Derivation of a modified paraxial formalism for two-dimensional trajectories in electron and ion sources of non-simple geometries

A modified paraxial formalism has been developed which describes two-dimensional charged particle beam dynamics in electron and ion sources with pointed or needle-type geometries. A Hamiltonian formalism, and a more exact treatment of energy conservation is used to derive the modified paraxial equation for two-dimensional trajectories in systems with axially symmetric prolate-spheroidal beams. Calculations have been done for a gallium field emission liquid metal ion source modeled by a hyperboloid of revolution and planar extractor. Two important conclusions emerge from these calculations: i) The dominant effect of space-charge, for source geometries with small radii of curvature, occurs in the region close to the apex (<0.05 n) and ii) beam divergence has a strong dependence on geometry. This latter effect is a consequence of large two-dimensional field gradients near the apex of sources with needle-type or pointed geometry.This work was supported in part by the Division of Materials Research, National Science Foundation, Grant No. DMR-8108829     

Investigation of a tin liquid metal ion source

In view of its importance in materials research, tin is a metal worth studying in a liquid metal ion source configuration, even if results complement or extend previous work. This is the more so if the new work corrects misconceptions of the past and adds to current thinking. We, therefore, prepared a Sn liquid metal ion source employing a Ni needle to anchor the liquid, cone-shaped, emitter. Source properties, such as the current–voltage curve, the mass spectra of the beam and the energy spread of the main ionic species, were studied in detail. The mass spectra show a considerable amount of Sn clusters, apart from the dominant species, Sn⁺ and Sn⁺⁺. The source was stable down to 1-A emission current, corresponding to an energy spread for the singly charged ions of 7 eV. Theoretical arguments, involving the peak energy deficit of the ion-energy distribution, strongly suggest that both Sn⁺ and Sn⁺⁺ are emitted by direct field evaporation from the liquid surface. The same conclusion is reached from a careful examination of the beam mass spectra of the source. PACS 07.77.Ka; 33.15.Ta; 61.25.Mv     

Computer simulation of liquid metal ion source optics

A numerical calculation of the electric field and current density distribution for a liquid metal ion (LMI) source has been carried out. If a field evaporation mechanism for ion formation is assumed an elongated Taylor cone shape emitter is required to account for the observed total currents. Trajectory calculations including the effect of uniform space charge have been carried out as a function of total current and particle mass. The predicted emission characteristics compare favorably with experimental results for Ga, however the homogeneous space charge model is unable to account for all of the experimentally observed increase in beam divergence with increasing mass and current.     

Contribution of field effects to the achievement of higher brightness ion sources

The use of field effects for the delivery of high brightness ion beams is considered. Two solutions have been experimentally investigated, which are intended to increase the supply function in a field ion microscope: a liquid fed field ionization source and a field desorption source. Their performances and characteristics have been compared and they suggest two different regimes of emission. The field desorption source seems however more likely to produce reliable results. Brightnesses on the source side of the order of 10⁸ to 10⁹A/cm²sr are expected but much care must be devoted to the design of the electrostatic transfer optics of the gun to take full benefit of the intrinsic properties of such large solid angle emitters.     

On the Current Intensity Limit of a Vacuum-Arc Ion Source

The maximum ion current that can be extracted as a high-energy beam from a metal-vapor vacuum-arc ion source is considered. Results are presented of measurements of the plasma ion current in the metal-vapor vacuum-arc II (MEVVA II) ion source. It is shown that this source is an efficient generator of metal ions, an intense flux of which is efficiently transported to the beam extractor. The maximum metal-ion current that is available for extraction at the extractor location is 5 percent of the arc current. The limitation to the intensity of the metal-ion beam that can be produced by this kind of ion source is found to be in the extractor design.     

An in-depth investigation into the temperature dependence of the mass spectra of an Au82Si18 liquid metal field emitter

The mass spectra of the beam emitted by a Au₈₂Si₁₈ eutectic molten metal ion source are investigated in detail as a function of emitter temperature. At the conclusion of the work it emerges that while Au⁺, Si⁺, and Si⁺⁺ are the result of direct field-evaporation from the liquid surface, Au⁺⁺ forms by the post-ionisation of Au⁺. Cluster ions are most probably the product of droplet break-up. PACS  07.77.Ka; 32.10.Bi; 61.25.Mv     

Construction and study of a magnetoactive plasma ion source

The interaction of intense electron beams with plasmas in a nonuniform magnetic field is studied for the purpose of obtaining a magnetoactive plasma ion source. The resulting experimental data are used to trace the dynamics of the changes in the basic parameters of the source, both during the stage where it interacts with the beam and in the cooling stage. It is found that the charged particles are contained for a long time and that the efficiency of energy transfer from the beam to the source is high. This source is intended to be used for shaping and accelerating multiampere ion beams. Zh. Tekh. Fiz. 69, 44–47 (April 1999)     

Field evaporation of impurity ions from a liquid gallium ion source

The charge states of ions emitted from a gallium liquid metal field ion (LMI) source contaminated with tin and copper have been measured. The results for tin show that the proportion of Sn²⁺ to Sn⁺ is much larger than is found for a LMI source of pure tin. A model in which Sn²⁺ is assumed to be produced by post-ionization of Sn⁺ is used to set an upper limit to the electric field at the point of emission. Its value is approximately equal to that predicted by field evaporation theory for a pure gallium source. Consequently the charge states of emitted impurity ions are determined by field strengths imposed by the main component.     

On the mechanism of liquid metal ion sources

We have developed a theoretical model of liquid metal ion source operation which consistently explains the shape and size of the ion emitting region, the mechanism of ion formation and properties of the ion beam. We find that field evaporation is the main current generating mechanism and that field evaporation and subsequent postionization produce the doubly and higher charged ions. Field ionization of thermally evaporated neutrals may make a significant, but not dominant, contribution to the current of singly charged ions. Our model is consistent with experimental results on energy spread, energy deficit and charge state ratios and we are able to explain the stability of the emitted ion current.     

The high-temperature behaviour of gallium and indium liquid metal ion sources

Energy distribution diagrams and derived data are presented for gallium and indium liquid metal ion sources operated at elevated temperatures. Results for the gallium source confirm that a secondary peak is formed on the energy distribution diagram at source temperatures above 250°C. Contrary to the findings of other research workers, data presented here show that the indium source displays similar characteristics to that of gallium. Off-axis data are also given, showing that secondary peak formation is not limited to the centre of the beam. Present hypotheses propose that secondary peak formation is the result of an increased contribution to emission by free-space field ionisation at elevated temperatures. Data presented here for the gallium and indium sources are discussed and the above hypotheses are examined. It is concluded that a field ionisation mechanism does not satisfactorily explain the form of the high temperature liquid metal ion source energy distributions.     

The post-ionisation of Pb ions from a molten Sn host field-ion emitter

A tin liquid metal ion source contaminated with lead has been studied. From a detailed analysis of the source mass spectra as a function of emission current, it is established that while Sn⁺, Sn⁺⁺ and Pb⁺ are directly field evaporated from the liquid surface, Pb⁺⁺ forms by the post-ionisation of Pb⁺. Moreover, our results for the Pb⁺⁺/Pb⁺ ratio are in excellent agreement with those of Komuro obtained with a pure lead LMIS. PACS 07.77.Ka; 33.15.Ta; 61.25.Mv     

Time-of-flight mass spectrometry studies of an ion beam generated by the titan source

The TITAN source generates wide-aperture beams of gas and metal ions of different materials. This is achieved because of two types of cold-cathode arc discharges, which operate simultaneously in the discharge system of the source. For metal ions to be obtained, use is made of the vacuum arc initiated between an ion-forming cathode and a hollow anode. To produce gas ions, a constricted low-pressure arc discharge is initiated with the same hollow anode. The constitution of ion beams generated by the TITAN source has been investigated using a homemade time-of-flight spectrometer. This paper describes the design of the latter and the principle of its operation, and discusses the physical peculiarities of the spectrometer operation, which affect the ion beam constitution. Institute of High Current Electronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 21–28, February, 2000.     

Trajectory analysis of ions formed in the field emitter inter-electrode region

Ion trajectories are three-dimensionally analyzed to determine the main source of ions striking the emitting area of a field emitter. Computer calculations of the ion trajectories are made for a field emitter in the shape of a sphere radius 1000 Å, mounted on an orthogonal cone, and anode radii of 3 cm and 1 cm centered at the emitter. Approximately 10^?10 to 10^?9fraction of the ions formed at the anode by electron impact desorption reach the emitter tip. Only those residual gas ions generated in the space of a few μm around the tip are able to impinge on the emitting area. Under typical operating conditions of 10^?9 Torr, the residual gas ions striking the tip are 10 to 10² times greater than those from the anode, assuming an ion ejection of one ion per 10⁵ electrons, and an anode potential of about 3 kV.     

15～20 mA多峰负氢离子源全三维数值模拟

对自主开发的全三维粒子模拟/蒙特卡罗(PIC/MCC)算法进行了阐述,包括算法流程设计、碰撞处理机制等。采用该算法对中国原子能科学研究院研制的15~20mA负氢离子源进行数值模拟研究,结果显示:该多峰会切离子源能得到空间分布均匀的离子束,采用优化后的虚拟过滤场,不仅能有效过滤高能电子,还将导致离子源下游区域低能电子增多,有效提高负氢离子体积产生效率。     

Xenon nano-beams for materials research

The advantage of focused Xe⁺ beams over other rare gas species have been investigated [Zhukov, V.; Kalbitzer, S. Russ. Microelectron. 2011, 40 (1), 17–24]. In particular, the higher operation temperature of a super-tip gas field ion source for xenon ions is one outstanding technical feature. The properties of focused Xe⁺ beams are estimated with special reference to optimised ion-optical transport systems. Applications to both ion beam materials modification and ion beam materials analysis are outlined.     

Discharge characteristics of the DUHOCAMIS with a high magnetic bottle-shaped field

For the purpose of producing high intensity, multiply charged metal ion beams, the dual hollow cathode ion source for metal ions (DUHOCAMIS) was derived from the hot cathode Penning ion source combined with the hollow cathode sputtering experiments in 2007. To investigate the behavior of this discharge geometry in a stronger magnetic bottle-shaped field, a new test bench for DUHOCAMIS with a high magnetic bottle-shaped field up to 0.6 T has been set up at the Peking University. The experiments with magnetic fields from 0.13 T to 0.52 T have indicated that the discharge behavior is very sensitive to the magnetic flux densities. The slope of discharge curves in a very wide range can be controlled by changing the magnetic field as well as regulated by adjusting the cathode heating power; the production of metallic ions would be much greater than gas ions with the increased magnetic flux density; and the magnetic field has a much higher influence on the DHCD mode than on the PIG mode.     

新型毫微秒强流脉冲电子束和离子束发生装置

本文讨论一种新的低气压放电型粒子源,用这种装置可以产生能量达70keV,电流密度超过10⁶A/cm²,电流为几百安培的脉冲电子流和安培级的脉冲离子流,作者建议用“电场递增效应”来解释这种多极板放电室的放电机制,由于它造价低、结构简单、重复频率高、寿命长,可以预期,这一装置将会得到广泛应用。 关键词：     

On the mechanism of liquid metal electron and ion sources

The mechanisms of electron and ion generation from Taylor cones of liquid metals are discussed. In the case of electron emission the vacuum arcing mechanism of Swanson and Schwind, which accounts for the observed high current repetitive pulsing is briefly reviewed. For ion emission mechanisms from onset to the high current regime are proposed. It is concluded that at onset ions are generated exclusively by field desorption. A theory to account for the observed emitter heating is advanced, and it is concluded that high currents result from field ionization of thermally evaporated atoms. It is shown that space charge becomes important even at very low ion currents and'is instrumental in providing stabilization in all regimes of ion emission.     

Technological Ion Sources Based on a Vacuum Arc Discharge

Technological ion sources of the TITAN type have been developed at the Institute of High Current Electronics, RAS, for the last ten years. These sources generate wide-aperture high-current beams of gas or metal ions and also mixed two-component gas-metal ion beams with a controllable component ratio. This is possible due to two discharge systems combined into one discharge system of the source. Metal ions are obtained with a vacuum arc discharge and gas ions are generated with a constricted hollow-cathode low-pressure arc discharge. This paper describes the principle and peculiarities of operation of the given sources, their design, parameters, and fields of application. A modified version of the Mevva ion source is considered. The design of this version is based on the results of studies conducted using the TITAN source.     

Resonant Ionization Laser Ion Source Project at TRIUMF

Resonant laser excitation and ionisation is one of the most successful tools for the selective production of radioactive ion beams (RIB) at on-line mass separator facilities. TRIUMF plans to augment the current ion sources with a resonant ionisation laser ion source (RILIS), to use the high production yields from the target, as shown by the delivery of 3*10⁴/s ¹¹Li ions from a standard target ion source with surface ionisation. The development and installation of TRIUMF's RILIS (TRILIS) is necessary to provide beams of short lived isotopes that conventional ion sources could not produce in sufficient intensity and purity for nuclear-, and nuclear astrophysics- experiments. A laser system consisting of three tunable titanium–sapphire (TiSa) lasers with frequency doubling and tripling was employed to demonstrate first off-line resonance ionisation of Ga, and is being installed for first on-line test and a run on ⁶²Ga in December 2004.