12MeV直线感应加速器轫致辐射靶靶面回流离子测量

强流直线感应加速器（LIA）能够产生2—3kA、10-20MeV、约80ns（FWHM）的强流高功率的脉冲电子束，经过数十米的传输、聚焦成毫米量级的束斑后打击到轫致辐射靶上，来产生高剂量的X光。同时电子束打靶使靶面沉积大量的能量导致靶面的温度骤然升高，引起靶表面物质或杂质（如碳、氢、水蒸气与靶材料等）被汽化电离而产生等离子体。强流电子束在靶前附近产生的强空间电荷电场（场强可达MV／cm）把离子从靶面拉出，以逆着电子束的方向前进，被称为回流离子。人们提出回流离子与电子束发生作用，会导致电子束被提前聚焦，在预定的靶面形成散焦。     

回流离子对强流相对论电子束传输的影响

 介绍了采用双膜法测量神龙一号直线感应加速器靶区回流离子效应的实验工作,通过一片厚度数十μm的靶膜产生回流离子,并采用基于光学渡越辐射的电子束剖面测量系统记录时间分辨的束斑,首次证实了神龙一号加速器靶区存在回流离子。通过采用不同材料的靶膜,实验观测到了不同离子发射情况下回流离子对强流相对论电子束传输的影响,结果发现采用金属靶膜时,回流离子导致电子束部分汇聚、部分发散,而采用聚合物薄膜时,回流离子会导致电子束剖面出现剧烈的变化。     

用集团运动的观点来研究回流离子问题

建立在单粒子运动学基础上的回流离子理论预测: 强流电子束轰击在 辐射转换靶上可能产生正离子. 这些离子在电子束空间电荷场作用下 回流, 会造成电子束过聚焦, 改变焦斑大小, 从而影响X光机的照相分辨率. 然而大量的实验没有发现相关的现象. 本文分析认为, 电子束 打靶时, 在靶表面可能会形成离子鞘层过渡区. 该区域可以抵消束流空间 电荷场对回流离子的驱动作用, 因此, 回流离子可能以等离子体集团扩散的 方式运动. 这种物理图像得到的结论是离子对聚焦的影响可忽略, 和已有的 实验结果相吻合.     

回流离子源特征分析

强流电子束轰击在辐射转换靶上可产生正离子. 这些离子在电子束空间电荷场作用下回流，对电子束聚焦会有影响，从而可能改变焦斑大小，影响X光机的照相分辨率. 离子种类、离子流的大小是很关注的参量. 本文分析了回流离子来源及其特征，认为回流离子以靶表面杂质热释放后被电子束直接电离生成的离子为主，离子发射受空间电荷限制而非受源限制.     

电子束在微束斑X射线源中运动轨迹的计算

轰击金属靶面的高能电子束束斑大小,是决定微束斑X射线源最终X射线束斑尺寸的关键因素之一.当LaB₆晶体阴极发射电流为60μA时,采用5点不等距有限差分法(FDM)计算了整个仪器内旋转对称电子光学系统电场的分布,并利用Runge Kutta法从LaB₆阴极发射端面开始追踪了电子束在整个系统内部的运动,经计算,聚焦在靶面上的电子束斑直径约为600~1000nm.     

电子束离子阱中高价态离子演化过程的数值模拟

讨论了电子束离子阱(EBIT)中决定高价态离子演化过程的主要物理机制.对EBIT中高价态离 子的演化过程进行了详细的数值计算并与实验进行了比较.讨论了EBIT各种不同的运行参数 对平衡时高价态离子相对丰度和温度的影响. 关键词： 电子束离子阱 高价态离子 数值模拟     

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.     

叠靶研究

 直线感应加速器（LIA）产生的高能、强流电子束与轫致辐射靶作用能够产生具有高剂量、小焦斑的X光，但伴随产生的回流离子会导致电子束束斑变大与X光分辨率降低，在多脉冲情况下更会影响到后续电子束的束靶作用等。叠靶结构能够增大束靶作用的立体空间，降低在靶面的能量沉积，可有效抑制回流离子的产生。对叠靶结构模型进行了理论计算与实验研究，并与单靶情况相比较，证实了在两种靶结构下所得到的X光照射量大小与角分布基本相同，但对于叠靶情况下靶面没有出现烧蚀现象，从而从根本上抑制了由靶面产生回流离子而对束流产生的过聚焦效应。     

神龙一号轫致辐射靶靶区回流离子测量

神龙一号直线感应加速器(LIA)产生的强流高功率的脉冲电子束与X光转换靶作用后可以产生高剂量的X光,同时由于转换靶的被烧蚀破坏在靶面产生回流离子,该回流离子的存在影响到电子束的聚焦。设计了4套法拉第筒及其对应的偏压电路,法拉第筒被放置在神龙一号X光转换靶上游不同位置,分布在电子束轴线两侧,电路设计最高偏压为1 kV;对神龙一号LIA的X光转换靶面产生的回流离子进行了实验测量,分别得到回流正离子密度约在1021/m3,离子运动速度可达2～3 mm/s。计算比较表明,该离子流强度与神龙一号靶前电子束流相差很大,只有电子束流强的0.27%,对神龙一号电子束聚焦不会造成影响。     

12MeV LIA轫致辐射靶面回流离子测量

 强流高功率脉冲电子束聚焦成mm量级的束斑后，打击到轫致辐射靶的过程中会产生回流离子，它会导致电子束被提前聚焦，在预定的靶面形成散焦。描述了法拉第筒对12MeV 直线感应加速器的轫致辐射靶面可能产生的离子及其参数进行的实验测量，并对实验结果进行了分析讨论。结果表明在靶前60°~70°方向未发现回流离子。     

电子束离子阱及高电荷态离子相关物理

文章简要介绍了电子束离子阱（EBIT）的发展背景及其在国际上的状况，较详细地解释了它的结构和工作原理，介绍了它在分解研究等离子体方面的特别优势以及在EBIT上能够实现的高电荷态离子相关的前沿物理学问题研究。     

A high-current electron beam ion trap as an on-line charge breeder for the high precision mass measurement TITAN experiment

The precision of atomic mass measurements in a Penning trap is directly proportional to the charge state q of the ion and, hence, can be increased by using highly charged ions (HCI). For this reason, charge breeding with an electron beam ion trap (EBIT) is employed at TRIUMF’s Ion Trap for Atomic and Nuclear science (TITAN) on-line facility in Vancouver, Canada. By bombarding the injected and trapped singly charged ions with an intense beam of electrons, the charge state of the ions is rapidly increased inside the EBIT. To be compatible with the on-line requirements of short-lived isotopes, very high electron beam current densities are needed. The TITAN EBIT includes a 6 Tesla superconducting magnet and is designed to have electron beam currents and energies of up to 5 A and 60 keV, respectively. Once operational at full capacity, most species can be bred into a He-like configuration within tens of ms. Subsequently, the HCI are extracted, pass a Wien filter to reduce isobaric contamination, are cooled, and injected into a precision Penning trap for mass measurement. We will present the first results and current status of the TITAN EBIT, which has recently been moved to TRIUMF after assembly and commissioning at the Max-Planck-Institute (MPI) for Nuclear Physics in Heidelberg, Germany.     

An EBIS/T with integrated Penning trap

A special problem in atomic physics research with highly charged ions is to prepare ions with a unique charge state inside of EBIS or EBIT devices. On the other hand, there are great losses resulting from the transport of the ions from the source to an external trap. Therefore we are setting up an EBIS/T with internal Penning trap. This new set-up will be able to study electron–ion interaction with well-defined initial and final charge states, distinguishing between single step successive ionisation and multiple step ionisation of charge states similar to the crossed beams method but for much higher charge states. Another feature of this system is to determine with high precision the ion charge state distribution in the EBIS/T by application of Fourier Transform Ion Cyclotron Resonance (FT-ICR). This method allows the on-line monitoring of the ion distribution and the evolution of the charge state population together with its dependence on the degree of space charge compensation of the electron beam in the EBIS/T. It will be possible to study ion dynamics in compensated space charge potentials. In case of high homogeneity of the magnetic field in the trap region, experiments may be considered to measure directly binding energies of highly-charged ions and other topics of high resolution mass spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electron beam density study using a portable slit imaging system at the Shanghai Electron Beam Ion Trap

In this work,a portable slit imaging system is developed to study both the electron beam diameter and the profile of the newly developed Shanghai Electron Beam Ion Trap (Shanghai EBIT).Images are detected by a charge coupled device (CCD) sensitive to both X rays and longer wavelength photons (up to visible).Large scale ray tracings were conducted for correcting the image broadening effects caused by the finite slit width and the finite width of the CCD pixels.A numerical de-convolution method was developed to analyse and reconstruct the electron beam density distribution in the EBIT.As an example of the measured beam diameter and current density,the FWHM (full width at half maximum) diameter of the electron beam at 81 keV and 120 mA is found to be 76.2 μm and the density 2.00 × 10 3 A·cm 2,under a magnetic field of 3 T,including all corrections.     

X-ray, visible and electron spectroscopy with the NIST EBIT

An overview is given of recent activities at the NIST electron beam ion trap (EBIT) facility. The machine has been operational for almost three years. Important characteristics and demonstrated capabilities of our EBIT are presented. Selected results include experiments with trapped highly charged ions (X-ray and visible spectroscopy), and with extracted ions (ion-surface collision studies). This revised version was published online in August 2006 with corrections to the Cover Date.     

不同磁路电子回旋共振离子源引出实验

空间推进所用的电子回旋共振离子源(ECRIS)应具有体积小、效率高的特点. 本文研究的ECRIS使用永磁体环产生磁场, 有效减小了体积, 该离子源利用微波在磁场中加热电子, 电子与中性气体发生电离碰撞产生等离子体. 磁场在微波加热电子的过程中起关键作用, 同时影响离子源内等离子体的约束和输运. 通过比较四种磁路结构离子源的离子电流引出特性来研究磁场对10 cm ECRIS性能的影响. 实验发现: 在使用氩气的条件下, 特定结构的离子源可引出160 mA的离子电流, 最高推进剂利用率达60%, 最小放电损耗为120 W·A^-1; 所有离子源均存在多个工作状态, 工作状态在微波功率、气体流量、引出电压变化时会发生突变. 离子源发生状态突变时的微波功率、气体流量的大小与离子源内磁体的位置有关. 通过比较不同离子源的引出离子束流、放电损耗、气体利用率、工作稳定性的差异, 归纳了磁场结构对此种ECRIS引出特性的影响规律, 分析了其中的机理. 实验结果表明: 保持输入微波功率、气体流量、引出电压不变时, 增大共振区的范围、减小共振区到栅极的距离, 离子源能引出更大的离子电流; 减小共振区到微波功率入口、气体入口的距离能降低维持离子源高状态所需的最小微波功率和最小气体流量, 提高气体利用率, 但会导致放电损耗增大. 研究结果有助于深化对此类离子源工作过程的认识, 为其设计和性能优化提供参考.     

EBIT as a versatile experimental facility

The Electron Beam Ion Trap (EBIT) produces ions, confined within the electron beam, with charges ranging up to U⁹²⁺ at near rest energies. This allows to study the interaction of a monoenergetic electron beam with any trapped ion to a high degree of precision via X-ray spectroscopy. The development of the EBIT into an ion (trap) source enables the possibility to perform for the first time studies of the interaction dynamics in strong fields of ions with matter where the ions carry hundreds of keV potential energy at very low kinetic energies (eV).     

考虑到束-波相互作用的速调管离子噪声二维模拟

采用粒子模拟的方法并考虑电子束与电磁波的相互作用，首次直接得到了速调管输出信号的离子噪声图像，阐述了束电子、二次电子、离子、电磁场之间的相互作用的动力学过程. 指出离子噪声所表现出来的相位波动是由电子束速度的波动引起的，电子束速度的变化来源于管内离子数量的变化，离子的数量的变化又与电子束状态变化相互影响，这是离子噪声产生的根本原因. 二次电子对离子噪声产生过程的影响甚微，但是其行为却反映了离子噪声的形成机理. 离子噪声引发的输出信号幅度波动取决于电子束速度和半径的改变，与离子行为密切相关. 关键词： 离子噪声 速调管 粒子模拟 电子束     

宽束冷阴极离子源分时中和技术的研究

离子束辅助镀膜沉积过程中,绝缘薄膜表面的电荷积累效应严重影响了薄膜质量。通过对宽束冷阴极离子源引出栅部分的改进,采用分时引出电子和离子方法,使正负电荷中和,以消除薄膜表面的放电现象,并对引出电子束的束流密度、能量、发射角等参数进行了测试。实验结果表明：在引出电压为600V时,电子的平均能量为100eV左右;引出电子束的发射角可以达到±40°,在±15°范围内的束流密度波动小于±5%。引出电子的束流密度较同参数下的离子束流密度小,通过调节脉冲电源的占空比,可达到很好的中和效果。     

Ionization of iridium ions in the Dresden EBIT studied by X-ray spectroscopy of direct excitation and radiative recombination processes

Ir^q+ ( 41≤q≤64) ions with open-shell configurations have been produced in the electron beam of the room-temperature Dresden Electron Beam Ion Trap (Dresden EBIT) at electron excitation energies from 2 keV to 13 keV. X-ray emission from direct excitation processes and radiative capture in krypton-like to aluminium-like iridium ions is measured with an energy dispersive Si(Li) detector. The detected X-ray lines are analyzed and compared with results from multiconfigurational Dirac-Fock (MCDF) atomic structure calculations. This allows to determine dominant produced ion charge states at different electron energies. The analysis shows that at the realized working gas pressure of 5×10^-9mbar for higher charged ions the maximum ion charge state is not preferently determined by the chosen electron beam energy needed for ionization of certain atomic substates, but by the balance between ionization and charge state reducing processes as charge exchange and radiative recombination. This behaviour is also discussed on the basis of model calculations for the resulting ion charge state distribution. Received 12 July 2001 and Received in final form 10 September 2001