Laser‐Triggered Proton Acceleration From Micro‐Structured thin Targets

By using relativistic massively parallel PIC code MANDOR, which features arbitrary target design including 3D micro‐structuring, a study of ion acceleration in short laser pulse interaction with different thin targets has been performed. Based on 3D simulation results it has been shown that micro‐structures on the front surface of thin plane targets increase a number and energy of hot electrons in comparison with that for the case of pure plain foils of optimal thickness. As a result, the energy of accelerated ions also increases up to 50%. However, the efficiency of ion acceleration from structured target reduces with laser pulse intensity increase, so that for laser pulses of ultra‐relativistic intensity a positive role of surface micro‐structuring diminishes. We have also studied to which extent a sub‐ps imperfection of the laser pulse shape, which smoothes the surface micro‐structures suppresses high‐energy ion generation. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observation of Raman forward scattering and electron accelerationin the relativistic regime

Raman forward scattering (RFS) is observed in the interaction of a high intensity (>10¹⁸ W/cm²) short pulse (<1 ps) laser with an underdense plasma (n_e~10¹⁹ cm ^-3). Electrons are trapped and accelerated up to 44 MeV by the high-amplitude plasma wave produced by RFS. The laser spectrum is strongly modulated by the interaction, showing sidebands at the plasma frequency. Furthermore, as the quiver velocity of the electrons in the high electric field of the laser beam becomes relativistic, various effects are observed which can be attributed to the variation of electron mass with laser intensity     

Novel features of non-linear Raman instability in a laser plasma

The electron phase space evolution in a non-relativistic and homogeneous laser plasma generated by a nanosecond laser in a near infrared region in the presence of stimulated Raman scattering is investigated by a numerical simulation. The mechanism of electron acceleration in the potential wells of the plasma wave accompanying the Raman back-scattering is analyzed in a 1D Vlasov-Maxwell model. The dominant wave modes are both the backward and the forward propagating Raman waves, each accompanied by a daughter electrostatic wave. In addition to a strong interaction of plasma electrons with the primary electrostatic wave in the case of back-scattering, a cascading is observed consisting in a secondary scattering of the primary Raman back-scattered wave. This phenomenon reduces the Raman reflectivity and causes an acceleration of electrons against the direction of the heating laser beam. Moreover, the strong trapping in the primary electrostatic wave generated by the Raman back-scattering leads due to the trapped particle instability to a significant spectral broadening of the original plasma wave and a subsequent intermittent behaviour of the scattering process. The high phase velocity electrostatic daughter wave of the forward Raman scattering cannot trap the electrons directly, but there is an indication of non-resonant quasi-modes combined of this wave and of the simultaneously existing electrostatic daughter wave accompanying the Raman back-scattering. The transform method is used for a solution of the set of partial differential equations, which consists of the Vlasov equation and of the full set of Maxwell equations in a 1D approximation. A simplified Fokker-Planck collision term is added to overcome the numerical instabilities during the simulation. The model has relevance to a long scale plasma geometry, such as occurring in the indirect drive experiments near the light entrance holes of target hohlraum.     

激光等离子体波电子加速器

本文分别用理论分析和粒子模拟方法讨论了等离子体尾波加速器和拍波加速器的物理机制。结果表明,只要激光等离子体波足够强,加上适当强度的横向磁场,就可以把MeV数量级的电子在公尺距离内加速到GeV数量级的能量。另外,还用粒子模拟方法,研究了激光对热等离子体受激向后喇曼散射产生低相速度的等离子体静电波对低能电子加速的问题,探讨了多级或多波加速的可能性。结果表明,利用激光等离子体波加速器,在一般的实验室条件下,就可获得GeV数量级的高能电子。 关键词：     

Dynamics of quadruple laser pulses in under‐dense plasmas

An analysis of dynamics of a quadruple laser pulse propagating through an under‐dense plasma is presented. The Drude model is used to derive the dielectric function of the plasma for relativistic non‐linearity in the electron mass. An approximate numerical solution of the nonlinear Schrödinger wave equation for the field of the laser beam is obtained with the help of the moment theory approach in the Wentzel–Kramers–Brillouin (WKB) approximation. Particular emphases are placed on the variations of spot size, pulse width, and longitudinal phase delay with the distance of propagation through the plasma. Self‐trapping of the laser pulse is also investigated.     

电子在激光驻波场中运动产生的太赫兹及X射线辐射研究

采用单电子模型和经典辐射理论分别对低能和高能电子在线偏振激光驻波场中的运动和辐射过程进行了研究. 结果表明: 垂直于激光电场方向入射的低速电子在激光驻波场中随着光强的增大, 逐渐从一维近周期运动演变为二维折叠运动, 并产生强的微米量级波长的太赫兹辐射; 高能电子垂直或者平行于激光电场方向入射到激光驻波场中, 都会产生波长在几个纳米的高频辐射; 低能电子与激光驻波场作用中, 激光强度影响着电子的运动形式、辐射频率以及辐射强度; 高能电子入射时, 激光强度影响了电子高频辐射的强度, 电子初始能量影响着辐射的频率; 电子能量越高, 产生的辐射频率越大. 研究表明可以由激光加速电子的方式得到不同能量的电子束, 并利用电子束在激光驻波场的辐射使之成为太赫兹和X射线波段的小型辐射源. 研究结果可以为实验研究和利用激光驻波场中的电子辐射提供依据.     

Collective properties of a relativistic electron beam injected into a high intensity optical lattice

Behaviour of a relativistic electron bunch, injected and trapped in a high intensity optical lattice resulting from the interference of two laser beams is studied. The optical lattice modifies the phase space distribution of the electron bunch due to the trapping and compression of the electrons by a ponderomotive force. High-frequency longitudinal beam eigenmodes of the trapped electron bunch are described in the framework of fluid and kinetic models. Such beam oscillations are expected to play a pivotal role in a stimulated Raman scattering of laser beams on the electrons.     

The nonlinear interaction of waves generated by the stimulated Raman scattering with plasma in the PALS experiment

This paper is devoted to the study of the nonlinear interaction of the waves generated by stimulated Raman scattering in plasma. The influence of nonlinear interaction of plasma wave with plasma electrons on the sum of action densities of waves generated by the laser wave is solved. The electron acceleration in the forward and backward wave fields is described. The change of the electric field of the quasimode of forward and backward plasma waves of Raman scattering given by trapping of plasma particles is calculated. Numerical results are calculated for typical parameters of the PALS experiment.     

On the theory of the acceleration of plasma electrons during stimulated scattering of an intense laser wave

The mechanism of electron acceleration in the stimulated Raman forward scattering of a monochromatic laser wave in a cold plasma is investigated theoretically. It is shown that as a result of the stochastic interaction of the electrons with the ponderomotive wave and with plasma waves excited in the scattering process, some of the electrons are accelerated to relativistic energies. Zh. Tekh. Fiz. 69, 3–8 (January 1999)     

利用超快电子探针诊断受激拉曼散射静电波的数值模拟

利用二维粒子模拟程序EPOCH验证了超快电子束探针诊断受激拉曼散射产生的静电波的可行性。结果表明,电子束探针穿过静电波电场后会在电子束探针的横向上产生密度调制,密度调制呈周期性分布且沿静电波的传播方向移动,密度调制的波数对应静电波的波数且移动速度对应静电波的相速度,因此特定条件下可用于反推电子的温度、密度等信息。在诊断静电波的过程中,电子束探针的束长必须小于静电波的波长或者诊断设备的曝光时间必须小于静电波的周期。本研究提供了一种新型的直接诊断静电波和电子温度、密度的方法,对于推动受激拉曼散射等激光等离子体不稳定性的实验研究具有重要意义。     

Acceleration of nonmonoenergetic electron bunches injected into a wake wave

The trapping and acceleration of nonmonoenergetic electron bunches in a wake field wave excited by a laser pulse in a plasma channel is studied. Electrons are injected into the region of the wake wave potential maximum at a velocity lower than the phase velocity of the wave. The paper analyzes the grouping of bunch electrons in the energy space emerging in the course of acceleration under certain conditions of their injection into the wake wave and minimizing the energy spread for such electrons. The factors determining the minimal energy spread between bunch electrons are analyzed. The possibility of monoenergetic acceleration of electron bunches generated by modern injectors in a wake wave is analyzed.     

On the production of flat electron bunches for laser wakefield acceleration

We suggest a novel method for the injection of electrons into the acceleration phase of particle accelerators, producing low-emittance beams appropriate even for the demanding high-energy linear collider specifications. We discuss the injection mechanism into the acceleration phase of the wakefield in a plasma behind a high-intensity laser pulse, which takes advantage of the laser polarization and focusing. The scheme uses the structurally stable regime of transverse wakewave breaking, when the electron trajectory self-intersection leads to the formation of a flat electron bunch. As shown in three-dimensional particle-in-cell simulations of the interaction of a laser pulse elongated in the transverse direction with an underdense plasma, the electrons injected via the transverse wakewave breaking and accelerated by the wakewave perform betatron oscillations with different amplitudes and frequencies along the two transverse coordinates. The polarization and focusing geometry lead to a way to produce relativistic electron bunches with an asymmetric emittance (flat beam). An approach for generating flat laser-accelerated ion beams is briefly discussed. The text was submitted by the authors in English.     

Electron acceleration by a short relativistic laser pulse at the front of solid targets

Acceleration of electrons in a low-density plasma in front of a solid target by a propagating short ultraintense laser pulse is studied. When the laser is reflected at the target surface the accelerated electrons, with energy scaling as the laser intensity, continue to move forward inertially and thus escape from the pulse. Electrons accelerated backwards by the reflected light can attain even higher energies due to their longer acceleration length and their high initial momentum from a relativistic return current.     

周期量级激光脉冲的Thomson散射

用经典辐射理论对线偏振周期量级激光脉冲的线性Thomson散射进行分析，从理论上得到它可产生亚阿秒脉冲的结论. 计算显示，在电子相对论因子为50、激光脉冲中心波长为1μm、归一化光场强度为0.01的情况下，用包含1.5个光周期的激光脉冲，可获得0.2as(半高全宽)的散射脉冲输出. 还对光场载波包络初相φ_ce和电子进入光场的初相φ_in对散射脉冲的影响作了分析讨论，结果表明，在适当的φ_ce和φ_in条件下，能实现单个阿秒脉冲输出，并可对脉冲宽度和频率进行调谐. 关键词： 线性Thomson散射 周期量级激光脉冲 载波包络初相 阿秒脉冲     

Investigation of a channeling high-intensity laser beam inunderdense plasmas

The interaction of an intense short pulse laser (>5×10 ¹⁸ Wcm^-2) with underdense plasma was extensively studied. The beam is found to be highly susceptible to the forward Raman scattering instability. At sufficiently high growth rates, this can lead to wavebreaking with the resultant production of a high flux of accelerated electrons (>10¹¹ for E>2 MeV). Some electrons are found to be accelerated well above the dephasing energy, up to 94 MeV. Self-scattered images intimate the presence of high-intensity channels that extend more than 3.5 mm or 12 Rayleigh lengths. These filaments do not follow the axis of laser propagation, but are seen to be emitted within an f4 cone centered around this axis. Spectra of the self-scattered light show that the main contribution of the scattering is not from light captured within these filaments. But there is evidence for self-phase modulation from effects such as ionization and relativistic self-focusing. However, no clear correlation is observed between channel length and the number or energies of accelerated electrons. Evidence for high intensities within the channels is given by small-angle Thomson scattering of the plasma wave generated therein, with this method, the intensity is found to be of the order of 10¹⁸ Wcm^-2 greater than 12 Rayleigh lengths from focus     

锥形激光等离子体中Compton 散射对电子的加速

应用相对论性电子与光子非弹性碰撞模型和经典相对论电动力学理论,结合锥形飞秒强激光等离子体中的光场特性和静电场能,分析、计算了入射的高能电子束与等离子体中的光子发生多光子非线性Compton散射时对电子的加速效应,发现等离子体中的光场会引起电子加速能量的振荡;等离子体中的静电场降低电子的加速效应。用高能电子束与锥形飞秒强激光等离子体中的光子发生双光子非线性Compton散射,是加速电子最为理想的情况。     

Second Harmonic Generation of Self‐Focused Cosh‐Gaussian Laser Beam in Thermal Quantum Plasma by Excitation of an Electron Plasma Wave

This paper presents a scheme for second harmonic generation (SHG) of an intense Cosh‐Gaussian (ChG) laser beam in thermal quantum plasmas. Moment theory approach in W.K.B approximation has been adopted in deriving the differential equation governing the propagation characteristics of the laser beam with distance of propagation. The effect of relativistic increase in electron mass on propagation dynamics of laser beam has been incorporated. Due to relativistic nonlinearity in the dielectric properties of the plasma, the laser beam gets self‐focused and produces density gradients in the transverse direction. The generated density gradients excite electron plasma wave (EPW) at pump frequency that interacts with the incident laser beam to produce its second harmonics. Numerical simulations have been carried out to investigate the effects of laser parameters on selffocusing of the laser beam and hence on the conversion efficiency of its second harmonics. Simulation results predict that within a specific range of decentered parameter the ChG laser beams show smaller divergence as they propagate and, thus, lead to enhanced conversion efficiency of second harmonics. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Generation of a quasimonoenergetic electron beam using a single laser pulse

The results of experiments are presented for a single laser pulse interaction with a very low density gas target under conditions when the generated wake wave is below the wave-breaking threshold and the laser pulse power is lower than the critical power for relativistic self-focusing. A quasimonoenergetic electron beam is found to be stably generated for various laser pulse intensity values by controlling the acceleration length.     

Optically controlled seeding of Raman forward scattering and injection of electrons in a self-modulated laser-wakefield accelerator

Optical seeding of plasma waves and the injection of electrons are key issues in self-modulated laser-wakefield accelerators. By implementing a copropagating laser prepulse with proper timing, we are able to control the growth of Raman forward scattering and the production of accelerated electrons. The dependence of the Raman intensity on prepulse timing indicates that the seeding of Raman forward scattering is dominated by the ionization-induced wakefield, and the dependence of the divergence and number of accelerated electrons further reveals that the stimulated Raman backward scattering of the prepulse plays the essential role of injecting hot electrons into the fast plasma wave driven by the main pulse.     

Pulse chirping effect on controlling the transverse cavity oscillations in nonlinear bubble regime

The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λp, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier–envelope phase(CEP) breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional(2D) particle-in-cell(PIC) simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.