高能电子束韧致辐射特性的理论研究

高能电子束轰击金属靶会产生韧致辐射X射线,为优化韧致辐射X射线品质,需要研究如何获取最佳辐射效率等韧致辐射规律.结合理论分析,并采用MCNP/4C对10,20?MeV电子的韧致辐射规律进行了模拟研究.讨论了不同靶材料产生的韧致辐射效率、角分布、能谱分布、准直锥孔内辐射效率等问题.通过对不同靶材料韧致辐射的模拟研究,给出了不同厚度靶与光子效率、注量分布、出射电子与角分布的关系与规律.由此得到不同靶材料对于10,20?MeV电子在最优韧致辐射效率下的一些边界条件与规律. 关键词： 韧致辐射 最佳效率 角分布 能谱     

双荧光层靶Kα线强度比诊断靶内超热电子温度

 利用双荧光层复合靶产生的Kα特征线强度比诊断了靶内超热电子的温度,即通过实验测量复合靶中两种不同材料荧光层辐射出的Kα特征线强度比,结合ITS3.0程序模拟结果,对超热电子温度进行诊断。将诊断结果与实验中利用电子磁谱仪测量的超热电子温度进行了比较,二者基本一致。结果表明,选取适当的荧光层靶厚,可以利用双荧光层复合靶产生的Kα特征线强度比对靶内的超热电子温度进行诊断。     

双荧光层靶Kα线强度比诊断靶内超热电子温度北大核心CSCD

利用双荧光层复合靶产生的Kα特征线强度比诊断了靶内超热电子的温度,即通过实验测量复合靶中两种不同材料荧光层辐射出的Kα特征线强度比,结合ITS3.0程序模拟结果,对超热电子温度进行诊断。将诊断结果与实验中利用电子磁谱仪测量的超热电子温度进行了比较,二者基本一致。结果表明,选取适当的荧光层靶厚,可以利用双荧光层复合靶产生的Kα特征线强度比对靶内的超热电子温度进行诊断。     

双荧光层靶Kα线强度比诊断靶内超热电子温度

利用双荧光层复合靶产生的Kα特征线强度比诊断了靶内超热电子的温度,即通过实验测量复合靶中两种不同材料荧光层辐射出的Kα特征线强度比,结合ITS3.0程序模拟结果,对超热电子温度进行诊断。将诊断结果与实验中利用电子磁谱仪测量的超热电子温度进行了比较,二者基本一致。结果表明,选取适当的荧光层靶厚,可以利用双荧光层复合靶产生的Kα特征线强度比对靶内的超热电子温度进行诊断。     

10-25 keV电子致厚W,Au靶韧致辐射谱的测量

测量了10-25 keV电子碰撞厚W, Au靶产生的韧致辐射谱, 并与Monte Carlo程序PENELOPE模拟的X射线谱进行了比较, 除在3 keV前实验谱略低于理论谱外, 整体上两者符合得很好. 在模拟电子与靶材料相互作用产生韧致辐射时, PENELOPE程序中只包含有普通韧制辐射的截面数据. 我们的实验结果表明, 在电子与固体靶相互作用时, 没有明显的极化韧致辐射产生, PENELOPE程序能够可靠地描述电子与固体厚靶相互作用产生的韧致辐射.     

利用轫致辐射谱回推超热电子温度

超短超强激光打靶产生的超热电子与固体靶相互作用时会产生轫致辐射X射线。利用蒙特卡罗方法,对电子在固体靶中传输产生的轫致辐射X射线进行了模拟。1 MeV电子束与固体靶作用产生的轫致辐射谱模拟结果表明,轫致辐射谱高能段斜率受靶厚度及靶材料的影响不明显。麦克斯韦分布的电子束及单能电子束与30 m铜靶作用的模拟结果显示,两种电子源产生的轫致辐射谱在电子束能量或温度较高时基本一致。给出了一种利用轫致辐射谱斜率反推超热电子温度的定标方法。模拟了不同温度下超热电子产生的轫致辐射光子的能量角分布及光子数角分布,结果显示辐射光子能量通量和光子数随着电子温度的提高越来越向前倾,并给出了另外一种由轫致辐射能量角分布反推超热电子温度的定标关系。     

利用轫致辐射谱回推超热电子温度

超短超强激光打靶产生的超热电子与固体靶相互作用时会产生轫致辐射X射线。利用蒙特卡罗方法,对电子在固体靶中传输产生的轫致辐射X射线进行了模拟。1 MeV电子束与固体靶作用产生的轫致辐射谱模拟结果表明,轫致辐射谱高能段斜率受靶厚度及靶材料的影响不明显。麦克斯韦分布的电子束及单能电子束与30 m铜靶作用的模拟结果显示,两种电子源产生的轫致辐射谱在电子束能量或温度较高时基本一致。给出了一种利用轫致辐射谱斜率反推超热电子温度的定标方法。模拟了不同温度下超热电子产生的轫致辐射光子的能量角分布及光子数角分布,结果显示辐射光子能量通量和光子数随着电子温度的提高越来越向前倾,并给出了另外一种由轫致辐射能量角分布反推超热电子温度的定标关系。     

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     

飞秒激光-金属薄膜靶相互作用中靶前后超热电子能谱的比较

采用电子谱仪测量了飞秒激光-金属薄膜靶相互作用中靶前和靶后产生的超热电子能谱.结果显示：靶前超热电子能谱的峰出现在约430 keV处，靶后超热电子能谱的峰出现在约175 keV处；靶前超热电子的有效温度分别为218 keV和425 keV，靶后超热电子能谱出现“软化”现象，其有效温度分别为96 keV和347 keV.靶前和靶后超热电子能谱明显不同是由于超热电子输运穿越过密等离子体和冷材料的靶，并在靶后建立Debye鞘，鞘电场使靶后超热电子能谱峰向低能端移动，鞘电场和自生磁场导致靶后超热电子能谱产生“软化”，估算出的鞘电场小于激光电场.     

超短超强激光与铜靶相互作用产生Kα源的蒙特卡罗模拟

超短超强激光打靶产生的超热电子，与固体靶相互作用时会产生Kα线辐射.由经典定标律给出了法线方向超热电子的温度.利用蒙特卡罗方法，对超热电子在固体靶中的传输进行了研究，模拟了不同靶厚度情况下Kα产额和角分布及不同电子温度下Kα光子的转化效率.计算结果与实验符合较好.结果表明：在一定电子温度下，随着靶厚度的增加Kα光子产额会达到饱和，并会使Kα光子发射的各向异性变得更加严重；存在最佳的电子温度，使Kα线转化效率最高. 关键词： 超短超强激光 超热电子 蒙特卡罗方法 Kα线')" href="#">Kα线     

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.     

Effects of Lower-Hybrid Heating on the Hot Electrons in an Electron-Cyclotron-Resonance Plasma

Experiments were performed on an electron cyclotron resonance plasma in a dc magnetic mirror to determine the effects of lower hybrid resonance radiation on the anisotropy of the plasma. It was found that the anisotropy of the plasma hot electrons decreased, the flux of hot electrons escaping through the mirror throats decreased and the midplane wall bremsstrahlung rate slightly increased as lower hybrid resonance power was increased. This is explained by observing that cold plasma, expelled by the lower hybrid radiation, decreases the number of scattering centers in the midplane, which results in a deeper diamagnetic well for the hot electrons.     

Effect of sheath potential on electromagnetic radiation emitted from the rear surface of a metallic foil target

In ultra-intense laser--matter interactions, intense electric fields formed at the rear surface of a foil target may have strong influences on the motion of energetic electrons, and thereby affect the electromagnetic emissions from the rear surface, usually ascribed to transition radiation. Due to the electric fields, transition radiation occurs twice and bremsstrahlung radiation also happens because the electrons will cross the rear surface twice and have large accelerations. In the optic region, transition radiation is dominant. The radiation spectrum depends on the electric field only when the electrons are monochromatic, and becomes independent of the electric field when the electrons have a broadband momentum distribution. Therefore, in an actual experiment, the electric field at the rear surface of a foil could not be studied just with the measurement of optic emissions. In the terahertz region, both bremsstrahlung and transition radiations should be taken into account, and the radiation power could be enhanced in comparison with that without the inclusion of bremsstrahlung radiation. The frequency at which the maximum terahertz radiation appears depends on the electric field.     

Photonuclear fission from high energy electrons from ultraintense laser-solid interactions

A new regime of laser-matter interactions in which the quiver motion of plasma electrons is fully relativistic, with energies extending well above the threshold for nuclear processes, is studied using a petawatt laser system. In solid target experiments with focused intensities exceeding 10(20) W/cm(2), high energy electron generation, hard bremsstrahlung, and nuclear phenomena have been observed. We report here a quantitative comparison of the high energy electrons and the bremsstrahlung spectrum, as measured by photonuclear reaction yields, including the photoinduced fission of 238U.     

Investigation of the effect of nucleus charge and atomic electron distributions on the processes of polarized bremsstrahlung of electrons in crystals

The process of polarized bremsstrahlung of electrons in crystals has been theoretically investigated with application of the new method taking into account the screening of the Coulomb nucleus field by atomic electrons. Real changes in the cross sections of both polarized and unpolarized bremsstrahlung and the degrees of particle polarization in a silicon crystal are obtained.     

Collective acceleration of xenon ions in a plasma-anode vircator

The collective acceleration of xenon ions in a plasma-anode vircator is studied. It is shown that the energy of accelerated ions may reach 900 MeV. The image of a bremsstrahlung source on the target suggests effective transport of relativistic electrons in the drift channel.     

Direct measurement of energetic electrons coupling to an imploding low-adiabat inertial confinement fusion capsule

We have imaged hard x-ray (>100 keV) bremsstrahlung emission from energetic electrons slowing in a plastic ablator shell during indirectly driven implosions at the National Ignition Facility. We measure 570 J in electrons with E>100 keV impinging on the fusion capsule under ignition drive conditions. This translates into an acceptable increase in the adiabat α, defined as the ratio of total deuterium-tritium fuel pressure to Fermi pressure, of 3.5%. The hard x-ray observables are consistent with detailed radiative-hydrodynamics simulations, including the sourcing and transport of these high energy electrons.