Backscatter reduction using combined spatial, temporal, and polarization beam smoothing in a long-scale-length laser plasma

Spatial, temporal, and polarization smoothing schemes are combined for the first time to reduce to a few percent the total stimulated backscatter of a NIF-like probe laser beam (2x10(15) W/cm (2), 351 nm, f/8) in a long-scale-length laser plasma. Combining temporal and polarization smoothing reduces simulated Brillouin scattering and simulated Raman scattering (SRS) up to an order of magnitude although neither smoothing scheme by itself is uniformly effective. The results agree with trends observed in simulations performed with the laser-plasma interaction code F3D simulations [R. L. Berger et al., Phys. Plasma 6, 1043 (1999)].     

Driven spatially autoresonant stimulated Raman scattering in the kinetic regime

The autoresonant behavior of Langmuir waves excited by stimulated Raman scattering (SRS) is clearly identified in particle-in-cell (PIC) simulations in an inhomogeneous plasma. As previously shown via a 3-wave coupling model [T. Chapman et al., Phys. Plasmas 17, 122317 (2010)], weakly kinetic effects such as trapping can be described via an amplitude-dependent frequency shift that compensates the dephasing of the resonance of SRS due to the inhomogeneity. The autoresonance (AR) leads to phase locking and to growth of the Langmuir wave beyond the spatial amplification expected from Rosenbluth's model in an inhomogeneous profile [M. N. Rosenbluth, Phys. Rev. Lett. 29, 565 (1972)]. Results from PIC simulations and from a 3-wave coupling code show very good agreement, leading to the conclusion that AR arises even beyond the so-called weakly kinetic regime.     

Trapped-particle instability leading to bursting in stimulated Raman scattering simulations

Nonlinear, kinetic simulations of stimulated Raman scattering (SRS) under laser-fusion conditions present a bursting behavior. Different explanations for this regime have been given in previous studies: saturation of SRS by increased nonlinear Landau damping [K. Estabrook et al., Phys. Fluids B 1, 1282 (1989)]], and detuning due to the nonlinear frequency shift of the plasma wave [H. X. Vu et al., Phys. Rev. Lett. 86, 4306 (2001)]]. Another mechanism, also assigning a key role to the trapped electrons is proposed here: the breakup of the plasma wave through the trapped-particle instability.     

Laser-beam smoothing induced by stimulated brillouin scattering in an inhomogeneous plasma

We present results of two-dimensional simulations of the stimulated Brillouin scattering (SBS) of an optically smoothed laser beam propagating in an expanding plasma. In the weak damping limit, both backscattered and transmitted light waves exhibit an additional spatiotemporal incoherence, which is shown to be induced by SBS taking place in an inhomogeneous plasma. This mechanism is not related to laser-beam self-focusing and is thus complementary to plasma-induced smoothing [A. J. Schmitt and B. B. Afeyan, Phys. Plasmas 5, 503 (1998)]. The incoherence induced by SBS in the entrance part of the plasma could reduce the growth of parametric instabilities developing further inside the plasma and is able to significantly enlarge the spreading angle of the transmitted light. The angular width of the backscattered light is also found to be significantly larger than the aperture angle of the incident beam.     

驱动激光束间相干性以及对背向散射影响的研究

早期的研究表明, 束匀滑技术能够有效地抑制高功率激光与等离子体相互作用中产生的各种参量不稳定性, 大大减少受激布里渊散射和受激拉曼散射. 但在NIF最近的实验研究中发现, 受激布里渊散射和受激拉曼散射份额远远高于预期, 原因可能与驱动激光束间的相干性有关. 本文利用"神光II"装置两路倍频纳秒激光辐照金平面靶, 研究了小口径取样情况下两者背向散射的能量和光谱在不同驱动激光相干条件下的变化情况. 初步结果表明, 激光束间存在着较强的相干性, 并且随着束间相干程度的增加, 背向散射也逐渐增强.     

三倍频Nd∶YAG激光抽运氧气中的受激拉曼和布里渊散射

报道了三倍频脉冲Nd∶YAG激光(355 nm)在两种不同带宽模式下抽运氧气中受激拉曼散射(SRS)和受激布里渊散射(SBS)的实验研究。在宽带(约1 cm-1)抽运模式下,只测到了前向受激拉曼散射,而没有观察到后向散射,其一级和二级斯托克斯最大能量转换效率可达22%和8%。在窄带(约0.003 cm-1)模式下,前向、后向受激拉曼散射和受激布里渊散射都测量到了,但大部分抽运能量都转换到受激布里渊散射,其转换效率可达67%。测量了两种带宽模式下各散射组分在它们最佳转换时的波形;窄带情况下后向受激拉曼散射和受激布里渊散射的脉宽分别可压窄至1.5 ns和2.3 ns,不到抽运脉宽的三分之一,使得受激布里渊散射峰值功率可大大高于抽运功率。对氧气中前向、后向受激拉曼散射和受激布里渊散射之间的竞争进行了讨论。     

三倍频Nd:YAG激光抽运氧气中的受激拉曼和布里渊散射

报道了三倍频脉冲Nd：YAG激光（355nm）在两种不同带宽模式下抽运氧气中受激拉曼散射（SRS）和受激布里渊散射（SBS）的实验研究。在宽带（约1cm^-1）抽运模式下,只测到了前向受激拉曼散射,而没有观察到后向散射,其一级和二级斯托克斯最大能量转换效率可达22％和8％。在窄带（约0．003cm^-1）模式下,前向、后向受激拉曼散射和受激布里渊散射都测量到了,但大部分抽运能量都转换到受激布里渊散射,其转换效率可达67％。测量了两种带宽模式下各散射组分在它们最佳转换时的波形;窄带情况下后向受激拉曼散射和受激布里渊散射的脉宽分别可压窄至1．5ns和2．3ns,不到抽运脉宽的三分之一,使得受激布里渊散射峰值功率可大大高于抽运功率。对氧气中前向、后向受激拉曼散射和受激布里渊散射之间的竞争进行了讨论。     

神光Ⅲ原型装置的黑腔靶背向散射光测量技术

在惯性约束聚变研究中,为研究激光等离子体互作用产生的受激布里渊与受激拉曼散射光(SRS),基于神光Ⅲ原型装置利用新的背向散射诊断系统对真空黑腔靶与充气黑腔靶以及有无束匀滑效果情况下的散射光光谱进行了测量。实验发现,在相同条件下,充气黑腔靶SRS散射光比真空黑腔靶大,在终端光学组件中使用束匀滑效果后也能降低散射光。诊断系统记录的散射光强度及光谱的时间变化过程对研究激光等离子体相互作用提供了重要的数据支撑。     

水中受激拉曼散射的能量增强及受激布里渊散射的光学抑制

为提高液体介质中受激拉曼散射的输出能量,提出了通过温度调控来抑制受激布里渊散射的方法,设计了532 nm多纵模宽带脉冲激光泵浦的受激拉曼散射发生系统,测量了不同温度下水中前向受激拉曼散射及后向受激布里渊散射的输出能量,分析了水温、泵浦激光线宽及热散焦效应对受激拉曼散射输出能量影响的物理机制.实验结果表明:通过降低水温可实现对受激布里渊散射过程的有效抑制,同时减小热散焦效应带来的光束畸变,从而有效提高受激拉曼散射的输出能量.研究结果对液体介质中的受激拉曼散射多波长转换具有重要意义.     

Phase conjugated wavefronts by stimulated brillouin and Raman scattering

Experimental investigations were made of wavefront reproduction (WFR) by backward stimulated Raman scattering (SRS) and backward stimulated Brillouin scattering (SBS) in CS₂ with a linearly polarized ruby pump beam. The experimental studies were carried out as a function of the optical waveguide diameter and photographic records comparing the pump beam with the backscattered SBS and SRS beams are presented. In all of the cases studied there was a high degree of replication between the SBS and pump beams; however, a deterioration in the quality of the replicated SRS beam was observed, and it increased as the cross-sectional area of the waveguide increased.     

黑腔等离子体中SRS与SBS过程的散射光谱分析

针对惯性约束聚变中激光通过等离子体时,在光路上激发的受激Raman散射(SRS)与受激Brillouin散射(SBS)过程进行了研究。介绍了用于分析两种不稳定性过程背向散射光谱特征的程序模型,数值模拟了不同等离子体状态参数条件下的SRS与SBS过程,讨论了黑腔内等离子体参数条件对两种不稳定性过程及其散射光谱特征的影响,提出了有益于抑制两种不稳定过程的等离子体参数条件。     

Volume reflection and volume refraction of relativistic particles in a uniformly bent crystal

The scattering of fast charged particles in a bent crystal has been analyzed in the framework of relativistic classical mechanics. The expressions obtained for the deflection function are in satisfactory agreement with the experimental data for the volume reflection of relativistic protons obtained by Yu. M. Ivanov et al., Phys. Rev. Lett. 97, 144801 (2006); Yu. M. Ivanov et al., Pis’ma Zh. Eksp. Teor. Fiz. 84, 445 (2006) [JETP Lett. 84, 372 (2006)]; and W. Scandale et al., Phys. Rev. Lett. 98, 154801 (2007). The features of the scattering of the particles on ring potentials are considered in a wide range of impact parameters.     

非均匀等离子体中受激Raman散射非线性行为

鉴于实际中等离子体不均匀性和非周期性边界条件，建立了受激Ｒａｍａｎ散射（ＳＲＳ）和受激Ｂｒｉｌｌｏｕｒｉｎ散射（ＳＢＳ）非线性耦合模型。ＳＢＳ对ＳＲＳ的影响主要表现在两方面：１）Ｌａｎｇｍｕｉｒ波与离子声波的非线性相互作用，２）ＳＢＳ与ＳＲＳ的竞争。本文研究了离子声衰变不稳定性、离子声波对Ｌａｎｇｍｕｉｒ波的非共振散射两种非线性过程在ＳＲＳ发展过程中的作用，给出一维不均匀等离子体中ＳＲＳ发展图象。Ｌａｎｇｍｕｉｒ波向短波转换，从而被强烈阻尼是抑制ＳＲＳ的重要机制。文中给出了ＳＢＳ／ＳＲＳ耦合过程中决定ＳＲ 关键词：     

Numerical analysis of stimulated inelastic scatterings in ytterbium-doped double-clad fiber amplifier with multi-ns-duration and multi-hundred-kW peak-power output

We investigate the stimulated Brillouin scattering (SBS) and the stimulated Raman scattering (SRS) in an ytterbium-doped double-clad fiber amplifier that outputs optical pulses with multi-ns-duration and multi-hundred-kW peak-power. The ytterbium-doped double-clad fiber amplifier is simulated by a model which is composed of a set of propagation-rate equations. The simulated results show that SBS and SRS will deform the output signal pulse in both the time domain and the spectral domain, and degrade the performance of the ytterbium-doped double-clad fiber amplifier seriously. It is shown in our simulation that the troublesome SBS can be effectively suppressed by broadening the signal linewidth to a critical value of 0.07 nm in our calculation, and the effect of SRS can be suppressed using a large-mode-area fiber with proper length. The model and the simulated results are very useful for designing an ultra-high-power ytterbium-doped double-clad fiber amplifier.     

Observations of wavefront reproduction by stimulated Brillouin and Raman scattering as a function of pump power and waveguide dimensions

Experimental investigations have been made of wavefront reproduction (WFR) by backward stimulated Raman scattering (SRS) and backward stimulated Brillouin scattering (SBS) in CS₂ and C₇H₆O using a linearly polarized ruby pump at λ=0.694 μm. The studies were carried out as a function of the length and cross-section of the optical waveguide and the pump power at the input to the nonlinear medium; curves showing the percentage of nonreproduced backscattered radiation versus power into the waveguide are presented. In all of the cases studied the degree of reproduction by SBS was higher than that by SRS and the efficiency of WFR for both SBS and SRS improved as the pump power into the waveguide was increased, the bore diameter of the lightpipe decreased, and the length of active media decreased.     

Three-dimensional simulations of ion acceleration from a foil irradiated by a short-pulse laser

Using 3D particle-in-cell simulations we study ion acceleration from a foil irradiated by a laser pulse at 10(19) W/cm(2) intensity. At the front side, the laser ponderomotive force pushes electrons inwards, thus creating the electric field by charge separation, which drags the ions. At the back side of the foil, the ions are accelerated by space charge of the hot electrons exiting into vacuum, as suggested by Hatchett et al. [Phys. Plasmas 7, 2076 (2000)]. The transport of hot electrons through the overdense plasma and their exit into vacuum are strongly affected by self-generated magnetic fields. The fast ions emerge from the rear surface in cones similar to those detected by Clark et al. [Phys. Rev. Lett. 84, 670 (2000)].     

Saturation of backward stimulated scattering of a laser beam in the kinetic regime

Stimulated Raman (SRS) and Brillouin scattering (SBS) are examined in the kinetic regime using particle-in-cell simulations. Wave front bowing of electron-plasma waves (ion-acoustic waves) from trapped particle nonlinear frequency shift is observed in the SRS (SBS) regime for the first time. Self-focusing from trapped particle modulational instability (TPMI) is shown to occur in 2D and 3D SRS simulations. The key physics of SRS saturation is identified as a combination of wave front bowing, TPMI, and self-focusing: Bowing marks the beginning of SRS saturation and self-focusing terminates SRS. Ion-acoustic wave bowing also contributes to SBS saturation. Velocity diffusion by transverse modes and rapid loss of hot electrons in regions of small transverse extent formed from self-focusing dissipate wave energy and increase Landau damping, despite trapping that reduces Landau damping initially.     

Acceleration of ultra-thin electron layer. Analytical treatment compared with 1D-PIC simulation

In this paper, we apply an analytical model [V.V. Kulagin et al., Phys. Plasmas 14, 113101 (2007)] to describe the acceleration of an ultra-thin electron layer by a schematic single-cycle laser pulse and compare with one-dimensional particle-in-cell (1D-PIC) simulations. This is in the context of creating a relativistic mirror for coherent backscattering and supplements two related papers in this EPJD volume. The model is shown to reproduce the 1D-PIC results almost quantitatively for the short time of a few laser periods sufficient for the backscattering of ultra-short probe pulses.     

Dust ion-acoustic shock-wave structures: Theory and laboratory experiments

An evolutionary theoretical model is developed that describes dust ion-acoustic shock waves in dusty plasma consisting of ions (treated in the hydrodynamic approximation), Boltzmann electrons, and variable-charge dust grains. Account is taken not only of ionization, absorption, momentum loss by electrons and ions in collisions with dust grains, and gas-kinetic pressure effects but also of the processes peculiar to laboratory plasmas. It is shown that the model is capable of describing all the main experimental results on dust ion-acoustic shock waves [Q.-Z. Luo et al., Phys. Plasmas 6, 3455 (1999); Y. Nakamura et al., Phys. Rev. Lett., 83, 1602 (1999)].     

Generalized statistical complexity measures: Geometrical and analytical properties

We discuss bounds on the values adopted by the generalized statistical complexity measures [M.T. Martin et al., Phys. Lett. A 311 (2003) 126; P.W. Lamberti et al., Physica A 334 (2004) 119] introduced by López Ruiz et al. [Phys. Lett. A 209 (1995) 321] and Shiner et al. [Phys. Rev. E 59 (1999) 1459]. Several new theorems are proved and illustrated with reference to the celebrated logistic map.