X-ray radiation from nonlinear Thomson scattering of an intense femtosecond laser on relativistic electrons in a helium plasma

We have generated x-ray radiation from the nonlinear Thomson scattering of a 30 fs/1.5 J laser beam on plasma electrons. A collimated x-ray radiation with a broad continuous spectrum peaked at 0.15 keV with a significant tail up to 2 keV has been observed. These characteristics are found to depend strongly on the laser strength parameter a(0). This radiative process is dominant for a(0) greater than unity at which point the relativistic scattering of the laser light originates from MeV energy electrons inside the plasma.     

Evidence of short-range screening in shock-compressed aluminum plasma

We have investigated the angular variation in elastic x-ray scattering from a dense, laser-shock-compressed aluminum foil. A comparison of the experiment with simulations using an embedded atom potential in a molecular dynamics simulation shows a significantly better agreement than simulations based on an unscreened one-component plasma model. These data illustrate, experimentally, the importance of screening for the dense plasma static structure factor.     

X-Ray diffraction from a dense plasma

We report measurements of x-ray scattering cross sections for dense plasmas created by subjecting aluminum foils to strong laser-driven shocks. A narrow cone of quasimonochromatic x-rays at approximately 4.75 keV was used to probe the shocked part of the foil and scattered photons were detected with a CCD camera. The scattering cross section shows a clear peak, indicating diffraction from the plasma. Analysis and simulation of the data suggest that radiative heating and electron-ion energy exchange are important factors in the plasma production.     

X-ray scattering measurements of radiative heating and cooling dynamics

Spectrally and time-resolved x-ray scattering is used to extract the temperature and charge state evolution in a near solid density carbon foam driven by a supersonic soft x-ray heat wave. The measurements show a rapid heating of the foam material (approximately 200 eV/ns) followed by a similarly fast decline in the electron temperature as the foam cools. The results are compared to an analytic power balance model and to results from radiation-hydrodynamics simulations. Finally, the combination of charge state and temperature extracted from this known density isochorically heated plasma is used to distinguish between dense plasma ionization balance models.     

Time-resolved X-Ray diffraction from coherent phonons during a laser-induced phase transition

Time-resolved x-ray diffraction with picosecond temporal resolution is used to observe scattering from impulsively generated coherent acoustic phonons in laser-excited InSb crystals. The observed frequencies and damping rates are in agreement with a model based on dynamical diffraction theory coupled to analytic solutions for the laser-induced strain profile. The results are consistent with a 12 ps thermal electron-acoustic phonon coupling time together with an instantaneous component from the deformation-potential interaction. Above a critical laser fluence, we show that the first step in the transition to a disordered state is the excitation of large amplitude, coherent atomic motion.     

电感耦合等离子体CVD低温生长硅薄膜过程中的铝诱导晶化

利用电感耦合等离子体CVD方法在350℃的低温下在镀Al玻璃衬底上制备出具有良好结晶性的Si薄膜.利用x射线衍射、紫外-可见分光椭圆偏振谱、原子力显微镜及x射线光电子谱等研究了薄膜的结构、表面形貌和成分分布等.结果表明，用这种方法制备的Si薄膜不但晶化程度高，而且具有良好的（111）结晶取向性，晶粒尺寸大于300nm，样品中无Al的残留.结合电感耦合等离子体的高电子密度特征讨论了低温生长过程中Al诱导Si薄膜晶化的机理. 关键词： 电感耦合等离子体CVD Al诱导晶化 Si薄膜 低温生长     

Observation of the second harmonic in Thomson scattering from relativistic electrons

A free relativistic electron in an electromagnetic field is a pure case of a light-matter interaction. In the laboratory environment, this interaction can be realized by colliding laser pulses with electron beams produced from particle accelerators. The process of single photon absorption and reemission by the electron, so-called linear Thomson scattering, results in radiation that is Doppler shifted into the x-ray and gamma-ray regions. At elevated laser intensity, nonlinear effects should come into play when the transverse motion of the electrons induced by the laser beam is relativistic. In the present experiment, we achieved this condition and characterized the second harmonic of Thomson x-ray scattering using the counterpropagation of a 60 MeV electron beam and a subterawatt CO2 laser beam.     

New light on disordered ensembles: ab initio structure determination of one particle from scattering fluctuations of many copies

We report on the first experimental ab?initio reconstruction of an image of a single particle from fluctuations in the scattering from an ensemble of copies, randomly oriented about an axis. The method is applicable to identical particles frozen in space or time (as by snapshot diffraction from an x-ray free electron laser). These fluctuations enhance information obtainable from an experiment such as conventional small angle x-ray scattering.     

Raman effect in the x-ray region

This paper presents a brief review of x-ray Raman scattering and some of our calculations on Raman scattered line shapes from light elements. We summarise the history of the Raman process in the x-ray region and present a detailed theory of the Raman scattering from an atomic many-electron system. Actual calculations of the Raman cross-section using this theory in single-particle approximation are given. The process of internal resonance Raman scattering is also discussed in the same formulation. The Raman cross-section is compared with the cross-sections of other x-ray scattering processes.     

Sum rules in x-ray resonant Raman scattering: recovering the Co ground state information in CoFe2O4 as a test case

We present an approach to x-ray resonant Raman scattering suitable for sum-rule analysis. In magnetic systems it gives ground state information as represented by an expansion in coupled multipoles of orbital and spin moments. The experiment is based on the angular dependence of the integrated peak intensities. In 3d transition elements one obtains information up to order 4 from the channel 2p(6)3d(n)-->2p(5)3d(n+1)-->2p(6)3s(1)3d(n+1). By combining these results with L(2,3) absorption dichroism, we show quantitatively the atomiclike properties of cobalt in CoFe2O4 up to high-order multipoles.     

A discotic disguised as a smectic: A hybrid columnar bragg glass

We show that discotics, lying deep in the columnar phase, can exhibit an x-ray scattering pattern which mimics that of a somewhat unusual smectic liquid crystal. This exotic, new glassy phase of columnar liquid crystals, which we call a "hybrid columnar Bragg glass," can be achieved by confining a columnar liquid crystal in an anisotropic random environment of, e.g., strained aerogel. Long-ranged orientational order in this phase makes single-domain x-ray scattering possible, from which a wealth of information could be extracted. We give detailed quantitative predictions for the scattering pattern in addition to exponents characterizing anomalous elasticity of the system.     

等离子体中X射线透过率分析及潜在通信应用研究

X射线具有波长短、光子能量高等特点,有望在等离子体环境中实现信息的有效传输.本文首先采用基于连续介质中的WKB分层法,研究了黑障条件下, X射线在非均匀等离子体鞘套中的透过率特性,仿真了不同等离子体电子密度和碰撞频率下X射线信号的透过率,理论上证明了X射线可用于黑障区信息传输的可行性.其次通过搭建环形扩散辉光放电等离子体发生器及实验验证系统,进行了国内外首次X射线穿过等离子体鞘套的验证实验.实验结果表明,等离子体对X射线信号的透过率存在一定程度的衰减,透过等离子体前后的X射线信号能谱轮廓相似度优于95.5%,能谱峰值点的偏移量小于1.3%.此外,在原有理论模型的基础上,考虑等离子中的粒子与X射线的碰撞、吸收效应,优化了X射线在等离子体中的透过率模型,与传统的理论方法相比,该模型可对实验现象进行更好的解释.同时计算了X射线在临近空间的透过率,并分析了X射线通信所能达到的潜在指标.这些结果有望为解决黑障区信号传输提供一定的理论与实验依据.     

超短超强激光与气体靶相互作用中的背向受激Raman散射

 实验诊断测量了超短超强激光与气体靶相互作用产生的背向受激Raman散射,在实验条件下呈现强耦合模式,背向受激Raman散射出现非线性Stokes多峰伴线结构,峰值的频率间隔小于等离子体波的频率,可以大致地推断出激光打靶过程中产生的等离子体密度偏低,其结果与等离子体强耦合理论计算结果一致。     

Picosecond time-resolved X-ray absorption spectroscopy of ultrafast aluminum plasmas

We have used point-projection K-shell absorption spectroscopy to infer the ionization and recombination dynamics of transient aluminum plasmas. Two femtosecond beams of the 100 TW laser at the LULI facility were used to produce an aluminum plasma on a thin aluminum foil (83 or 50 nm), and a picosecond x-ray backlighter source. The short-pulse backlighter probed the aluminum plasma at different times by adjusting the delay between the two femtosecond driving beams. Absorption x-ray spectra at early times are characteristic of a dense and rather homogeneous plasma. Collisional-radiative atomic physics coupled with hydrodynamic simulations reproduce fairly well the measured average ionization as a function of time.     

Fine structure of the diffraction image of an edge dislocation in high-resolution section topography

The propagation of an x-ray wave field in an elastic field of an edge dislocation crossing a scattering triangle exactly along the bisector of the scattering angle has been considered. The scattering of the x-ray wave field by a complex elastic field of the edge dislocation has been analyzed using the methods of geometrical optics. It has been established that the fine structure of a diffraction image of defects in thick crystals is determined by the differences in scattering of the normal and anomalous modes of the x-ray wave field in the vicinity of the Bragg reflection. In the case of thick crystals, the x-ray diffraction image of defects can have a symmetry different from the symmetry of the function of local misorientations of the crystal lattice. X-ray wave scattering by local distortions of the crystal lattice can occur according to two different mechanisms depending on the gradient of space changes in the deformation field. In the crystal regions where the elastic field varies slowly with a change in the distance, the x-ray wave field has had time to adjust itself to follow the course of deviations of the crystal lattice from the exact Bragg condition. In the crystal region where the elastic field changes significantly at distances of the order of the extinction length, this region leaves the reflecting position and interference scattering occurs at the interface of the region. It is important that the form of the deformation field in this case is of no significance.     

Surface and bulk spin ordering of antiferromagnetic materials: NiO(111)

We present a study of the prototypical NiO(111) antiferromagnet by nonresonant surface x-ray magnetic scattering. Direct access to the antiferromagnetic surface and bulk spin ordering is demonstrated. Our data support a first order antiferromagnetic to paramagnetic transition. A quantitative determination of the magnetization profile is proposed. It is shown that the NiO(111) surface spins remain ordered at higher temperatures than in the bulk and that the blocking temperature in exchange coupled ferromagnetic-NiO interfaces is most likely related to an S-domain structure loss occurring 25 K below the Ne el temperature.     

Nonperturbative heavy-quark diffusion in the quark-gluon plasma

We evaluate heavy-quark (HQ) transport properties in a quark-gluon plasma (QGP) within a Brueckner many-body scheme employing interaction potentials extracted from thermal lattice QCD. The in-medium T matrices for elastic charm- and bottom-quark scattering off light quarks in the QGP are dominated by attractive meson and diquark channels which support resonance states up to temperatures of ~1.5T(c). The resulting drag coefficient increases with decreasing temperature, contrary to expectations based on perturbative QCD scattering. Employing relativistic Langevin simulations we compute HQ spectra and elliptic flow in sqrt[s(NN)]=200 GeV Au-Au collisions. A good agreement with electron decay data supports our nonperturbative computation of HQ diffusion, indicative for a strongly coupled QGP.     

An x-ray biplanar photodiode and the x-ray emission from magnetically confined laser produced plasma

Development of a single and multichannel biplanar vacuum photodiode for x-ray detection is reported, which has been used to study the x-ray emission from laser produced plasma expanding across an externally applied magnetic field. Two to three times enhancement in x-ray emission has been observed which was found correlated with decrease in size of the x-ray emitting plasma plume (expansion velocity of plasma). Experimental observations were found in close agreement with the analytical model based on an increase in plasma density as a result of plasma confinement in magnetic field. Temporal evolution of x-ray emission indicates that recombination radiation seems to be playing an important role in x-ray enhancement.