强场下真空中粒子对产生的研究进展

随着激光技术的飞快发展,激光强度不断提高,超强外场下真空中正负电子对产生的过程,即能量向质量转化过程,已经成为一个研究热点。主要综述了近几年量子Vlasov方程方法和计算量子场论(数值求解Dirac方程)方法在研究强场下真空中正负电子对产生方面的进展,分别介绍了空间均匀场和空间不均匀场下的粒子对产生的情况。第一种情况主要介绍双脉冲结构振荡电场中电子-正电子对的产生、强双频振荡电场中非微扰电子-正电子对的产生、频率调制的激光场中电子-正电子对的产生和Dirac真空对啁啾外场的快速分辨。第二种情况主要介绍优化空间局域电场提高粒子对的产生率、多个势阱-垒结构的振荡场对粒子对产生的增强、振荡 Sauter 电势中正负电子对产生的问题、操纵Dirac真空以控制其在场诱导下的衰变、作为信息传输介质的Dirac真空还有正负电子对产生中的相干和非相干啁啾机制的转变。     

强场X射线激光物理

相干X光,特别是X射线自由电子激光技术的发展提供了一种新的产生超强光场的途径.由于其较高的光子能量、高峰值功率密度与超短的脉冲长度,有望将强场激光物理从可见光波段推进到X光波段.目前,基于X射线的非线性原子分子物理已取得了初步进展,随着X射线光强的提升,相互作用将进入相对论物理、强场量子电动力学(quantum electrodynamics,QED)物理等领域,为激光驱动加速与辐射、QED真空、暗物质的产生与探测等带来新的科学发现机会.本文对强场X射线激光在固体中的尾场加速、真空极化、轴子的产生与探测等方面进行介绍,旨在阐明X射线波段强场物理在若干基础前沿与关键应用方面的独特优势,并对未来的发展方向进行展望.     

Electron-Positron Pair Production in a Strong Laser Field Enhanced by an Assisted High Frequency Weak Field

Electron-positron pair production in a strong laser field enhanced by an assisted high frequency weak field is investigated by solving the quantum Vlasov equation.The average and residual pair number densities are obtained for sinusoid electric field and it is found that the high frequency assisted weak field will enhance pair production significantly.There exists an optimal frequency of assisted field that makes the pair production number density get a maximum one,which is a few orders of higher than that without assisted field.We also discuss the other possible assisted fields.     

A simple method for timing an XFEL source to high-power lasers

We propose a technique for timing an X-ray free-electron laser (XFEL) to a high-power conventional laser with femtosecond accuracy, yielding the relative jitter between pump and X-ray probe, and allowing sorting of experimental results over a certain time window. The same electron bunch is used to produce both an XFEL pulse and an ultrashort optical pulse by means of an optical radiator downstream of the X-ray undulator. Being produced by the same electron bunch, these pulses are perfectly synchronized. Cross-correlation techniques will allow to determine relative jitter between the optical pulse (and, thus, the XFEL pulse) and a pulse from an external pump-laser. Technical realization of the proposed timing scheme uses an optical-replica synthesizer setup to be installed after the final bunch-compression stage of the XFEL for electron bunch diagnostics purposes. A number of critical issues are quantitatively analyzed.     

Investigation of self-focusing a Gaussian laser pulse in a weakly relativistic and ponderomotive regime inside a collisionless warm quantum plasma

This article investigates nonlinear self-focusing of an intense right hand circularly polarized Gaussian profile laser pulse in a weakly relativistic and ponderomotive regime inside a collisionless and unmagnetized warm quantum plasma. The nonlinear propagation equation for laser pulse in plasma has been derived. Then, the evolution differential equation for laser spot-size was obtained with considering the parabolic equation approach under the Wentzel-Kramers-Brillouin and paraxial ray approximations. This differential equation was solved numerically by fourth-order Runge-Kutta method. It is shown that our solution confirms the results of the self-focusing of the laser pulse in a weakly relativistic ponderomotive regime in cold quantum plasma in extreme conditions. Numerical results indicate that self-focusing of the laser pulse in the presence of relativistic and ponderomotive nonlinearity inside warm quantum plasma is improved in comparison with relativistic and ponderomotive cold quantum plasma.     

Pair production and optical lasers

Electron-positron pair creation in a standing wave is explored using a parameter-free quantum kinetic equation. Field strengths and frequencies corresponding to modern optical lasers induce a material polarization of the QED vacuum, which may be characterized as a plasma of e+e- quasiparticle pairs with a density of approximately 10(20) cm-3. The plasma vanishes almost completely when the laser field is zero, leaving a very small residual pair density n(r) which is the true manifestation of vacuum decay. The average pair density per period is proportional to the laser intensity but independent of the frequency nu. The density of residual pairs also grows with laser intensity but n(r) proportional to nu(2). With optical lasers at the forefront of the current generation, these dynamical QED vacuum effects can plausibly generate 5-10 observable two-photon annihilation events per laser pulse.     

Pair production from vacuum at the focus of an X-ray free electron laser

There are definite plans for the construction of X-ray free electron lasers (FEL), both at DESY, where the so-called XFEL is part of the design of the electron–positron linear collider TESLA, as well as at SLAC, where the so-called Linac Coherent Light Source (LCLS) has been proposed. Such an X-ray laser would allow for high-field science applications: one could make use of not only the high energy and transverse coherence of the X-ray beam, but also of the possibility of focusing it to a spot with a small radius, hopefully in the range of the laser wavelength. Along this route one obtains very large electric fields, much larger than those obtainable with any optical laser of the same power. In this Letter we discuss the possibility of obtaining an electric field so high that electron–positron pairs are spontaneously produced in vacuum (Schwinger pair production). We find that if X-ray optics can be improved to approach the diffraction limit of focusing, and if the power of the planned X-ray FELs can be increased to the terawatt region, then there is ample room for an investigation of the Schwinger pair production mechanism.     

Quantum processes in short and intensive electromagnetic fields

This work provides an overview of our recent results in studying two most important and widely discussed quantum processes: electron-positron pairs production off a probe photon propagating through a polarized short-pulsed electromagnetic (e.g. laser) wave field or generalized Breit–Wheeler process, and a single a photon emission off an electron interacting with the laser pules, so-called non-linear Compton scattering. We show that the probabilities of particle production in both processes are determined by interplay of two dynamical effects, where the first one is related to the shape and duration of the pulse and the second one is non-linear dynamics of the interaction of charged fermions with a strong electromagnetic field. We elaborate suitable expressions for the production probabilities and cross sections, convenient for studying evolution of the plasma in presence of strong electromagnetic fields.     

Schwinger limit attainability with extreme power lasers

High intensity colliding laser pulses can create abundant electron-positron pair plasma [A. R. Bell and J. G. Kirk, Phys. Rev. Lett. 101, 200403 (2008)], which can scatter the incoming electromagnetic waves. This process can prevent one from reaching the critical field of quantum electrodynamics at which vacuum breakdown and polarization occur. Considering the pairs are seeded by the Schwinger mechanism, it is shown that the effects of radiation friction and the electron-positron avalanche development in vacuum depend on the electromagnetic wave polarization. For circularly polarized colliding pulses, these effects dominate not only the particle motion but also the evolution of the pulses. For linearly polarized pulses, these effects are not as strong. There is an apparent analogy of these cases with circular and linear electron accelerators to the corresponding constraining and reduced roles of synchrotron radiation losses.     

量子电动力学—粒子模拟极端等离子体动力学研究

新一代拍瓦激光装置有望将激光强度提升至10²³~10²⁴ W·cm^-2,在此极端强场条件下非线性量子电动力学效应对等离子体动力学过程产生重要影响.相对论电子在强电磁场作用下会同步辐射大量伽马光子,当后者穿过超强电磁场时会级联产生正负电子对.与此同时,这些量子电动力学效应也会反作用于激光等离子体相互作用过程,如辐射阻尼严重影响电子运动过程.为了研究这样极端的等离子体动力学,我们介绍最近几年发展的量子电动力学数值模拟模块,并将其耦合到传统的粒子模拟程序中,即量子电动力学-粒子模拟程序.由于大量新辐射的光子和产生的正负电子对会造成模拟粒子数目的不断增加,我们发展了粒子融合技术来减小模拟规模.利用此量子电动力学-粒子模拟程序,我们对极端强场激光物质相互作用以及极端天体物理现象开展了数值模拟研究.     

Geodesic deviation and particle creation in curved spacetimes

A quantum mechanical picture, relating accelerated geodesic deviation to creation of massive particles via quantum tunneling in curved background spacetimes, is presented. The effect is analogous to pair production by an electric field and leads naturally to production of massive particles in de Sitter and superluminal FRW spacetimes. The probability of particle production in de Sitter space per unit volume and time is computed in a leading semiclassical approximation and shown to coincide with the previously obtained expression.     

Generation of doublet spectral lines at self-seeded X-ray FELs

Self-seeding schemes, consisting of two undulators with a monochromator in between, aim to reduce the bandwidth of SASE X-ray FELs. We recently proposed to use a new method of monochromatization exploiting a single crystal in Bragg transmission geometry for self-seeding in the hard X-ray range. The obvious and technically possible extension is to use such kind of monochromator setup with two (or more) crystals arranged in a series to spectrally filter the SASE radiation at two (or more) closely-spaced wavelengths within the FEL gain band. This allows for the production of doublet (or multiplet) spectral lines. Exploitations of such mode of operation involve any situation where there is a large change in cross-section over a narrow wavelength range. In this paper we consider the simultaneous operation of the LCLS hard X-ray FEL at two closely spaced wavelengths. We present simulation results for the LCLS baseline, and we show that this method can produce fully coherent radiation shared between two longitudinal modes. Mode spacing can be easily tuned within the FEL gain band, i.e. within 10 eV. An interesting aspect of the proposed scheme is a way of modulating the electron bunch at optical frequencies without a seed quantum laser. In fact, the XFEL output intensity contains an oscillating “mode-beat” component whose frequency is related to the frequency difference between the pair of longitudinal modes considered. Thus, at saturation one obtains FEL-induced modulations of energy loss and energy spread in the electron bunch at optical frequency. These modulations can be converted into density modulation at the same optical frequency with the help of a weak chicane installed behind the baseline undulator. Powerful coherent radiation can then be generated with the help of an optical transition radiation (OTR) station, which have important applications. In this paper we briefly consider how the doublet structure of the XFEL generation spectra can be monitored by an optical spectrometer. Furthermore, the OTR coherent radiation pulse is naturally synchronized with the X-ray pulses, and can be used for timing the XFEL to high power conventional lasers with femtosecond accuracy for pump-probe applications.     

Electron-positron pair production in a strong asymmetric laser electric field

By solving the quantum Vlasov equation, electron positron pair production in a strong electric field with asymmetric laser pulses has been investigated. We consider three different situations of subcycle, cycle and supercycle laser pulses. It is found that in asymmetric laser pulse field, i.e.. when the pulse length of one rising or falling side is fixed while the pulse length of the other side is changed, the pair production rate and mnnber density can be significantly modified comparable to symmetric situation. For each ca,se of these three different cycle pulses, when one side pulse length is constant and the other side pulse length becomes shorter, i.e., the whole pulse is compressed, the more pairs can be produced than that in tile vice versa case, i.e., the whole pulse is elongated. In compressed pulse case there exists an optimum pulse length ratio of asylnmetric pulse lengths which makes the pair number density maximunn. Moreover, the created maximum pair number density by subcycle pulse is larger than that by cycle or/and supercycle pulse. In elongated pulse case, however, only for supercycle laser pulse the created pairs is enhanced and there exists also an optimum asymmetric pulse length ratio that maximizes the pair number density. On the other hand. surprisingly, in both cases of subcycle and cycle elongated laser pulses, the pair number density is monotonically decreasing as the asymmetry of pulse increases.     

上海激光康普顿散射伽马源的发展和展望

激光康普顿散射(Laser Campton Scattering, LCS)光源,是一种基于相对论电子束与激光光子相互作用的新型X-ray或Gamma-ray光源。它具有能量高、波长短、脉冲快和峰值亮度高的特性,已成为国际先进光源技术的重要选项之一。本文介绍了激光康普顿散射光源的产生原理、国内外发展现状以及目前国际上运行和在建的激光康普顿散射光源装置,其中重点介绍了上海光源二期正在建设的上海激光电子伽马源(Shanghai Laser Electron Gamma Source, SLEGS)装置,以及在这一光源装置上可以开展的核物理、核天体物理、核废料处理及核医学应用等研究。随着上海软X射线自由电子激光试验装置(Soft X-ray Free Electron Laser, SXFEL)升级为用户装置,以及未来十三五国家重大科技基础设施-硬X射线自由电子装置(Shanghai HIgh repetition rate XFEL aNd Extreme light facility,SHINE)的建设完成,基于直线电子加速器(LINear ACcelator, LINAC)的康普顿散射光源的伽马能量将会达到GeV量级的高能量。超短脉冲、高极化度、高通量的激光康普顿散射光源将迎来新的发展机遇,基于康普顿伽马光源的核物理、天体物理、粒子物理及应用基础研究也必将迈上一个新台阶。     

光泵三能级激光的全量子理论

本文采用Langevin量子理论处理光泵三能级激光体系。在激光场的光子数不太高时导出一个普适的激光场Van der Pol运动方程。它既包括了非相干和相干泵浦,又包括了强泵浦和弱泵浦情况。若对热库取平均,该方程将给出半经典理论的全部结果。本文的结果表明,激光增益来源于两个基本过程:两个光子过程和两步过程。当泵浦强度大于某阈值时,将出现动态斯塔克效应。激光线宽主要来源于热噪声、真空涨落、激光能级的自发辐射以及自发Raman散射。 关键词：     

Ultrarelativistic electron-positron plasma

Ultrarelativistic electron-positron plasmas can be produced in high-intensity laser fields and play a role in various astrophysical situations. Their properties can be calculated using QED at finite temperature. Here we will use perturbative QED at finite temperature for calculating various important properties, such as the equation of state, dispersion relations of collective plasma modes of photons and electrons, Debye screening, damping rates, mean free paths, collision times, transport coefficients, and particle production rates, of ultrarelativistic electron-positron plasmas. In particular, we will focus on electron-positron plasmas produced with ultra-strong lasers.     

Approximated Vector Potential of Intense Laser Pulses in a Pair Plasma

A （3＋1 ）-dimensional Kadomtse-Petviashvili （KP） equation for nonlinearly interacting intense laser pulses with an electron-positron （e-p） plasma is derived. Taking into account the combined action of the relativistic particle mass increase and the relativistic light ponderomotive force, using the perturbation method, and allowing different types solution, we discuss the analytical solution of （3＋1）-dimensional KP-I equation, and give the approximate solutions of vector potential of the intense laser pulse in e-p plasma. Our results may be significantly useful in understanding the nonlinear wave propagation and interaction of intense laser beams in an e-p plasma.     

含时量子蒙特卡罗方法研究两电子原子在强激光作用下电子的动力学行为

通过改进含时量子蒙特卡罗方法研究了一维模型双电子原 子在强激光作用下的电子动力学过程. 与准确的数值积分求解含时薛定谔方程相比, 计算得到的波包对应的量子系综中粒子的动力学变化和含时波包演化结果定性一致, 且大幅度地提高了计算效率. 根据计算得到的经典粒子系综的动力学演化行为, 分析了原子在强激光作用下的激发、电离、重散射等非线性过程.     

Polarized inelastic electromagnetic processes in crystals with allowance for a new real distribution of nucleus charges and atomic electrons

The process of polarized electron-positron pair photoproduction via a polarized γ quantum in a crystalline medium is investigated using a new method that considers the distribution of nucleus charges and atomic electrons. Real changes in the cross sections of both polarized and nonpolarized electron pair production, and in the degrees of particle polarization in silicon crystal, are obtained.     

Dense pair plasma generation by two laser pulses colliding in a cylinder channel

An all-optical scheme for high-density pair plasmas generation is proposed by two laser pulses colliding in a cylinder channel. Two dimensional particle-in-cell simulations show that, when the first laser pulse propagates in the cylinder,electrons are extracted out of the cylinder inner wall and accelerated to high energies. These energetic electrons later run into the second counter-propagating laser pulse, radiating a large amount of high-energy gamma photons via the Compton back-scattering process. The emitted gamma photons then collide with the second laser pulse to initiate the Breit–Wheeler process for pairs production. Due to the strong self-generated fields in the cylinder, positrons are confined in the channel to form dense pair plasmas. Totally, the maximum density of pair plasmas can be 4.60 × 10~(27)m~(-3), for lasers with an intensity of 4×10~(22)W·cm~(-2). Both the positron yield and density are tunable by changing the cylinder radius and the laser parameters. The generated dense pair plasmas can further facilitate investigations related to astrophysics and particle physics.