超短超强激光与薄膜靶相互作用中不同价态碳离子的来源

研究了超短超强激光与不同厚度薄膜Al靶相互作用中靶背法线方向碳离子的最初来源. 通过对比分析碰撞电离率和场致电离率所起的作用, 发现C⁴⁺ 及更低价态的碳离子主要由场致电离产生, 而高价态的C⁵⁺和C⁶⁺ 离子主要来自于超热电子与靶表面的碰撞电离. 关键词： 超短超强激光与等离子体相互作用 离子加速 场致电离 碰撞电离     

电离效率对激光电离团簇的高价离子产物的影响

纳秒激光与团簇相互作用产生高价离子逐渐成为分子物理界的热点之一, 为了深入研究团簇电离的本质, 本文以分子结构相似、元素组成相同的苯、环己烯和环己烷的分子团簇为对象, 利用飞行时间质谱研究了其与5 ns的532 nm激光相互作用时电离产物的价态和强度分布. 结果表明: 这三种化合物多光子电离效率苯>环己烯>环己烷, 但其高价离子的价态和比值苯是最低的, 环己烷的碳离子最高价态为4价, C³⁺和C²⁺的比值为1.1; 环己烯电离产物C³⁺和C²⁺ 的比值降低为0.6; 苯团簇的最高价态只有3价, C³⁺和C²⁺的比值约为0.4. 引起这种现象的原因可以归结于高的多光子电离效率会导致团簇多位点的电离, 引起团簇在电子加热到发生碰撞电离之前发生解离, 减小了团簇的尺寸, 进而减少了离子发生碰撞电离产生高价离子的反应时间, 最终阻碍了高价态离子的产生.     

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     

ECR源的新工作模式

叙述了兰州重离子加速器ＥＣＲ２源所使用的新工作模式的条件、特点及高电荷态离子束的输出性能，给出了对该模式特点的初步分析．     

Characteristics of highly charged ions produced at Kobe EBIS under modulated operation

The electron beam ion source (Kobe EBIS) has been developed to perform modification of surfaces using highly charged ions (HCIs) at the Kobe University, Japan. Recent study revealed that periodic intermission of electron beam improves charge state distribution of extracted ions. The period of intermission is in the order of 100 ms, and the width of beam-off time is 1 ms or less. This operational mode (pulse mode) makes it possible to produce Ar¹⁵⁺ to Ar¹⁷⁺ effectively, whereas the charge is limited less than 14+ under the ordinary operational mode with direct current (DC) electron beam. A spike of HCIs with a peak current in the order of nA is also observed at each moment of electron beam off. The measurement of the time evolution of Ar¹⁶⁺ intensity around the timing of mode change revealed that the intensity of extracted Ar¹⁶⁺ changes slowly after mode change with a time constant of few seconds.     

Ion storage in Kingdon trap

Abstract

The characteristics of stored ions in a Kingdon trap have been investigated. The charge distribution of stored ions was measured by a time of flight (TOF) mass spectrometer. Storage of Ar^q+ (q = 1, 2, 3, 4) produced by electron beam irradiation has been confirmed. The dependences of Ar ion yields on the trapping potential and storage time have been systematically studied.

Applying a voltage to end plates is very important for the storage of ions. Remarkable oscillations of the ion yields are found in the decay curves as a function of storage time for Ar⁺, Ar²⁺ and Ar³⁺ indicating periodical motion of each ion group about the central wire. The three dimensional orbits of ions in the trap are analysed by a computer calculation to understand the experimental results.     

Study on charge equilibration time of highly charged ions in carbon foils

Charge state distribution of 0.8MeV/u uranium ions after transmission through a thin carbon foil has been studied. It is observed that the charge state distribution is equilibrated after the uranium ions have passed through a 15μg/cm^2 carbon foil. The equilibrated average charge state is 33.72 and the charge equilibration time of uranium ions in carbon foil is less than 5.4fs.     

Study on charge equilibration time of highly charged ions in carbon foils

Charge state distribution of 0.8MeV/u uranium ions after transmission through a thin carbon foil has been studied. It is observed that the charge state distribution is equilibrated after the uranium ions have passed through a 15 $\mu $g/cm$^{2}$ carbon foil. The equilibrated average charge state is 33.72 and the charge equilibration time of uranium ions in carbon foil is less than 5.4fs.     

Probing a cooled beam of decelerated highly-charged heavy ions extracted out of the ESR by a channeling experiment

A cooled beam of decelerated highly-charged heavy ions is slowly extracted out of the cooler and storage ring ESR, by combining the deceleration technique and the charge exchange extraction mode. The quality of the external ion beam is tested by a channeling experiment. Bare Au⁷⁹⁺ ions are injected into the ESR at an energy of 360 MeV/u, decelerated to 53 MeV/u, and finally cooled strongly in the electron cooler. By breeding of neighboring charge state ions via radiative recombination in the electron cooler H-like ions are produced. The H-like ion fraction is extracted out of the storage ring. This extracted Au⁷⁸⁺ ion beam is probed by a channeling experiment measuring the extinction rate of the projectile Kα X-ray yield around the [110] axis of a thin silicon crystal. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measurement of the transverse kinetic energies of argon recoil ions produced in 120 MeV -Ar collisions

Measurements were carried out to deduce the transverse kinetic energies of highly charged argon recoil ions produced in single collisions of 120 MeV ions with argon atoms in which the post collision charge states of the projectiles were not determined. A time of flight spectrometer was designed and fabricated to detect the charge states of recoils. Experimental procedures for optimizing the spectrometer for extraction, transmission and detection of recoils are described. A simple approach for determining the transverse kinetic energy of the recoil ions from FWHM of the peaks is reported. This method is shown to be independent of the choice of collision partners and requires only the knowledge of the physical values of “optimized parameters” of time-of-flight spectrometer used in the experiment. The transverse kinetic energy of the recoil ions determined from the present approach is found to vary from 0.03 eV for to 4.02 eV for Ar¹⁰⁺. These values are compared with the results reported by earlier workers and are shown to follow a q²-behaviour up to a charge state q =8+ of the recoil ions. Received: 5 February 1998 / Revised: 8 June 1998 / Accepted: 11 June 1998     

Cluster-assisted generation of multi-charged ions in nanosecond laser ionization of pulsed hydrogen sulfide beam at 1064 and 532nm

The multi-charged sulfur ions of S^q= (q\le 6) have been generated when hydrogen sulfide cluster beams are irradiated by a nanosecond laser of 1064 and 532,nm with an intensity of 10¹⁰\sim 10¹²W1\cdotcm^-2. S⁶⁺ is the dominant multi-charged species at 1064nm, while S⁴⁺, S³⁺ and S²⁺ ions are the main multi-charged species at 532nm. A three-step model (i.e., multiphoton ionization triggering, inverse bremsstrahlung heating, electron collision ionizing) is proposed to explain the generation of these multi-charged ions at the laser intensity stated above. The high ionization level of the clusters and the increasing charge state of the ion products with increasing laser wavelength are supposed mainly due to the rate-limiting step, i.e., electron heating by absorption energy from the laser field via inverse bremsstrahlung, which is proportional to \lambda ², \lambda being the laser wavelength.     

类氢氖基态电离势随等离子体温度密度的变化

采用自洽场离子球模型,研究类氢氖基态1s的电离势随等离子体电子温度及电子密度的变化规律,计算得到基态电离势的百分偏移量随等离子体电子密度的变化关系,拟合结果表明两者的对数值满足很好的线性关系.该结果对计算等离子体电离态分布及光谱模拟具有一定意义.     

Tungsten ions produced by infrared pulsed laser irradiation

Energetic ions have been obtained irradiating a tungsten target with a Q-switched Nd:Yag laser, 1064?nm wavelength, 9?ns pulse width, 900?mJ maximum pulse energy and power density of the order of 10¹⁰?W/cm². The laser-target interaction induces a strong metal etching with production of plasma in front of the target. The plasma contains neutrals and ions with high charge state. Time-of-flight measurements are presented for qualitative analysis of the ion production. A cylindrical electrostatic ion analyzer permits measuring of the yield of emitted ions, the charge state of detected ions and the ion energy distribution. Measurements indicate that, at a laser fluence of the order of 100?J/cm², the charge state may reach 9+ and the ion energy reaches about 5?keV. The ion energy distribution is given as a function of the charge state. Experimental results indicate that an electrical field is developed along the normal to the plane of the target surface, which accelerates the ions up to high velocity. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the author has corrected for the Coulomb interactions occurring inside the plasma.     

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.     

强激光与高密度气体相互作用中电子和离子加速的数值模拟

在现有的一维粒子模拟程序的基础上发展了带光电离和碰撞电离及蒙特卡罗两体碰撞的模拟程序(1D PIC-MCC). 用此程序模拟研究了短脉冲激光与He气靶相互作用时电子和离子的加速过程. 研究表明当强激光与过临界密度的微米厚度的平面靶相互作用时，靶前表面物质将被激光脉冲前沿迅速离化；新生的电子被激光场有质动力加速成为高能电子，这些电子穿入到靶内，通过电子碰撞电离离化靶内物质；一部分高能电子穿透靶后，会在靶的后表面形成强的电荷分离场，该场迅速离化靶后表面物质，同时使得后表面离子得到加速. 部分穿透靶的超热电子将被电荷分离场重新拉回靶内，在靶的前后表面振荡. 一些振荡电子在此过程中得到电荷分离场加速，离开前表面，在前表面也形成电荷分离场，使前表面离子得到加速. 关键词： 激光等离子体 光电离和碰撞电离 电子加速 离子加速     

光场电离气体原子体系的剩余电子能量

总结并推导了在考虑激光脉冲包络时超短强激光脉冲光场电离多电子气体原子体系的阈上电离能量的计算公式,消除了以往研究中的唯象假设。发展了产生各阶离化态离子的阈值光强和饱和光强的计算方法。数值模拟表明,随激光能量升高,平均剩余电子能量呈现出类似阶梯型平台的饱和行为。     

激光金等离子体的电子离子碰撞电离速率系数

电子离子碰撞电离过程在超组态碰撞辐射(SCROLL)模型中真实模拟非局域热动力学平衡(non-LTE)高Z材料Au激光等离子体M带谱5f-3d跃迁中各种复杂离子的电离态特性,诸如离子的平均电离度和电荷态分布是一个主要过程.基于准相对论扭曲波玻恩交换近似,采用组态平均的方法,从头计算了金M带类铁金离子-类锗金离子的电子离子碰撞电离速率系数,其中电离截面的高能行为由Bethe系数决定.结果表明:在"神光Ⅱ"实验装置诊断的电子温度～2keV,电子密度～6×1021cm-3范围内,这些参数有利于使用超组态碰撞辐射模型拟Au的激光等离子体M带细致谱5f-3d跃迁的平均电离度和电荷态分布.     

Investigation of the transfer ionization process in collisions of partially stripped ions on He

In this paper a projectile ion-recoil ion coincidence technique is used to investigate the transfer ionization processes in collisions of 0.22--6.30~MeV C^{q + } ions and 0.25--6.35~MeV O^{q + } ions (q=1, 2, 3, 4) with the He atom separately. The cross section ratio f of transfer ionization to single electron transfer is measured, and the dependence of f on both charge state q and energy E of the projectiles is investigated. The electron-structure and the mechanisms leading to transfer ionization affect the dependence of f on q and E. Our measurements, along with other data published previously, suggest a similar dependence of f on charge state and energy of projectile for partially stripped ions over a large energy range. The maximum value of f is approximately 0.17q^{0.60}; the energy corresponding to maximum f is about 160q^{0.60}~keV/u.     

Preparation of Macroscopic Quantum-Interference States for a Collection of Trapped Ions Via a Single Geometric Operation

We describe a scheme for the generation of macroscopic quantum-interference states for a collection of trapped ions by a single geometric phase operation. In the scheme the vibrational mode is displaced along a circle with the radius proportional to the number of ions in a certain ground electronic state. For a given interaction time, the vibrational mode returns to the original state, and the ionic system acquires a geometric phase proportional to the area of the circle, evolving from a coherent state to a superposition of two coherent states. The ions undergo no electronic transitions during the operation. Taking advantage of the inherent fault-tolerant feature of the geometric operation, our scheme is robust against decoherence.     

托卡马克离子温度梯度湍流输运同位素定标修正中杂质的影响

托卡马克实验发现,在不同参数条件下,等离子体能量约束经验定标律会有或大或小的修正.为解释这种修正现象发生的原因,应用回旋动理学方法,对含重(钨)杂质等离子体离子温度梯度(ITG)(包括杂质模)湍流输运的同位素效应进行了数值研究.结果表明钨杂质效应极大地修改了同位素定标律和有效电荷效应.随着杂质离子电荷数Z和电荷集中度f_z的变化,同位素定标律在较大范围内变化. ITG模最大增长率定标大约为M_i~(-0.48→-0.12),杂质模的定标为M_i~(-0.46→-0.3),其中, M_i表示主离子质量数.在ITG模湍流中,有效电荷数越大,关于M_i的拟合指数偏离-0.5越远,表现为同位素质量依赖减弱.在两种模中,杂质电荷集中度越大,同位素质量依赖越弱.研究了杂质效应使定标关系发生偏离的原因,证实杂质种类、杂质电荷数和杂质浓度的不同,是引起同位素质量依赖发生改变的重要原因.结果证实并解释了不同参数条件下托卡马克同位素定标的差异性.研究成果可以为ITER实验安排及杂质相关输运实验中选择装置材料、工作气体和设置其他参数提供理论参考.