Dense electron-positron plasmas and ultraintense γ rays from laser-irradiated solids

In simulations of a 10 PW laser striking a solid, we demonstrate the possibility of producing a pure electron-positron plasma by the same processes as those thought to operate in high-energy astrophysical environments. A maximum positron density of 10(26) m(-3) can be achieved, 7 orders of magnitude greater than achieved in previous experiments. Additionally, 35% of the laser energy is converted to a burst of γ rays of intensity 10(22) W cm(-2), potentially the most intense γ-ray source available in the laboratory. This absorption results in a strong feedback between both pair and γ-ray production and classical plasma physics in the new "QED-plasma" regime.     

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。     

Near QED regime of laser interaction with overdense plasmas

Interaction of laser plulses with intensities up to 10²⁵?W/cm² with overdense plasma targets is investigated via three-dimensional particle-in-cell simulations. At these intensities, radiation of electrons in the laser field becomes important. Electrons transfer a significant fraction of their energy to γ-photons and obtain strong feedbacks due to radiation reaction (RR) force. The RR effect on the distribution of laser energies among three main species: electrons, ions and photons is studied. The RR and electron-positron pair creation are implemented by a QED model. As the laser intensity inreases, the ratio of laser energy coupled to electrons drops while the one for γ-photons reaches up to 35%. Two distinctive plasma density regimes of the high-density carbon target and low-density solid hydrogen target are identified from the laser energy partitions and angular distributions of photons. The power-laws of absorption efficiency versus laser intensity and the transition of photon divergence are revealed. These show enhanced generation of γ-photon beams with improved collimation in the relativistically transparent regime. A new effect of transverse trapping of electrons inside the laser field caused by the RR force is observed: electrons can be unexpectedly confined by the intense laser field when the RR force is comparable to the Lorentz force. Finally, the RR effect and different regions of photon emission in laser-foil interactions are clarified.     

激光参数对非理想真空激光光强极限的影响

人类在实验室可实现的激光强度极限是强场量子电动力学（QED）的重要问题。在非理想真空条件下,极端超强激光与残留的电子相互作用触发伽马光子辐射与正负电子对产生的QED级联效应,从而显著消耗激光能量,大幅降低可实现的激光峰值强度。考虑到QED级联效应与激光偏振、焦斑尺寸、脉宽长度有着密切的关系,基于囊括QED过程的粒子网格模拟方法（Particle-in-cell, PIC）对上述参数的效应进行分析,同时构建了激光场演化的自洽方程来进行解释,二者结果基本保持一致,获得的强度极限在考虑的参数范围内为1026～1027 W/cm?2。结果表明,同等情形下,圆偏振激光可激发更强的QED级联,使得激光强度上限略低于线偏振。此外,紧聚焦激光由于QED级联发生的时空间尺度更小,从而激光的吸收效应被显著抑制,进而可以实现更强的聚焦强度。对于更长脉宽的激光,由于正负电子对吸收的能量区域更加弥散,使得可实现的激光强度上限阈值有所提升。但对于超短脉宽情形（如单周期）,由于QED级联的种子源电子束不能很好地被约束在激光区域,理论分析耗散的激光能量偏高。此外,在高真空度的情形下,残余电子的随机性也会对可实现激光强度产生一定的影响。研究结果可为后续开展极端强场QED实验和数100 PW级超强超短激光装置建设提供指导。     

Absorption of gamma-ray photons in a vacuum neutron star magnetosphere: I. Electron-positron pair production

The production of electron-positron pairs in a vacuum neutron star magnetosphere is investigated for both low (compared to the Schwinger one) and high magnetic fields. The case of a strong longitudinal electric field where the produced electrons and positrons acquire a stationary Lorentz factor in a short time is considered. The source of electron-positron pairs has been calculated with allowance made for the pair production by curvature and synchrotron photons. Synchrotron photons are shown to make a major contribution to the total pair production rate in a weak magnetic field. At the same time, the contribution from bremsstrahlung photons may be neglected. The existence of a time delay due to the finiteness of the electron and positron acceleration time leads to a great reduction in the electron-positron plasma generation rate compared to the case of a zero time delay. The effective local source of electron-positron pairs has been constructed. It can be used in the hydrodynamic equations that describe the development of a cascade after the absorption of a photon from the cosmic gamma-ray background in a neutron star magnetosphere.     

Electron-positron pair creation by a strong tightly focused laser field

The analytical calculation of the electron-positron pair creation yield by an extremely strong focused laser pulse in the vacuum is presented. In particular, dependence of the total yield of the pair production on the focusing parameter and the polarization properties of the laser pulse are analyzed. Comparison with the previous consideration of N.B. Narozhny, V.S. Popov, V.D. Mur, and S.S. Bulanov is given and discussed. As a result, we confirm their conclusion that the electron-positron pair production can be observed in principle at the level of intensity of 10²⁷ W/cm².     

Electron-positron pair creation in the electric fields generated by micro-bubble implosions

We show that electron-positron pair production from the vacuum is possible via the strong Coulomb fields generated by micro-bubble implosions induced by ultra-high intensity lasers. Even in the case where the Coulomb fields are lower than the pair creation threshold, externally injected high energy electrons or photons could be used to generate pairs.     

超强线极化激光在e-p等离子体中传播的成丝不稳定性

基于超强线极化激光在正负电子对(e-p)等离子体中的传播方程,讨论了相互作用过程中的成丝不稳定性。得到了电磁波的非线性色散关系和不稳定性增长率,并讨论了传播过程中激光强度和等离子体温度对成丝不稳定性的影响。结果表明,成丝不稳定性的强弱主要由相互作用的非线性效应与受有质动力作用后等离子体密度的减小之间的竞争所决定。     

极强激光场驱动超亮伽马辐射和正负电子对产生的研究进展

高功率超短超强激光脉冲的诞生开启了相对论非线性光学、高强场物理、新型激光聚变、实验室天体物理等前沿领域.近年来,随着数拍瓦级乃至更高峰值功率激光装置的建成,超强激光与等离子体相互作用进入到一个全新的高强场范畴.这种极强激光场与等离子体相互作用蕴含着丰富的物理过程,除了经典的波与粒子作用、相对论效应、有质动力效应等非线性物理过程外,量子电动力学(QED)效应变得格外重要,例如辐射阻尼效应、正负电子对产生、强伽马射线辐射、QED级联、真空极化等.本文主要介绍我们近年来在极端强激光场与等离子体相互作用中激发的QED效应以及伴随的超亮强伽马射线辐射和稠密正负电子对产生等方面的研究进展.     

Efficient propagation of polarization from laser photons to positrons through compton scattering and electron-positron pair creation

We have demonstrated for the first time the production of highly polarized short-pulse positrons with a finite energy spread in accordance with a new scheme that consists of two-quantum processes, such as inverse Compton scattering and electron-positron pair creation. Using a circularly polarized laser beam of 532 nm scattered off a high-quality, 1.28 GeV electron beam, we have obtained polarized positrons with an intensity of 2 x 10(4) e+ /bunch. The magnitude of positron polarization has been determined to be 73 +/- 15(stat) +/- 19(syst)% by means of a newly designed positron polarimeter.     

Photon bremsstrahlung from an extreme relativistic hydrogen plasma

The effect of the electron-positron pair creation on the intensity of radiation by an extreme relativistic plasma is studied. The degeneracy of the electron and positron gases is taken into account. It is found that the pair creation is of considerable importance for both the quantity and temperature dependence of the radiation intensity.Dedicated to Professor V. Votruba on his sixtieth birthday.     

Analytical Study of Pair Production Rate from Vacuum in an Elliptic Polarized Field by a Two-Level Transition Technique

Electron-positron pair production rate created from vacuum in the presence of an elliptically polarized laser field is investigated.By applying the technique of two level transition amplitude,a routine for obtaining pair production rate is presented,and approximate analytical expressions are given both for the low frequency strong field regime and the high frequency weak field regime.We found that for an elliptically polarized field,the electron-positron pair production rate decrease when the elliptic eccentricity increase in the high frequency weak field regime,however,in the low frequency strong field regime,there is almost the same electron-positron pair production rate as in the constant electric field case.     

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.     

强激光场中真空极化效应

 通过求解狄拉克方程,对强激光场下真空极化问题进行了研究。理论计算结果表明：在仅随时间变化的电场下,要激发狄拉克海中负能级的电子,需要两个阈值条件,即激光场的电场强度大于等于10¹⁶ V/cm和激光场的持续时间大于等于10^-21 s。前者主要保证负能态电子有足够的能量跃迁到正能态,后者主要是保证电子在跃迁过程中动量亏损得以补偿。     

Modulation instability of intense laser beam in an electron-positron plasma

Modulation instability of an intense right-hand elliptically polarized laser beam propagating through an electron-positron plasma is investigated by a new method. The nonlinear dispersion relation, in which the relativistic and ponderomotive nonlinearities are taken into account, is obtained for the laser radiation in electron-positron plasma by the Lorentz transformation. The Karpman equation is generalized to the case of three dimensions with three field components. When the nonlinear frequency shift of the electromagnetic field in plasma is involved, the nonlinear evolution equation for the slowly varying envelope of the laser field is obtained. Thus, modulation instability of the intense laser beam in electron-positron plasma is studied and the temporal growth rate of the instability is derived. The analysis shows that the growth rate of modulation instability is increased significantly near the critical surface in a laser-plasma.     

强激光等离子体耦合效应的数值模拟

 研究了高强度（10¹²(10^{14W/cm2）,纳秒脉冲(高斯型)激光与AI、 CH等离子体的耦合效应。采用一维双温、单流体力学方程组,数值模拟研究激光强度和波长对靶表面能量沉积和对等离子体特征参数的影响。激光等离子体耦合的主要机制有:轫致辐射、逆轫致辐射吸收、热扩散和电子、离子之间碰撞能量交换。给出了电子最高温度与光强的近似定量关系。}     

Seed and vacuum pair production in strong laser field

Researches on the electron-positron pair production in the presence of the intense laser field are reviewed, motivated by the theoretical importance of the nonperturbative QED problem and the worldwide development of high power laser facilities. According to the distinct experimental requirements and theoretical methods, two types of pair production are elaborated, which are, respectively, pair production in the combination of a seed particle and the strong laser, and vacuum pair production without a seed particle. The origin of the nonperturbative problem caused by the strong field is analyzed. The main ideas, realisation, achievements, validity, challenges and bottleneck problems of the nonperturbative methods developed for each type of the pair production problem are discussed.     

Bi靶激光等离子体极紫外光源以及碎屑辐射特性研究

激光等离子体极紫外光源具有体积小、稳定性高和输出波长可调节等优势,在极紫外光刻领域发挥着重要的作用。Bi靶激光等离子体极紫外光源在波长9～17 nm范围内具有较宽的光谱,可应用于制造极紫外光刻机过程中所需的极紫外计量学领域。利用平像场光谱仪和法拉第杯对Bi靶激光等离子体极紫外光源以及离子碎屑辐射特性进行了实验研究。在单脉冲激光打靶条件下,实验中观察到Bi靶激光等离子极紫外光谱在波长12.3 nm处出现了一个明显的凹陷,其对应着Si L-edge的吸收,是Bi元素光谱的固有属性。相应地在波长为11.8和12.5 nm位置处产生了两个宽带的辐射峰。研究了两波长光谱特性以及辐射强度随激光功率密度的变化。结果表明,在改变聚焦光斑大小实现不同激光功率密度（0.7×1010～3.1×1010 W·cm-2）过程中,当功率密度为2.0×1010 W·cm-2时两波长处的光辐射最强,其原因归结为Bi靶极紫外光辐射强度受激光能量用于支撑等离子膨胀的损失和极紫外光被等离子体再吸收之间的平衡制约所致。在改变激光能量实现不同激光功率密度过程中,由于烧蚀材料和产生两波长所需高阶离子随着功率密度的增加而增加,增强了两波长处的光辐射。进一步,研究了双脉冲激光对Bi靶极紫外光谱辐射特性影响,实验发现双脉冲打靶下原来在单脉冲打靶时出现在波长13～14 nm范围内的凹陷消失。最后,对单脉冲激光作用Bi靶产生极紫外光源碎屑角分布进行了测量。结果表明,当探测方向从靶面法线方向移动到沿着靶面方向上的过程中,探测到Bi离子动能依次减小,并且离子动能随激光脉冲能量降低而呈线性减小。此项研究有望为我国在研制极紫外光刻机过程所需的计量学领域提供技术支持和打下夯实的基础。     

Interference effect in the photoproduction of electron-positron pairs on a nucleus in the field of two light waves

In the general relativistic case, the nonresonance photoproduction of electron-positron pairs on a nucleus in the field of two circularly polarized light waves propagating in one direction is theoretically investigated. It is shown that there are two essentially different kinematical regions: the noninterference region, where the Bunkin-Fedorov quantum parameters play the role of multiphoton parameters, and the interference region, where interference effects are important and where quantum interference parameters are treated as multiphoton parameters. The interference effect is found in the photoproduction of an electron-positron pair on a nucleus in the field of two light waves. This effect occurs in the interference region and is due to the production of an electron-positron pair in the plane spanned by the light wave vector and the incident-photon momentum and to stimulated correlated emission and absorption of photons of the two waves. The cross sections for pair photoproduction on a nucleus in the above kinematical regions are determined for moderately strong fields. It is shown that, in the interference region, the partial cross sections for the case where there is a correlated emission (absorption) of the same number of photons of the two waves can significantly exceed the corresponding cross sections in any other geometry.     

Solitary wave propagation in quantum electron-positron plasmas

Existence of large amplitude stationary solitary wave structures in an unmagnetized electron-positron (e-p) plasma is studied using a quantum hydrodynamic (QHD) model that includes the quantum force (tunnelling) associated with the Bohm potential and the Fermi-dirac pressure law. It is found that in a quasi-neutral pair (e-p) plasma, where the dispersion is only due to the the quantum tunnelling effects, the large amplitude stationary solitary structure exists only when the normalized Mach speed,M <√2. Such solitary structures do not exist in absence of the Bohm potential term in an unmagnetized quasineutral pair (e-p) plasma. The system is shown to support only rarefactive stationary solitary waves. For such waves the amplitude, being independent of the quantum parameter H (the ratio of the electron plasmon to electron Fermi energy), decreases with the Mach number M, whereas the width increases with both M and H. The present theory is applicable to analyze the formation of localized coherent solitary structures at quantum scales in dense astrophysical objects as well as in intense laser fields.