微束斑X射线源LaB6阴极电子枪的设计

LaB6阴极电子枪是微束斑X射线源的关键组成部件之一。详细的讨论了电子枪的各组成电极 ,即LaB6阴极、Wehnelt栅极和阳极的设计。采用边界元法 (BEM )对电子枪内部电场电位和场强分布进行了有效的数值计算 ,并从阴极开始起利用四阶Runge Kutta法追踪电子的运动轨迹。给出了当阳极端口处电子束半径为设计值时各电极电压、电极结构等工作参量 ,利用该LaB6阴极电子枪可以满足微束斑X射线源需要高亮度、微细电子束发射的要求。     

微束斑X射线源及X射线光学元件

高质量的X射线源,尤其高亮度的微纳束斑X射线源是现代X射线光学高清晰成像最为关键的部件之一,在工业无损探伤、生命科学、材料科学等科学研究和实际应用中具有重要的意义。简单介绍了微束斑X射线源的产生方法及发展历史,并对微束X射线光学涉及到的聚焦X射线光学元件（如X射线掠入射反射镜、布拉格法反射镜、多层膜反射镜、多层膜光栅、X射线波带片、毛细管聚焦透镜和复合折射透镜等）的主要特点作了简要的系统介绍。最后展望了微细束X射线在微纳检测与分析等方面的应用前景。     

微束斑X射线源聚焦系统的数值研究

本文研究了由两个磁透镜组成的电子束打靶微束斑X射线源的聚焦系统.首先利用有限元法计算了磁透镜的磁感应强度的轴上分布,在此基础上用三次样条函数插值计算了磁场的空间分布,最后用四阶Runge-Kutta法追踪了电子的运动轨迹,并给出了入射电子束经过聚焦系统后的缩小倍率.     

一种新型微束斑X射线源的研究

采用高效长寿命的LaB6单晶阴极电子枪作为电子束发射系统、轻便的等径双圆筒静电系统作为聚焦系统等技术研究了一种新型台式微束斑X射线源.通过在综合测试仪上的初步实验研究,表明该微束斑X射线源不仅可以连续辐射,而且可以脉冲辐射X射线,输出X射线束斑小、亮度高.此外,该台式X射线源具有体积小、重量轻,使用寿命长、造价低,可灵活移动等优点.     

聚焦慢速高荷态重离子束微束斑X射线源

利用电子束离子源(EBIS)或者电子束离子陷阱(EBIT)产生的慢速高电荷态重离子束轰击金属靶面,离子束与靶面作用并复合辐射特征X射线;并将高荷态离子束采用离子光学系统会聚为微细束后再与靶面作用,能够辐射出微米甚至亚微米级、纳米级的微束斑X射线.本文介绍这一新型微束斑X射线源的结构、机理及其特性等.     

微束斑X射线源发射系统的数值计算

本文采用边界元法(BEM)对高亮度微束斑X射线源的LaB₆阴极、Wehnet栅极和阳极等发射系统的电位和场强分布进行了数值计算,首先计算了系统对称轴上的电位,再利用三次样条插值法和谢赫尔公式等计算了系统在考虑三次相差情况下的傍轴电场强度分布.     

基于空间应用的透射式微型微束调制X射线源

针对空间应用对X射线源的需求, 提出了一种透射式微型微束调制X射线源的设计方案, 建立了调制X射线源的理论模型. 相较传统的X射线源, 增加了栅极电压控制和多个聚焦极微束聚焦功能, 通过改变栅极电压实现X射线的幅度调制和脉冲调制. 利用带电粒子光学仿真软件SIMION, 模拟计算了不同管电压下透射式阳极靶的最佳靶厚, 仿真分析了不同栅极电压对电子束运动轨迹的影响, 最终得到了150 μm的微束焦斑直径. 完成了原理样机的加工镀膜和真空密封, 搭建了调制X射线源的测试装置, 实验报道了阳极钨靶的谱线特性, 分析了栅极电压影响出射X射线强度的原因, 讨论了栅极幅度调制的可能性, 完成了调制X射线源栅极脉冲调制的验证.     

准单色近平行光束的X射线源

针对准单色近平行光束X射线背光成像诊断需求,提出了一种用球面弯晶进行X射线衍射选单从而获取准直光束的新方案.在神光Ⅱ装置上,设计了基于球面弯晶X射线衍射选单准直光束系统,完成了该系统的安装、调试和实验应用,获得了准单色(10~(-3)△λ/λ10~(-2))、小发散角(2 mrad)和大辐照匀斑(直径φ500 m)的X射线光源.同时基于衍射光学和球面镜成像理论,研究了不同布拉格角对球面弯晶X射线衍射光束发散角及其像散差的影响.结果表明,布拉格角会影响球面弯晶X射线衍射光束的发散角.用控制布拉格角范围的方法有望获得发散角优于1 mrad的近平行光束X射线光源.这种准单色、极小发散度和均匀角分布的X射线光源可应用于高分辨X射线成像诊断.     

9MeV,小于0.1mm微焦点X射线源验证实验

密度高、成像分辨率高、成像速度快的X射线数字成像检测需要高能微焦点大剂量X射线源,高品质电子源是实现这一X射线源的关键手段。基于中国工程物理研究院太赫兹自由电子激光的主加速器,验证了低发射度、低能散度的高亮度电子束实现高能微焦点的可行性,得到电子束半高全宽尺寸小于70μm的9 MeV微焦点,并初步开展成像实验,双丝像质计焦斑清晰分辨9D号丝,丝直径0.13 mm。     

微结构X射线源中电子与阳极作用的蒙特卡罗模拟

根据电子多重散射理论,基于蒙特卡罗方法研究不同能量电子垂直入射不同材料的阳极靶后,电子在阳极靶内的沉积能量分布,及X射线发射位置的能量分布.结果表明：电子在靶内的轨迹扩展和入射电子能量、靶材料有关,99%电子能量沉积在近似圆柱形区域内.且电子在入射方向上的沉积能量分布不是直接递减,而是先递增到一定深度后再递减,符合电子背散射理论.说明X射线产生区域是在距表面一定深度区域内.另外,通过对比分析发现金刚石膜作为电子吸收光栅是不可行的,但作为热沉材料有很大的潜力.这些结论可为微结构X射线源研究及高亮度高相干X射线源设计提供参考.     

水窗波段附近激光等离子体X光辐射产生的途径

讨论了利用激光等离子体产生水窗波段Ｘ光辐射的可能途径。靶材，即原子序数的选取是决定产生这一波段辐射的基本因素，利用激光等离子体的平衡态和非平衡态特性都可在水窗波段产生亮度Ｘ光辐射，提出利用黑体辐射来实现水窗波段Ｘ光显微成像研究的方案。     

高精度有限差分法模拟Kelvin-Helmholtz不稳定性

 WENO有限差分格式有较高的分辨精度,适合复杂流场的计算,在国际上被广泛采用。本文利用WENO有限差分格式求解2维守恒型欧拉方程,实现了对无粘流体中Kelvin-Helmholtz不稳定性的数值模拟。速度剪切方向采用周期边界条件;扰动增长方向采用嵌边出流边界条件,一个不稳定波长分布64个网格。数值模拟给出的扰动幅值线性增长率与线性稳定性分析给出的结果很好符合,显示了该格式的有效性和精度。数值模拟给出了清晰的密度等值线,表明该方法还具有较好的界面变形捕捉能力。     

First experiments on the Australian Synchrotron Imaging and Medical beamline,including investigations of the effective source size in respect of X‐ray imaging

The Imaging and Medical beamline at the Australian Synchrotron achieved `first light' in December 2008. Here, the first experiments performed on the beamline are reported, which involved both X‐ray imaging and tomography studies for a range of samples. The use of a plastic‐edge phantom for quantitative measurements of contrast and resolution proved to be very instructive and helped to confirm certain parameter values such as the effective horizontal source size, detector resolution and average X‐ray energy for the polychromatic beam.     

A high‐accuracy complex‐phase method of simulating X‐ray propagation through a multi‐lens system

The propagation of X‐ray waves through an optical system consisting of many X‐ray refractive lenses is considered. For solving the problem for an electromagnetic wave, a finite‐difference method is applied. The error of simulation is analytically estimated and investigated. It was found that a very detailed difference grid is required for reliable and accurate calculations of the propagation of X‐ray waves through a multi‐lens system. The reasons for using a very detailed difference grid are investigated. It was shown that the wave phase becomes a function, very quickly increasing with increasing distance from the optical axis, after the wave has passed through the multi‐lens system. If the phase is a quickly increasing function of the coordinates perpendicular to the optical axis, then the electric field of the wave is a quickly oscillating function of these coordinates, and thus a very detailed difference grid becomes necessary to describe such a wavefield. To avoid this difficulty, an equation for the phase function is proposed as an alternative to the equation of the electric field. This allows reliable and accurate simulations to be carried out when using the multi‐lens system. An equation for the phase function is derived and used for accurate simulations. The numerical error of the suggested method is estimated. It is shown that the equation for the phase function allows efficient simulations to be fulfilled for the multi‐lens system.     

利用拉曼自由电子激光测量目标毫米波散射特性的初步研究

介绍拉曼自由电子激光器应用于目标的毫米波散射特性研究所做的一些探索性工作。自由电子激光器是七十年代中期出现的一种新型强相干辐射源。拉曼自由电子激光输出在毫米波段具有高功率、可调谐等优点。利用自由电子激光辐射高能量和宽频带的特点，研究及材料的毫米波散射特性，开创了新的研究方向和研究途径，具有重大的应用前景。本文在这方面进行了探索性研究，得到了一些初步结果。     

Simulation of the Formation of the Squeezed State of an Electron Beam with a Sublimiting Beam Current in a Closed Drift Tube

Technical Physics - The dynamics of formation of a virtual cathode and the squeezed beam state (SBS) in a setup with a sublimiting relativistic electron beam under repeated input of the electron...     

Enhancement of measurement accuracy of X‐ray PIV in comparison with the micro‐PIV technique

The X‐ray PIV (particle image velocimetry) technique has been used as a non‐invasive measurement modality to investigate the haemodynamic features of blood flow. However, the extraction of two‐dimensional velocity field data from the three‐dimensional volumetric information contained in X‐ray images is technically unclear. In this study, a new two‐dimensional velocity field extraction technique is proposed to overcome technological limitations. To resolve the problem of finding a correction coefficient, the velocity field information obtained by X‐ray PIV and micro‐PIV techniques for disturbed flow in a concentric stenosis with 50% severity was quantitatively compared. Micro‐PIV experiments were conducted for single‐plane and summation images, which provide similar positional information of particles as X‐ray images. The correction coefficient was obtained by establishing the relationship between velocity data obtained from summation images (V_S) and centre‐plane images (V_C). The velocity differences between V_S and V_C along the vertical and horizontal directions were quantitatively analysed as a function of the geometric angle of the test model for applying the present two‐dimensional velocity field extraction technique to a conduit of arbitrary geometry. Finally, the two‐dimensional velocity field information at arbitrary positions could be successfully extracted from X‐ray images by using the correction coefficient and several velocity parameters derived from V_S.     

Lattice Boltzmann Simulation of Mixed Convection Heat Transfer in a Driven Cavity with Non-uniform Heating of the Bottom Wall

The goal of this article is to study numerically the mixed convection in a differentially heated lid-driven cavity with non-uniform heating of the bottom wall. The velocity field is solved by a hybrid scheme with multiple relaxation time Lattice Boltzmann(MRT-LBM) model, while the temperature field is obtained by resolution of the energy balance equation using the finite difference method(FDM). First, the model is checked and validated using data from the literature. Validation of the present results with those available in the literature shows a good agreement.A good efficiency in time simulation is confirmed. Thereafter, the model has been applied to mixed convection in a driven cavity with non-uniform heating wall at the fixed Grashof number Gr = 106. It is found that, the heat transfer is weakened as the Richardson number is augmented. For Gr = 106, we note the appearance of secondary vortices at different positions of the cavity corners.     

基于道格拉斯格式的宽角有限差分光束传播法分析大角度交叉波导

交叉波导是集成光学中的一种重要的器件单元，针对大角度交叉波导的结构及折射率分布，运用基于广义道格拉斯算子的宽角多步有限差分光束传播法模拟大角度交叉波导的传输特性，数值计算出波导中的光场的分布。与Crank-Nicholson格式的有限差分光束传播法相比，该算法在几乎不增加计算时间和计算机内存资源的情况下，结合了广义道格拉斯格式的有限差分光束传播法截断误差小[只有o(△x)^4]和宽角多步法所具有较高精度的优点，是光波导器件理想的数值模拟工具。在此基础上计算分析了大角度交叉波导的输出分支波导的能量与交叉角度的关系，以及交叉角度对波导间能量耦合的影响。这为实际的设计制作光器件的工作提供了极好的基础。