Electron acceleration by an intense short-pulse laser in underdense plasma

Electron acceleration from the interaction of an intense short-pulse laser with low density plasma is considered. The relation between direct electron acceleration within the laser pulse and that in the wake is investigated analytically. The magnitude and location of the ponderomotive-force-caused charge separation field with respect to that of the pulse determine the relative effectiveness of the two acceleration mechanisms. It is shown that there is an optimum condition for acceleration in the wake. Electron acceleration within the pulse dominates as the pulse becomes sufficiently short, and the latter directly drives and even traps the electrons. The latter can reach ultrahigh energies and can be extracted by impinging the pulse on a solid target. (C) 2003 American Institute of Physics.

Electron self-injection and acceleration driven by a tightly focused intense laser beam in an underdense plasma

A scheme for electron self-injection in the laser wakefield acceleration is proposed. In this scheme, the transverse wave breaking of the wakefield and the tightly focused geometry of the laser beam play important roles. A large number of the background electrons are self-injected into the acceleration phase of the wakefield during the defocusing of the tightly focused laser beam as it propagates through an underdense plasma. Particle-in-cell simulations performed using a 2D3V code have shown generation of a collimated electron bunch with a total number of 1.4 x 109 and energies up to 8 MeV. (C) 2005 American Institute of Physics.

Observation of Postsoliton Expansion Following Laser Propagation through an Underdense Plasma

The expansion of electromagnetic postsolitons emerging from the interaction of a 30 ps, 3 x 10(18) W cm(-2) laser pulse with an underdense deuterium plasma has been observed up to 100 ps after the pulse propagation, when large numbers of postsolitons were seen to remain in the plasma. The temporal evolution of the postsolitons has been accurately characterized with a high spatial and temporal resolution. The observed expansion is compared to analytical models and three-dimensional particle-in-cell results, revealing a polarization dependence of the postsoliton dynamics.

Dynamics of intense laser propagation in underdense plasma: Polarization dependence

We present a comprehensive numerical study of the dynamics of an intense laser pulse as it propagates through an underdense plasma in two and three dimensions. By varying the background plasma density and the polarization of the laser beam, significant differences are found in terms of energy transport and dissipation, in agreement with recently reported experimental results. Below the threshold for relativistic self-focusing, the plasma and laser dynamics are observed to be substantially insensitive to the initial laser polarization, since laser transport is dominated by ponderomotive effects. Above this threshold, relativistic effects become important, and laser energy is dissipated either by plasma heating (p-polarization) or by trapping of electromagnetic energy into plasma cavities (s-polarization) or by a combination of both (circular polarization). Besides the fundamental interest of this study, the results presented are relevant to applications such as plasma-based accelerators, x-ray lasers, and fast-ignition inertial confinement fusion. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737151]

Anticorrelation between Ion Acceleration and Nonlinear Coherent Structures from Laser-Underdense Plasma Interaction

In laser-plasma experiments, we observed that ion acceleration from the Coulomb explosion of the plasma channel bored by the laser, is prevented when multiple plasma instabilities such as filamentation and hosing, and nonlinear coherent structures (vortices/post-solitons) appear in the wake of an ultrashort laser pulse. The tailoring of the longitudinal plasma density ramp allows us to control the onset of these insabilities. We deduced that the laser pulse is depleted into these structures in our conditions, when a plasma at about 10% of the critical density exhibits a gradient on the order of 250 {\mu}m (gaussian fit), thus hindering the acceleration. A promising experimental setup with a long pulse is demonstrated enabling the excitation of an isolated coherent structure for polarimetric measurements and, in further perspectives, parametric studies of ion plasma acceleration efficiency.

Magnetic field generation during intense laser channelling in underdense plasma

Channel formation during the propagation of a high-energy (120 J) and long duration (30 ps) laser pulse through an underdense deuterium plasma has been spatially and temporally resolved via means of a proton imaging technique, with intrinsic resolutions of a few micron and a few ps, respectively. Conclusive proof is provided that strong azimuthally symmetric magnetic fields with a strength of around 0.5 MG are created inside the channel, consistent with the generation of a collimated beam of
relativistic electrons. The inferred electron beam characteristics may have implications for the cone-free fast-ignition scheme of inertial confinement fusion

Intense local plasma heating by stopping of ultrashort ultraintense laser pulse in dense plasma

Self-trapping, stopping, and absorption of an ultrashort ultraintense linearly polarized laser pulse in a finite plasma slab of near-critical density is investigated by particle-in-cell simulation. As in the underdense plasma, an electron cavity is created by the pressure of the transmitted part of the light pulse and it traps the latter. Since the background plasma is at near-critical density, no wake plasma oscillation is created. The propagating self-trapped light rapidly comes to a stop inside the slab. Subsequent ion Coulomb explosion of the stopped cavity leads to explosive expulsion of its ions and formation of an extended channel having extremely low plasma density. The energetic Coulomb-exploded ions form shock layers of high density and temperature at the channel boundary. In contrast to a propagating pulse in a lower density plasma, here the energy of the trapped light is deposited onto a stationary and highly localized region of the plasma. This highly localized energy-deposition process can be relevant to the fast ignition scheme of inertial fusion.

Macroscopic evidence of soliton formation in multiterawatt laser-plasma interaction

A novel physical phenomenon has been observed following the interaction of an intense (10(19) W/cm(2)) laser pulse with an underdense plasma. Long-lived, macroscopic bubblelike structures have been detected through the deflection that the associated electric charge separation causes in a proton probe beam. These structures are interpreted as the remnants of a cloud of relativistic solitons generated in the plasma by the ultraintense laser pulse. This interpretation is supported by an analytical study of the soliton cloud evolution, by particle-in-cell simulations, and by a reconstruction of the proton-beam deflection.

再入尾迹电磁特性的湍流效应研究

本文旨意在于通过探讨高超声速再入尾迹中的湍流等离子体与电磁波相互作用的机理，以及建立能反映此机理的应用性理论模型，从而提供一套可进行目标特性分析的方法，以便为工程部门的突防技术服务。本题目在再入气动物理现象研究中具有重要意义。综合分析指出，地面雷达观测到的非相干散射信号主要来源于再入尾迹的亚密湍流区产生的体积散射。因此，电磁散射特性分析主要针对尾迹亚密湍流等离子体。并且，这里所有的分析都是根据在工程应用中最成熟的一阶畸变波Born近似理论模型。再入尾迹电磁特性的湍流效应研究，着眼点就在于湍流等离子体场的研究。对湍流等离子体场理论模型，本文试图通过模式理论来表达，即求解平均化的全Navier-Stokes方程及其封闭方程k-ε-g模型，从而准确获得流动平均场和脉动场信息。这种表达方式较以前有了较大改进。注意到高超声速流动具有强烈可压缩性的特点，故使用的N-S平均方程由质量加权平均过程产生，湍流模型方程也经过可压缩性修正。方程的离散求解方法，都是运用带矢通量分裂的二阶TVD格式的有限体积法。再入尾迹湍流场的初始条件由近尾迹（底部）流动经N-S方程求解给定，初始值更加准确可靠。尾迹从层流到湍流的转捩过程采用相对成熟的半经验公式确定。飞行器的高超声速再入过程必然导致它周围的空气温度升高，使得流动表现出真实气体效应。对重点考察的湍流流动而言，真实气体效应主要表现为气体处于热化学平衡状态。就工程部门面临的实际问题，把一阶畸变波Born近似的解算方法做些改进，使其能够处理的范围从轴对称尾迹扩展到三维湍流等离子体场是必要的。这为深入的理论分析提供了有力的保障。在能够准确模拟湍流流动的刻划雷达散射截面的基础上，考察亚密湍流等离子体对电磁散射的影响。通过选择的几个有代表性的因素进行讨论，初步结果表明：湍流转捩方式、湍流尺度对尾迹雷达散射截面值计算影响不大，而电子组份脉动能初始值影响较明显，且在特定条件下湍流模型的影响亦不大。但由于湍流模型涉及脉动初始值，其影响需进一步确定。同时，一些今后开展继续此项研究工作的有益建议也提了出来。

Self-focusing, Raman, and modulation instability of shaped relativistic laser pulse in plasmas

The interaction of shaped laser pulses with plasmas is studied in a strict theoretical framework without adopting the slow-varying envelope approximation (SVEA). Any physical quantities involved in the interaction are denoted as a summation of different real quantities of respective phases. The relationships among the phases of those real quantities and their moduli are strictly analyzed. Such strict analyses lead to a more exact equation set for the three-dimensional envelope of the laser pulse, which is not based on SVEA. Based on this equation set, self-focusing, Raman, and modulation instabilities could be discussed in a unified framework. The solutions of this equation set for the laser envelope reveal many possible multicolor laser modes in plasmas. The energy and the shape of a pulse determine its propagation through plasmas in a multicolor mode or in a monochromic mode. A global growth rate is introduced to measure the speed of the transition from the monochromic mode in vacuum to a possible mode in plasmas. (c) 2006 American Institute of Physics.

Soft X-ray harmonic comb from relativistic electron spikes

We demonstrate a new high-order harmonic generation mechanism reaching the "water window" spectral region in experiments with multiterawatt femtosecond lasers irradiating gas jets. A few hundred harmonic orders are resolved, giving mu J/sr pulses. Harmonics are collectively emitted by an oscillating electron spike formed at the joint of the boundaries of a cavity and bow wave created by a relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by particle-in-cell simulations.

Relativistic High Harmonic Generation In Gas Jet Targets

We experimentally demonstrate a new regime of high-order harmonic generation by relativistic-irradiance lasers in gas jet targets. Bright harmonics with both odd and even orders, generated by linearly as well as circularly polarized pulses, are emitted in the forward direction, while the base harmonic frequency is downshifted. A 9 TW laser generates harmonics up to 360 eV, within the 'water window' spectral region. With a 120 TW laser producing 40 uJ/sr per harmonic at 120 eV, we demonstrate the photon number scalability. The observed harmonics cannot be explained by previously suggested scenarios. A novel high-order harmonics generation mechanism [T. Zh. Esirkepov et al., AIP Proceedings, this volume], which explains our experimental findings, is based on the phenomena inherent in the relativistic laser - underdense plasma interactions (self-focusing, cavity evacuation, and bow wave generation), mathematical catastrophe theory which explains formation of electron density singularities (cusps), and collective radiation due to nonlinear oscillations of a compact charge.

High-Order Harmonics from Bow Wave Caustics Driven by a High-Intensity Laser

We propose a new mechanism of high-order harmonic generation during an interaction of a high-intensity laser pulse with underdense plasma. A tightly focused laser pulse creates a cavity in plasma pushing electrons aside and exciting the wake wave and the bow wave. At the joint of the cavity wall and the bow wave boundary, an annular spike of electron density is formed. This spike surrounds the cavity and moves together with the laser pulse. Collective motion of electrons in the spike driven by the laser field generates high-order harmonics. A strong localization of the electron spike, its robustness to oscillations imposed by the laser field and, consequently, its ability to produce high-order harmonics is explained by catastrophe theory. The proposed mechanism explains the experimental observations of high-order harmonics with the 9 TW J-KAREN laser (JAEA, Japan) and the 120 TW Astra Gemini laser (CLF RAL, UK) [A. S. Pirozhkov, et al., arXiv:1004.4514 (2010); A. S. Pirozhkov et al, AIP Proceedings, this volume]. The theory is corroborated by high-resolution two- and three-dimensional particle-in-cell simulations.

Demonstration of laser pulse amplication by stimulated Brillouin scattering

The energy transfer by stimulated Brillouin backscatter from a long pump pulse (15 ps) to a short seed pulse (1 ps)has been investigated in a proof-of-principle demonstration experiment. The two pulses were both amplified in differentbeamlines of a Nd:glass laser system, had a central wavelength of 1054 nm and a spectral bandwidth of 2 nm, and crossedeach other in an underdense plasma in a counter-propagating geometry, off-set by 10◦. It is shown that the energy transferand the wavelength of the generated Brillouin peak depend on the plasma density, the intensity of the laser pulses, and thecompetition between two-plasmon decay and stimulated Raman scatter instabilities. The highest obtained energy transferfrom pump to probe pulse is 2.5%, at a plasma density of 0.17ncr, and this energy transfer increases significantly withplasma density. Therefore, our results suggest that much higher efficiencies can be obtained when higher densities (above0.25ncr) are used.