Controlled betatron x-ray radiation from tunable optically injected electrons

The features of Betatron x-ray emission produced in a laser-plasma accelerator are closely linked to the properties of the relativistic electrons which are at the origin of the radiation. While in interaction regimes explored previously the source was by nature unstable, following the fluctuations of the electron beam, we demonstrate in this Letter the possibility to generate x-ray Betatron radiation with controlled and reproducible features, allowing fine studies of its properties. To do so, Betatron radiation is produced using monoenergetic electrons with tunable energies from a laser-plasma accelerator with colliding pulse injection [J. Faure et al., Nature (London) 444, 737 (2006)]. The presented study provides evidence of the correlations between electrons and x-rays, and the obtained results open significant perspectives toward the production of a stable and controlled femtosecond Betatron x-ray source in the keV range.     

飞秒激光-薄膜靶相互作用中超热电子产额和激光转化效率

 在激光能量130 mJ（靶面），脉宽60 fs，波长800 nm，对比度1∶10^-6，激光与靶法线成45°夹角，P偏振，靶面激光峰值功率密度约为7.0×10¹⁷ W·cm^-2，无预脉冲的条件下，采用电子谱仪与经γ标准源标定的LiF热释光探测器（TLD）相配合，测量了飞秒激光-薄膜靶相互作用中产生的超热电子能谱。根据所测的能谱，推算出超热电子的产额和激光能量转化为超热电子能量的效率，在靶法线方向分别为1.19×10¹⁰/sr和4.55%/sr，在激光反射方向分别为1.83×10⁹/sr和0.76%/sr。结果显示，不同方向的超热电子产额和激光转化效率有所不同，原因在于激光-等离子体相互作用产生的超热电子构成各向异性的分布。     

激光-近临界等离子体高能高亮度X射线的产生

基于激光驱动超热电子产生的高品质X射线源是高能量密度实验中有效的诊断技术手段,对辐射源亮度、穿透性和时空分辨率等特性具有极高的要求。结合粒子模拟和蒙特卡罗模拟研究,首先利用近临界密度等离子体实现了激光自聚焦通道中的大电量高能电子加速,通过直接加速机制产生了电量超过600 nC、有效温度可达15 MeV的高能电子;以此为基础提高电子-光子能量转换率,有效优化了光子能量和产额,并通过一定的转换靶参数优化方案产生了微焦点(FWHM小于200 m)、高能、高亮度X射线,为高空间分辨(小于200 m)成像诊断提供了很好的途径,有望早日实现激光等离子体轫致辐射单脉冲瞬态照相的实际应用。     

Generation and propagation of hot electrons in laser-plasmas

Hot electrons are generated in the interaction between intense ultrashort laser pulses with targets. The process depends on the laser intensity, polarization, incident angle, scale length of plasmas and target materials. In this paper, the recent progress on generation and propagation of hot electrons in non-relativistic and relativistic laser-plasma interactions at the Institute of Physics, Chinese Academy of Sciences, are reviewed.     

Relativistic laser plasmas for novel radiation sources

Relativistic laser-plasma interaction results in new sources of short-pulsed x-ray radiation. Here we consider two options. The first one is betatron radiation of electrons accelerated in underdense plasmas and oscillating in transverse fields of the laser wake. This radiation is incoherent and broadband, the pulse duration is comparable with that of the driving laser. The second option is the high harmonic generation (HHG) from overdense plasma surfaces. This radiation is coherent. The relativistic high harmonics are phase locked and emerge in the form of (sub-)attosecond pulses. One- and three-dimensional regimes of relativistic HHG from overdense plasmas are considered.     

Deducing the electron-beam diameter in a laser-plasma accelerator using x-ray betatron radiation

We investigate the properties of a laser-plasma electron accelerator as a bright source of keV x-ray radiation. During the interaction, the electrons undergo betatron oscillations and from the carefully measured x-ray spectrum the oscillation amplitude of the electrons can be deduced which decreases with increasing electron energies. From the oscillation amplitude and the independently measured x-ray source size of (1.8±0.3) μm we are able to estimate the electron bunch diameter to be (1.6±0.3) μm.     

超热电子产生的靶后相干渡越辐射光谱实验研究

利用OMA光学多道分析仪测量了激光与薄膜靶相互作用中产生的辐射光谱，在靶后观察到红移的二次谐波发射. 这种二次谐波是v×B加热产生的、具有微脉冲结构的超热电子束在等离子体-真空边界产生的相干渡越辐射(CTR). 随着激光能量的增大，红移峰向长波方向移动，光谱同时发生展宽. 分析认为，等离子体临界面的迅速膨胀是导致二次谐波红移的主要原因. 随着预脉冲能量的增大，临界面膨胀速度增大，导致了发射峰更大的红移. 实验还测量了靶面法线方向的辐射光谱，观察到基频辐射的红移和展宽. CTR为诊断临界面的运动方向和速度提供了一种新的方法. 关键词： 相干渡越辐射 超热电子 超短超强激光 等离子体相互作用     

利用光核反应对激光等离子体中超热电子温度诊断的理论研究

超短超强激光与等离子体相互作用可以产生高能的超热电子, 利用光核反应的方法可以对这部分超热电子的温度进行诊断. 本文通过粒子输运程序(MCNP), 模拟了超热电子通过轫致辐射产生γ 光子, γ 光子再分别与⁶³Cu, ¹⁰⁷Ag, ¹²C等活化材料发生光核反应的物理模型, 并根据核素的活化截面数据, 计算了不同活化片的放射性活度, 得到了¹¹C/⁶²Cu, ¹¹C/¹⁰⁶Ag活度比与电子温度关系曲线, 采用理论模拟的方法实现了激光等离子体产生的超热电子的温度诊断. 关键词： 超热电子 轫致辐射光子 光核反应 MCNP程序     

利用韧致辐射诊断激光等离子体相互作用产生的超热电子

超短脉冲强激光与等离子体相互作用时产生的向靶内传输的超热电子在“快点火”方案中起着极其重要的作用.对于这部分向靶内传输的超热电子，韧致辐射方法是一种能有效、全面获得超热电子各方面信息的诊断方法.通过三维蒙特卡罗程序MCNP模拟了超热电子在靶材料中的输运以及它们在靶后方向产生的韧致辐射的性质，论证了韧致辐射诊断方法的可行性. 关键词： 超热电子 韧致辐射 诊断 MCNP程序     

Stimulated Emission of Gamma Photon from Ultrashort Pulse Intense Laser-Solid-Target Interaction

The efficient production of energetic γ photons is a significant physical process in the relativistic ultrashortpulse laser-plasma inducing photonuclear action. Based on the interaction of laser-solid-target, an analytical theory onstimulated γ photon emission from a hot electron firing the target-nucleus is developed by a relativistic full quantummethod. The emitting power or probability of γ photon in arbitrary space direction can be calculated for laser irradiatingsolid-target normally. It is valid only if the scatter-centre is immovable or its motion can be neglected compared withthat of the scattered electrons.     

超强激光驱动的辐射反作用力效应与极化粒子加速

光强超过10~(22) W/cm~2的极端超强激光将光与物质的相互作用推进到辐射主导区域,激发高能伽马光子辐射,产生明显的辐射反作用力效应.辐射反作用力可以显著影响强场中带电粒子的动力学行为,并从根本上改变了极端强场区域的激光等离子体相互作用规律.如何理解和验证辐射反作用力效应是强场物理研究的核心内容之一.本文结合该方向的国内外研究进展,论述了辐射反作用力的经典形式与强场量子电动力学的理论计算与模拟方法,详细讨论了单粒子在强场中的反射、量子随机辐射、自旋-辐射耦合等效应,介绍了激光等离子体相互作用中的电子冷却、辐射俘获、高效伽马辐射等机制,并给出了目前辐射反作用力效应的实验验证方法与进展.针对自旋在强场量子电动力学方面的效应,介绍了激光加速产生极化粒子源的方法.     

激光功率密度对Al膜靶后表面快电子发射的影响

 报道了在20 TW皮秒激光器上完成的p偏振激光与等离子体相互作用过程中产生的快电子的角分布和能谱测量结果。实验得到：当激光功率密度小于10¹⁷ W/cm²时,电子发射没有明显定向性,在激光入射面内多峰发射;当激光功率密度大于10¹⁷ W/cm2,小于10¹⁸ W/cm²时,电子主要沿靶面法线方向发射;当激光功率密度达到相对论强度时,电子主要沿激光传播方向发射;激光功率密度未达到相对论强度时,靶后表面法线方向快电子能谱拟合平均温度符合共振吸收温度定标率;激光功率密度达相对论强度以上时,靶后表面法线方向快电子能谱拟合平均温度高于已有的温度定标率。     

Formation and dynamics of relativistic electromagnetic solitons in plasmas containing high-and low-energy electron components

We present theoretical and numerical studies of the nonlinear interactions between intense electromagnetic waves in plasmas containing high-and low-energy electron components. Such plasmas are frequently observed in laser-plasma experiments, where the hot electron component is created by the acceleration of electrons by strong electrostatic waves that are created by the laser-induced Raman forward and backward instabilities. The two-component electron plasma is described by the Vlasov equation for the hot electrons and the hydrodynamic equations for the cold electrons, which are coupled nonlinearly to the electromagnetic wave equation and the Poisson equation for the potential. The present nonlinear system is shown to admit electromagnetic solitary waves correlated with a positive potential and trapped electron islands from the hot electron population. The text was submitted by the authors in English.     

超热电子产生Kα辐射的参数影响

 超短超强激光与固体靶相互作用,可以产生大量的Kα特征谱线发射。这种Kα线光源在背光照相、医学成像和超快诊断等方面具有显著优势。为了寻求获得Kα线高产额的方法,利用ITS蒙卡模拟程序,对超短超强激光产生的超热电子在固体靶中产生的Kα线发射进行了模拟。系统研究了Kα线发射强度随出射角度、靶厚度和超热电子温度等参数的变化情况。研究发现在恒定的激光功率密度或超热电子温度下,存在最佳的靶厚度,使得Kα线光子产额最大,并用模拟结果对已有实验数据进行了解释。     

超热电子的产生与输运背向辐射的实验研究

报道了在100TW fs激光器上采用电子磁谱仪和光学CCD积分成像相机分别对激光-固体靶相互作用在靶背方向产生的超热电子能谱及其光学渡越辐射进行的测量。能谱测量结果显示：超热电子能谱呈单温类-麦克斯韦分布,拟合的温度为107keV;光学渡越辐射(OTR)测量结果显示：OTR是由于超热电子输运穿越固体靶所致,而辐射区域呈圆盘状、有发散角、有光强分布;如果考虑超热电子的产生和加热机制,则占主导地位的加热机制是共振吸收对电子的加热。     

X射线二极管的灵敏度标定和应用

阐述了Ｘ射线二极管铝阴极光电转换量子效率，并在单色亚千Ｘ射线光源装置上对铝阴极的灵敏度进行了绝对标定，最后介绍了采用Ｘ射线二极管探头来测量激光－等离子体相互作用中产生的亚千Ｘ射线能量。     

Dose estimation and shielding calculation for X-ray hazard at high intensity laser facilities

An ionizing radiation hazard produced from the interaction between high intensity lasers and solid targets has been observed. Laser-plasma interactions create "hot" electrons, which generate bremsstrahlung X-rays when they interact with ions in the target. However, up to now only limited studies have been conducted on this laser-induced radiological protection issue. In this paper, the physical process and characteristics of the interaction between high intensity lasers and solid targets are analyzed. The parameters of the radiation sources are discussed, including the energy conversion efficiency from laser to hot electrons, hot electron energy spectrum and electron temperature, and the bremsstrahlung X-ray energy spectrum produced by hot electrons. Based on this information, the X-ray dose generated with high-Z targets for laser intensities between 10¹⁴ and 10²⁰ W/cm² is estimated. The shielding effects of common shielding items such as the glass view port, aluminum chamber wall and concrete wall are also studied using the FLUKA Monte Carlo code. This study provides a reference for the dose estimation and the shielding design of high intensity laser facilities.     

超热电子的产生与输运背向辐射的实验研究

报道了在100TW fs激光器上采用电子磁谱仪和光学CCD积分成像相机分别对激光-固体靶相互作用在靶背方向产生的超热电子能谱及其光学渡越辐射进行的测量.能谱测量结果显示:超热电子能谱呈单温类-麦克斯韦分布,拟合的温度为107 keV;光学渡越辐射(OTR)测量结果显示:OTR是由于超热电子输运穿越固体靶所致,而辐射区域呈圆盘状、有发散角、有光强分布;如果考虑超热电子的产生和加热机制,则占主导地位的加热机制是共振吸收对电子的加热.     

用于电子能谱测量的LiF热释光探测器标定

 用γ标准源对LiF热释光探测器（TLD）的灵敏度因子、线性吸收系数、离散性、重复性进行了测定。在此基础上建立了电子质量阻止本领的修正模型，理论上计算出电子等效质量阻止本领，给出单能电子的注量。由LiF TLD阵列可测量激光等离子体相互作用中发射的超热电子能谱（能量-注量关系）。     

Dispersion and transport of energetic particles due to the interaction of intense laser pulses with overdense plasmas

We study the angular distribution of relativistic electrons generated through laser-plasma interaction with pulse intensity varying from 10(18) W/cm2 up to 10(21) W/cm2 and plasma density ranging from 10 times up to 160 times critical density with the help of 2D and 3D particle-in-cell simulations. This study gives clear evidence that the divergence of the beam is an intrinsic property of the interaction of a laser pulse with a sharp density gradient. It is entirely due to the excitation of large static magnetic fields in the layer of interaction. The energy deposited in this layer increases drastically the temperature of the plasma independently of the initial temperature. This makes the plasma locally collisionless and the simulation relevant for the current experiments.