Interaction of ultraintense laser pulses with an underdense,preformed plasma channel

We report on experimental results regarding the propagation of ultraintense laser pulses in a preformed plasma channel. In this experiment, the long (4-mm) fully ionized plasma channel created by the amplified spontaneous emission (ASE) was measured by interferometry before and after the propagation of the short laser pulse. Forward spectra show a cascade of Raman satellites, which merge with one another when the laser power was increased up to critical power for relativistic self-focusing P_c. The number of filaments measured by interferometry increases when the laser power increases. High conversion efficiency (≈10%) of second harmonic generation was observed in the interaction     

Experimental study of relativistic self-focusing andself-channeling of an intense laser pulse in an underdense plasma

This paper reports on experimental investigations on relativistic self-focusing and self-channeling of a terawatt laser pulse (0.7 TW⩽P⩽15 TW) in an underdense plasma. We present results obtained with picosecond (τ=1 ps) and subpicosecond (τ=0.4 ps) pulses. In the “long pulse” regime, modifications in the laser propagation are observed for Pc, the critical power for self-focusing. By contrast, self-guiding of subpicosecond pulses is observed for P≈P_c. Using a paraxial envelope model describing the laser propagation and taking into account the plasma response to the ponderomotive force, it is shown that a maximum laser intensity of 5-15 times that reached in vacuum may be achieved when P is in the (1.25-4)×P_c range. It is also demonstrated that ion motion may significantly reduce the effective P_c     

Experimental evidence of plasma-induced incoherence of an intense laser beam propagating in an underdense plasma

Time dependent large angular spreading and spectral broadening of an intense randomized laser beam propagating in an underdense, well-characterized plasma is measured. The two features are correlated and increase with laser intensity or plasma density. This spatial and temporal incoherence imposed upon the beam via the coupling with the plasma is interpreted, in agreement with recent numerical simulations, as due to the interplay between dynamical filamentation and strongly driven stimulated Brillouin forward scattering.     

Opacity of an underdense plasma slab due to the parametric instabilities of an ultraintense laser pulse

The interaction of ultraintense laser beams with underdense plasma slabs has been investigated with two-dimensional particle-in-cell numerical simulations, showing a strong absorption and a correlatively low transmission. Energetic electrons in the multi-MeV range are produced. At very high intensities the plasma transparency is recovered. These results are interpreted in terms of the development of electron parametric instabilities in the self-consistently heated plasma.     

High-power, kilojoule class laser channeling in millimeter-scale underdense plasma

Experiments were performed using the Omega EP laser, operating at 740 J of energy in 8 ps (90 TW), which provides extreme conditions relevant to fast ignition studies. A carbon and hydrogen plasma plume was used as the underdense target and the interaction of the laser pulse propagating and channeling through the plasma was imaged using proton radiography. The early time expansion, channel evolution, filamentation, and self-correction of the channel was measured on a single shot via this method. A channel wall modulation was observed and attributed to surface waves. After around 50 ps, the channel had evolved to show bubblelike structures, which may be due to postsoliton remnants.     

Investigation of stimulated brillouin backscattering of nanosecond CO2 laser radiation from an extended underdense plasma

Stimulated Brillouin backscattering (SBS) of nanosecond CO₂ laser radiation is investigated from an homogeneous extended plasma for well defined interaction conditions. It is found that SBS saturates at the theoretical limit near 100% Growth of SBS ion-waves starts from near thermal-wave amplitudes. Theoretical and observed SBS threshold fluxes agree within the 50% error bounds of these values. It is pointed out that self focusing does not greatly affect SBS under the conditions investigated. SBS thresholds depend on plasma density and focal spot diameter. It is indicated that an equilibrium between inverse bremsstrahlung absorption and classical heat conduction can explain several aspects.     

Collimated multi-MeV ion beams from high-intensity laser interactions with underdense plasma

A beam of multi-MeV helium ions has been observed from the interaction of a short-pulse high-intensity laser pulse with underdense helium plasma. The ion beam was found to have a maximum energy for He2+ of (40(+3)(-8)) MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10 degrees. 2D particle-in-cell simulations show that the ions are accelerated by a sheath electric field that is produced at the back of the gas target. This electric field is generated by transfer of laser energy to a hot electron beam, which exits the target generating large space-charge fields normal to its boundary.     

亚密度冷等离子体的低功率激光吸收特性分析

当电子密度远低于入射小功率激光所对应的临界密度时,冷等离子体对激光的吸收特性与高密度热等离子体将有很大的差别。在亚密度冷等离子体中,电子与中性粒子间的碰撞将占主导地位,对应的频率远大于电子-离子碰撞频率,这使得碰撞吸收本质发生了变化。亚密度等离子体的电子密度和碰撞频率均较小,它在单位长度传输路径内对常用的工作在可见光、红外波段内的小功率激光的碰撞吸收可以忽略不计。但是对于非正常吸收机制的影响尚需深入研究。     

CO2 laser induced compton effect in underdense hydrogen plasma

Recent experimental observation of induced scattering of CO₂ laser radiation by an underdense hydrogen plasma can be explained theoretically by non-linear and Compton scattering from ions in the highly cooperative plasma conditions prevailing (kλ_D? 1).     

Stimulated Raman backscattering from an ultrashort laser interacting with underdense plasmas

We present experimental results of the stimulated Raman backscattering instability (BSRS) in an ultrashort intense (45fs, 5.7×10¹⁷Wcm^-2) laser pulse interacting with an optically ionized helium gas. We have studied the stimulated Raman backscattering reflectivity and the phenomenon of the transition from strongly coupled BSRS to weakly coupled BSRS. We have obtained a good agreement between our experimental results and the theory of BSRS.     

Compton散射对短脉冲强激光在次临界等离子体中自聚焦的影响

应用相对论理论和多光子非线性Compton散射模型,研究了Compton散射对短脉冲强激光在次临界等离子体中自聚焦的影响,提出了将入射光和Compton散射光作为形成激光自聚焦的新机制,给出了三级电流密度满足的修正方程,并进行了数值模拟。结果表明,相对论效应使等离子体中的短脉冲强激光自聚焦趋势减缓,而散射则加速了自聚焦的发展,总自聚焦趋势比无相对论时来得快一些,主要原因是由于散射光和入射光形成的耦合光频率较入射光频率增大,散射效应补偿了因相对论效应引起的自聚焦减缓效应的缘故。     

Nonlinear interaction of an intense laser beam with millimeter sized underdense plasmas

Experimental results are reported on the nonlinear interaction of a 0.5 μm laser light with long scalelength preformed plasmas. Spectra from stimulated Brillouin and Raman scattering, as well as measurements of the transmitted laser light clearly indicate the occurence of strong nonlinearities driven by filamentation of the laser beam.     

激光大气传输湍流扰动仿真技术

为了研究激光大气传输时湍流效应对激光应用技术的影响,对湍流扰动的仿真进行了分析。介绍了在实验室内进行激光大气传输湍流扰动研究的数值仿真技术和仿真系统。阐述了快速傅里叶变换（FFT）和Zernike多项式两种湍流扰动数值仿真方法,并且对比了两种方法的优劣。利用物理相位屏搭建了实物湍流仿真系统,介绍了其理论模型并进行了仿真实验,对激光经湍流系统传输后的光强能量分布进行了研究分析。结果显示,室内湍流仿真系统能够准确地模拟弱起伏条件下湍流对激光传输的影响。     

Observation of magnetized soliton remnants in the wake of intense laser pulse propagation through plasmas

Slowly evolving, regularly spaced patterns have been observed in proton projection images of plasma channels drilled by intense (?101? W?cm?2) short (～1 ps) laser pulses propagating in an ionized gas jet. The nature and geometry of the electromagnetic fields generating such patterns have been inferred by simulating the laser-plasma interaction and the following plasma evolution with a two-dimensional particle-in-cell code and the probe proton deflections by particle tracing. The analysis suggests the formation of rows of magnetized soliton remnants, with a quasistatic magnetic field associated with vortexlike electron currents resembling those of magnetic vortices.     

Quasistatic magnetic fields generated by a nonrelativistic intense laser pulse in uniform underdense plasma

Quasistatic magnetic fields generated by nonrelativistic intense linearly polarized (LP) and circularly polarized (CP) laser pulses in an initially uniform underdense plasma in the collision-dominated limit are investigated analytically. Using a selfconsistent analytical model, we perform a detailed derivation of quasistatic magnetic fields in the laser pulse envelope in the collision-dominated limit to obtain exact analytical expressions for magnetic fields and discuss the dependence of magnetic fields on laser and plasma parameters. Equations for quasistatic magnetic fields including both axial component B_z and the azimuthal one B_θ are derived simultaneously from such a selfconsistent model. The dependence of quasistatic magnetic field on incident laser intensity, transverse focused radius of laser pulse, electron density and electron temperature is discussed.     

Microwave propagation through modulated air plasma

When a microwave propagates through a plasma in which electron density and electron collision frequency periodically vary, the propagating wave is modulated in amplitude and phase. An approximate theory is derived to suit the laboratory experimental conditions. Introducing the amplitude and phase difference, the dependence of electron density and electron collision frequency has been derived for different radio frequency modulation and frequency parameter. A scanning double probe technique is used to measure the exact time variation in the plasma parameters at any fixed position during a single cycle of the applied field. Theoretical values agree with those of experiment.