Material dependence of total electron emission yields following slow highly charged ion impact

The total electron emission yields following the interaction of “Slow (2 keV/a.u.) Highly Charged Ions” (SHCI) (O³⁺⁷⁺, Xe¹²⁺⁵²⁺, Au⁵⁴⁺⁶⁹⁺) with different target surfaces (highly-oriented pyrolytic graphite (HOPG), Au and SiO2) have been measured. The emission yields increase with charge state, and is found to be highest for carbon, the HOPG target, and lowest for the SiO₂ target. An empirical formula for the electron emission is including recent results from investigations of plasmon excitation following SHCI impact are used to interpret the results.     

Electron emission yields from boron-like Ar ions impinging on Au(1 0 0)

Using a new experimental station to be installed at the HITRAP facility at GSI we studied electron emission yields of Ar¹³⁺ ions impinging on a clean Au(1 0 0) surface. By taking data under different incidence angles and at different initial kinetic energies, contributions from kinetic and potential electron emission are separated. The number distributions of the emitted electrons exhibit signatures of specific trajectory classes contributing differently to the electron emission yields. Support for the identification of the different trajectory classes is obtained from SRIM simulations.     

Critical capture distances for highly charged ions above dielectric covered metal surfaces

We model the first stage of the electronic interaction between an ion and a metal surface covered with a thin dielectric layer. Specifically, we seek to answer two questions. (i) As an ion approaches the surface from far away, does the first electron that it captures originate from the exposed dielectric layer or the metal underneath it? (ii) What is the ion’s distance from the metal when the first electron is captured? To answer these questions, the classical potential that an electron is subject to during the interaction is calculated. The dielectric film is treated as a continuum with simple band structure. We input the parameters from recent experiments (Co with 1.5 nm thick Al₂O₃ film) and found that (i) the first capture proceeds from the metal, and (ii) the dielectric film extends the distance threshold for first capture compared to a metal with no film.     

Simulation of transmission of slow highly charged ions through insulating tapered macro-capillaries

The field of charged-particle transmission through insulating nanocapillaries has expanded its scope within the last few years. Originally motivated by research on elementary ion-insulator interactions recent work has shifted the focus on the development of tools for ion-beam shaping and guiding. The design of tapered macrocapillaries has attracted growing interest and has found first applications in directing ions to targeted regions of biological cells for microsurgery. Due to the large dimensions of these capillaries, simulation of such systems faces considerable difficulties which we address in this paper. A first proof-of-principle simulation is presented.     

Electron emission from tungsten induced by slow, fusion-relevant ions

Tungsten has recently been introduced as a new wall material for fusion, because it exhibits favourably low sputtering yields and a very low tritium retention as compared to the commonly used graphite wall and divertor tiles. We measure total electron emission yields due to impact of slow singly and multiply charged ions (deuterium, helium and carbon) on sputter-cleaned polycrystalline tungsten surfaces by using a current method in combination with a retarding grid. Results are presented in the eV to keV impact energy region as typical for fusion edge plasma conditions and discussed in terms of potential and kinetic electron emission.     

A deceleration system at the Heidelberg EBIT providing very slow highly charged ions for surface nanostructuring

Recently, it has been demonstrated that each single-impact of a slow (typically 1-2 keV/u) highly charged ion (HCI) creates truly topographic and non-erasable nanostructures on CaF₂ surfaces. To further explore the possibility of nanostructuring various surfaces, using mainly the potential energy stored in such HCIs, projectiles with kinetic energies as low as possible are required. For this purpose a new apparatus, capable of focusing and decelerating an incoming ion beam onto a solid or gaseous target, has been installed at the Heidelberg electron beam ion trap (EBIT). An X-ray detector and a position-sensitive particle detector are utilized to analyze the beam and collision products. First experiments have already succeeded in lowering the kinetic energy of HCIs from 10 keV/q, down to ∼30 eV/q, and in focusing the decelerated beam to spot sizes of less than 1 mm², while maintaining the kinetic energy spread below ∼20 eV/q.     

Nanostructuring of BaF2 (1 1 1) surfaces by single slow highly charged ions

The creation of surface nanostructures in BaF₂ (1 1 1) surfaces was studied after irradiation with slow highly charged Xe ions from the Dresden-EBIT (electron beam ion trap). After irradiation, the crystals were investigated by scanning force microscopy (SFM). Using specific ion parameters, the topographic images show nanohillocks emerging from the surface. Additionally, we used the technique of selective chemical etching to reveal the lattice damage created by ion energy deposition below and above threshold needed for surface hillocks formation. The role of both potential and kinetic energy as well as a comparison with results for swift heavy ion irradiations of BaF₂ single crystals are presented.     

Potential energy threshold of surface erosion on GaN by impact of slow highly charged heavy ions

In the present work the erosion behavior on the surface of GaN epi-layer by the impact of various slow highly charged heavy ions (SHCIs, including Ar^q+, Xe^q+ and Pb^q+, in two incidence geometries) was investigated. Atomic force microscopy reveals a well-defined threshold of potential energy carried by the incident heavy ions accounting for the surface erosion. This threshold also depends on the projected range of the SHCIs, the longer the projected range, the higher the potential energy required for the onset of surface erosion. And the etched depth is close to a linear function of potential energy deposited, increasing with the potential energy increases. Moreover, the etching rate for 60° off normal incidence is by more than a factor of 2 larger than etching rate for normal incidence, and the etch rate by Xe^q+ is larger than by Pb^q+ under the same potential energy and incident direction. And a mechanism is discussed.     

Using a current method for measuring ion-induced electron emission from LiF

The interaction of ions with an insulating substrate leads to surface charging. For intense ion beams these charges are not sufficiently fast removed between successive ion impact events. As a result the trajectories of the incident beam and the electron emission yield may be altered in a hardly predictable way. We demonstrate, that by heating an alkali-halide sample to a sufficiently high temperature, this macroscopic charging can be avoided and a simple current method can be used to study electron emission from such a surface. Measured electron emission yields from a LiF(1 0 0) single crystal due to impact of singly and multiply charged heavy noble gas ions (argon and xenon) are presented. For argon projectiles our results compare well with previous data, which were obtained using a more sophisticated electron statistics method and six orders of magnitude less intense ion beams.     

PL and XPS study of radiation damage created by various slow highly charged heavy ions on GaN epitaxial layers

Photoluminescence (PL) spectrum, in conjunction with X-ray photoelectron spectroscopy (XPS) is used to evaluate the surface damage of GaN layer by highly-charged Xe^q+ (18 ? q ? 30), Ar^q+ (6 ? q ? 16) and Pb^q+ (q = 25,35) ions. The intensity of PL emission of GaN layer, including near band-edge peak and yellow luminescence, decreases with increasing fluence and charge state of the incident ions. Finally the PL emission is completely quenched after irradiation to high fluences at high charge state. A new peak at 450 nm appeared in PL spectra of the specimens irradiated with Xe¹⁸⁺, Ar⁶⁺ and Ar¹¹⁺, indicating that radioactive recombination within donor-acceptor pairs (DAPs) during irradiation. After irradiation, XPS spectra show N deficient or Ga rich on GaN surface and XPS spectra of Ga3d core levels indicate spectral peak evidently shifts from a Ga-N to Ga-Ga and Ga-O bond. The relative content of Ga-N bond decreases and the content of Ga-Ga bond increases with the increase of ion fluence and ion charge state. The binding energy of Ga3d_5/2 electron corresponding to Ga-Ga bond of the irradiated GaN film is found to be smaller than that of metallic Gallium (Ga⁰), which can be attributed to irradiation damage.     

Surface nanostructuring of SrTiO3 single crystals by slow highly charged ions and swift heavy ions

Single crystals of strontium titanate have been irradiated with both slow highly charged Xe ions extracted from an Electron Beam Ion Trap and swift heavy Xe ions. After irradiation, the crystals were investigated by scanning force microscopy in air. In both cases nanohillocks due to impact of individual projectiles were observed. This similarity originates from the fact that both swift heavy ions and slow highly charged ions initially transfer their energy to the electronic system of the target, leading to a localized region of high electronic excitation. This electronic excitation is subsequently transferred to the lattice atoms by electron-phonon coupling, leading to pronounced lattice heating. The formation of surface hillocks can then be ascribed to a melting process. We also present first evidence for the existence of a potential energy threshold for nanohillock formation on strontium titanate surfaces by slow highly charged ions.     

Production of intense highly charged ion beams by IMP 14.5 GHz electron cyclotron resonance ion source

A new 14.5GHz Electron Cyclotron Resonance(ECR) ion source has been constructed over the last two years.The source was designed and tested by making use of the latest results from ECR ion source development ,such as high mirror magnetic filed,large plasma volume,and biased probe.140μA of O^7 ,185μA of Ar^11 and 50 μA of Xe^26 could be produced with a RF power of 800W, The intense beams of highly charged metallic ions are produced by means of the method of a metal evaporation oven and volatile compound throuth axial access,The test results are 130μA of Ca^11 ,70μA of Ca^12 and 65 μA of Fe^10 ,The ion source has been put into operation for the cyclotron at the Institute of Moderm Physics(IMP).     

Temporal and spatial study of differently charged ions emitted by ns-laser-produced tungsten plasmas using time-of-flight mass spectroscopy

Tungsten (W) is an important material in tokamak walls and divertors.The W ion charge state distribution and the dynamic behavior of ions play important roles in the investigation of plasma-wall interactions using laser-ablation-based diagnostics such as laser-induced breakdown spectroscopy and laser-induced ablation spectroscopy.In this work,we investigate the temporal and spatial evolutions of differently charged ions in a nanosecond-laser-produced W plasma in vacuum using time-of-flight mass spectroscopy.Ions with different charge states from 1 to 7 (W+ to W7+) are all observed.The temporal evolutions of the differently charged ions show that ions with higher charge states have higher velocities,indicating that space separation occurs between the differently charged ion groups.Spatially-resolved mass spectroscopy measurements further demonstrate the separation phenomenon.The temporal profile can be accurately fitted by a shifted Maxwell-Boltzmann distribution,and the velocities of the differently charged ions are also obtained from the fittings.It is found that the ion velocities increase continuously from the measured position of 0.75 cm to 2.25 cm away from the target surface,which indicates that the acceleration process lasts through the period of plasma expansion.The acceleration and space separation of the differently charged ions confirm that there is a dynamic plasma sheath in the laser-produced plasma,which provides essential information for the theoretical laser-ablation model with plasma formation and expansion.     

Electron beam ion traps and their applications

A brief introduction to the historical background and current status of electron beam ion traps (EBITs) is presented. The structure and principles of an EBIT for producing highly charged ions are described. Finally, EBITs as a potential tool in hot-plasma diagnostics and in studying frontier problems of highly charged ion physics are discussed.     

ECR离子源高电荷态金属离子产生的MIVOC法实验研究

为满足兰州重离子加速器的实验要求,先后在中国科学院近代物理研究所14．5GHz（LECR2）及10GHz＋14．5GHz（LECR3）高电荷态ECR离子源上使用MIVOC（Metallic ion from volatile compounds）方法进行了高电荷态金属离子产生的实验研究。主要研究了铁和镍的各种高电荷态离子的产生,具有代表性的是210eμA的Fe^11＋、175eμA的Fe^12＋、142eμA的Fe^13＋、25eμA的Fe^16＋、64eμA的Ni^10＋、57eμA的Ni^13＋、31eμA的Ni^15＋和15eμA的Ni^16＋。本文将分别给出两种金属离子产生的多电荷态束流谱图,并对实验装置的安排、实验现象及结果进行讨论与总结。     

Highly charged ion interactions with thin insulating films

The electrical conductance of magnetic tunnel junction (MTJ) devices whose ultra-thin aluminum oxide tunnel barriers were irradiated by highly charged ions (HCIs) increases linearly with the fluence of HCIs, while retaining a current-voltage relationship indicative of a tunnel junction. The slope of the MTJ conductance σ_c as a function of fluence varies with different tunnel barrier thicknesses d, levels of oxidation (stoichiometry) and charge state q. Since the MTJ conductance after HCI irradiation is due to tunneling, the increased conductance can result from thinning the barrier, reducing the effective tunnel barrier height φ, or both. Measurements of the current-voltage profile provide sufficient degrees of freedom to substantially constrain d and φ provided the reduction of the barrier remains within the assumptions of the commonly used WKB (Wentzel-Kramers-Brillouin) tunneling formalism. For the Xe³²⁺ ions discussed here the perturbation of the tunnel barrier is much weaker than in our previously reported measurements of Xe⁴⁴⁺ and application of WKB is still reasonable. This analysis reveals a trend of decreasing d while φ changes little.     

上海EBIT装置漂移管组件研制

漂移管组件为EBIT提供约束离子的轴向电势阱,是EBIT装置的核心区域.组件工作在低温、强磁场、高电压、超高真空的环境中,这对工程设计提出了很高的要求.本文介绍了上海EBIT装置的电势阱设计、漂移管组件的结构设计和一些特殊工程问题的处理方法,并对组件进行了低温和高压试验.本文的研究工作,对上海EBIT装置的成功研制有着重要的意义,目前上海EBIT可以稳定运行在100kV/110mA.     

Molybdenum L-shell X-ray production by 350-600 keV Xe (q = 25-30) ions

Molybdenum L-shell X-rays were produced by Xe^q+ (q = 25-30) bombardment at low energies from 2.65 to 4.55 keV/amu (350-600 keV). We observed a kinetic energy threshold of Mo L-shell ionization down to 2.65-3.03 keV/amu (350-400 keV). The charge state effect of the incident ions was not observed which shows that the ions were neutralized, reaching an equilibrium charge state and losing their initial charge state memory before production of L-shell vacancies resulted in X-ray production. The experimental ionization cross sections were compared with those from Binary Encounter Approximation theory. Taking into account projectile deflection in the target nuclear Coulomb field, the ionization cross section of Mo L-shell near the kinetic energy threshold was well described.