targets in the precision machining process.