Field emission energy distribution (clean surfaces)

The emission of electrons from a cold cathode upon application of a strong electric field is called field emission. Since the electrons must tunnel through the classically forbidden barrier outside the solid, field emission was one of the first confirmations of the new quantum theory of the 1920's.¹ A field of tens of million volts per centimeter is required to obtain a reasonable current. In order to achieve such hgh fields at reasonable voltages, the cathode or emitter is usually etched to a very sharp point (～1,000 Å in radius). Therefore, several thousands of volts applied to the anode will produce the desired field. In 1937, Miiller2 developed what is known as the field emission microscope. The success of Miiller's microscope was a consequence of his realization that, if he produced a small hemispherically shaped tip that was thermally smoothed and cleaned, he would project a greatly magnified image of the spatial distribution of electrons tunneling from the emitter onto a fluorescent screen. Such a field emission pattern for clean tungsten is shown in Figure 1. The image on the screen is a nearly stereographic projection of the hemispherical end of the emitter. Because of its small size, the emitter is usually part of a single crystal and thus exposes all crystallographic orientations, so that individual crystal planes can be located and identified in the field emission pattern. The changes in the field emission pattern with exposure to adsorbed atoms or molecules have been used very successfully to study surface processes, such as diffusion, adsorption and desorption kinetics, or work function changes.³     

The quantum yield of a metallic nanoantenna

The local-field factor and quantum yield of a metallic nanoantenna are studied to identify its enhancement of an emitter’s emission within the feed gap. For simplicity, a two-dimensional model, an Au nanoantenna with an emitter at the center, is studied. The electromagnetic field is solved by a set of surface integral equations. An incident plane wave irradiating the nanoantenna is modeled to simulate the excitation of the emitter by illuminating light, and the local-field factor is used to evaluate the amplification of the electric field in the feed gap of the metallic nanoantenna. Once the emitter becomes excited, a model of an electric dipole interacting with the nanoantenna is used for calculating the radiative and nonradiative powers to obtain the quantum yield of the excited emitter in the presence of the nanoantenna. The numerical results of quantum yield indicate that an Au nanoantenna acts as a low-pass filter for the emission of the emitter. Moreover, the smaller the feed gap, the larger the local-field factor but the less the quantum yield. PACS 78.67.-n; 33.80.-b; 33.50.-j; 42.30.-d; 42.50.Hz; 81.07.Pr     

空间电荷效应对热场致发射中诺廷汉效应的影响

热场致发射阴极所产生的强流电子束具有很强的空间电荷效应,为研究该效应对热场致发射过程中诺廷汉（Nottingham）效应的影响机理,在理论分析的基础上,用数值方法研究了不同逸出功和多个外加电场条件下考虑空间电荷效应对诺廷汉效应结果的影响,并与不考虑空间电荷效应时的情形进行了对比. 结果表明：空间电荷效应的强弱会显著影响到阴极表面的稳态电场,进而对诺廷汉效应产生不可忽略的影响;当逸出功在3.0–4.52 eV、外加电场在3×10⁹–9×10⁹ V/m范围内时,考虑空间电荷效应的影响后,热场致发射电子所带走的平均能量较不考虑空间电荷效应时增加0–2.5 eV,且温度越高或外加电场越大时,该增加值越大;考虑空间电荷效应对诺廷汉效应的影响后,热场致发射电子从阴极带走的平均能量随外加电场的增加呈非线性下降规律;当阴极表面温度较高时,诺廷汉效应中的冷却效应随二极管间隙距离的变大而增强. 关键词： 热场致发射 诺廷汉效应 空间电荷效应 阴极表面电场     

Processing and identification of field-emission images of metallic surfaces

Techniques and algorithms for recording and processing field-emission images have been proposed. An original method of identifying individual crystal faces and directions in the surface structure based on fieldemission images and information on the emitter lattice type is described. The results may be used for detailed investigation of field electron emission under various impacts on the metallic surface.     

Effect of dipole structures on field emission of wide-gap semiconductor emitters

It is shown that dipole structures placed in a thin (less than 1 nm) near-surface layer of a high-resistivity field emitter produce small domains on the emitting surface in which the electric field may exceed 10⁸ V/cm. In these domains, the emitter surface potential is positive, providing effective electron transport from inside the emitter to the emission boundary. Optimal dipole orientations ensuring maximal electric fields at the surface are found. When the surface density of dipoles localized in the near-surface layer is on the order of 10⁶ cm⁻², one can expect an emitter-averaged emission current density of higher than 1 A/cm². The dipole structures in the near-surface layer may persist owing to incorporated impurity molecules having a dipole moment or result from a random combination of positively charged ionized impurities and electrons captured by deep traps. Trap charging/discharging asymmetry accounts for the hysteresis of the emission I–V characteristics.     

Use of the BFCPIC and BFCRAY codes to describe space charge limited emission in electron guns for gyrotrons

Numerical modelling of an electron gun in the space charge limited regime requires determining the current density distribution as well as the electric fields and electron trajectories. This is a rather complicated self-consistent problem, since the space charge influences the electric field, which in turn influences the electron trajectories. Previous simulations of magnetron electron guns using the BFCPIC and BFCRAY codes used a simple emission model (constant current density) that is approximately valid for thermionic emission. The code has been modified to include space charge limited emission. Several different ways of doing this are considered. One of the models considered uses Gauss’s law to force the electric field on the emitter to vanish; it was used in the original version of BFCPIC for the simulation of ion diodes. A second is based on the use of Child’s law (locally), which may be more appropriate for extension to fully electromagnetic particle-in-cell (PIC) codes. Calculations were performed with both models, and the results compared with each other and with experiments performed at FZK.     

Electric-field effect in a mixed-valence ion pair Cr3+−Cr2+ in a KZnF3 crystal

The influence of an external axial electric field on the absorption spectrum of a Cr³⁺−Cr²⁺ mixed-valence pair center coupled by the double-exchange mechanism in a KZnF₃ crystal is investigated. It is shown experimentally that the Cr³⁺−Cr²⁺ pair has an electric dipole moment. The migration of an electron is accompanied by local lattice strain. At the minima of the adiabatic potential, the intermediate fluorine atom is displaced from the lattice site. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 147–149 (25 July 1998)     

Shape of a magnetic resonance line in a thin film on the surface of anisotropic superconductor with irregularly distributed Abrikosov’s vortices

The form of an electron paramagnetic resonance (EPR) in a thin paramagnetic film (λ/10, λ-London’s depth of magnetic field penetration into superconductor) overlying the surface of an anisotropic superconductor is calculated taking into account the local magnetic field non-uniformity of an irregular Abrikosov’s vortex lattice. It is shown that the form of EPR is noticeably varied with the degree of irregularity of the superconductor vortex lattice. It is suggested that an inclusion of this circumstance into consideration may essentially change the conclusions made on the lattice type and parameters of this superconductor, which are typically derived from the analysis of the EPR form. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 49–52, January, 2007.     

爆炸电子发射初期阴极表面电场的研究

为了研究二极管爆炸电子发射初始阶段阴极表面复杂的物理现象及规律, 建立了由场致电子发射阴极构成的一维平板真空二极管物理模型,通过自行编程数值求解泊松方程, 考虑了发射出的电子对阴极表面电场的非线性影响,自洽模拟得到了阴极表面电场随时间的变化情况. 模拟结果表明,爆炸电子发射初期,阴极表面电场随时间的增加而呈现出不断振荡的规律, 且振荡幅度越来越小,最终到达一个稳态的值,二极管两极板之间的外加电场越大, 阴极表面稳态电场的绝对值越大;电场增强系数越大,阴极表面稳态电场的绝对值越大. 在整个时间演变过程中,阴极表面的实际电场强度决定着阴极发射的电流密度大小, 反过来阴极发射的电流密度又会影响到阴极表面的电场.     

钛基底的纳米金刚石掺混纳米碳管的场发射特性

在纳米金刚石场发射的基础上, 研究了纳米金刚石掺混纳米碳管的场发射特性。采用电泳沉积法形成了纳米金刚石与纳米碳管的复合涂层, 经热处理后制备出阴极样品, 然后进行微观表征, 再进行场发射特性测试与发光测试。结果表明, 与未掺混的纳米金刚石阴极样品相比, 复合涂层阴极样品的场发射开启电场明显减小, 场发射电流提高, 在较低的电场下阳极表面荧光粉就可以发光, 但发光不均匀, 出现了"边沿发光"的现象。分析了纳米金刚石掺混纳米碳管场发射性能提高的机理, 是由于纳米碳管掺入之后, 涂层的电子输运能力得到增强, 涂层中有效发射体的数目增加。最后, 解释了"边沿发光"现象的成因。     

端窗透射微型X光管的结构优化

为了得到场约束方式下,金属陶瓷封装端窗透射微型X光管发射电流的最大化和焦斑尺寸的最小化,对热阴极发射体的几何结构进行仿真计算,为设计实物模型提供技术方案。首先,从理论上推导了直热式阴极发射体、发射电流密度与几何结构的关系,其次讨论了有限积分算法在求解电场分布数值解过程中的离散化思路,最后利用CST粒子工作室软件,建立了几何模型,对阴极发射体的几何结构进行了优化。在灯丝距离控制极0.4 mm、控制极开孔直径为0.6 mm的位置,能够得到较小的焦斑和较高的电荷密度分布。试制的阴极发射体最大发射电流可达85 A。     

Similarities of dielectric surface flashover under atmospheric conditions for pulsed unipolar and RF excitation

Mechanisms in vacuum surface flashover caused by rf (f < 10 GHz) or unipolar voltages are virtually identical. Similarities between rf (representing high-power microwave window breakdown on the high-pressure side) and unipolar surface flashover are expected in an atmospheric environment as well. Two separate experimental setups were utilized to investigate both unipolar flashover and rf window flashover under atmospheric conditions while controlling excitation, temperature, pressure, humidity, and type of gas present, all under a similar electric field-surface geometry. The local electric field at the flashover initiating points has been numerically calculated in detail for all test geometries. For both rf and unipolar pulsed excitation, the flashover dynamics are changed by the application of UV light to the dielectric surface. A UV prepulse has a distinct impact on the arc’s path and a tendency to decrease the hold-off electric field. The effect of humidity on the hold-off electric field for both pulsed unipolar and rf excitations, along with temporally resolved emission spectroscopy of the flashover event, is discussed. This work was funded by the Cathode and HPM Breakdown MURI program and managed by the Air Force Office of Scientific Research (AFOSR) and by Sandia National Laboratories.     

A measurement of the local electric field on field emitter crystals using T-F emission

A method is introduced to measure in situ the absolute value of the local field strength on the surface of a field emitter tip by using T?F emission. The method is based on the unified theory of electron emission (Christov) which is now experimentally well confirmed. The method is tested on several single crystal faces of tungsten tips. The absolute value of the local field can be determined within an error of about 5%. Relative field strengths at different points on one single crystal face can be measured with an error of 2%. Once the absolute value of the field strength is thus measured, the absolute value of the work function can be obtained additionally, but so far only with a fairly great error.     

Incommensurateness effects in a lattice Ginzburg-Landau model

This paper discusses the effect of magnetic translational symmetry on the vortex structure in superconducting crystals with a large basis in artificial Josephson media (regular lattices of superconducting clusters) prepared with opal as the base material. For external magnetic fields lower than the upper critical field, the lattice Ginzburg-Landau model reduces to the two-dimensional Frenkel’-Kontorova model which in some cases is exactly solvable, in which the crystal lattice plays the role of an “hard sublattice” while the deformable vortex lattice plays the role of a “soft sublattice.” It is shown that static shear waves in the vortex lattice are solutions to the two-dimensional sine-Gordon equation with an additional condition of incompressibility implied by flux quantization. The pinning energy is found as a function of the magnetic field, nearness to the transition line, and the crystal lattice constant. Fiz. Tverd. Tela (St. Petersburg) 39, 1158–1162 (July 1997)     

Field emission of electrons from cathodes made of carbon fibers with a nanostructured emitting surface

Field electron emission from cathodes made of a bunch of carbon fibers under the condition of technical vacuum is studied experimentally. A model to optimize the field emission properties of the cathode by optimizing its macrogeometry with regard to the emitting surface structure is suggested. The current-voltage characteristics of the cathode are taken in the working voltage range 1–3 kV and for anode-cathode spacings varying from 1 to 10 mm. The current density from the cathode may reach 10 A/cm² or more.     

Electron field emission properties of highly dense carbon nanotube arrays

In this paper, we have studied field emission properties of highly dense arrays of multi-walled carbon nanotubes (CNTs) used as cathodes in diode-type field emission devices with a phosphor screen. For the high-density CNT emitters it is demonstrated that the emission sites are located on the CNT-cathode edges, which is direct experimental evidence of the ‘edge effect’. The results of computer simulations (using ‘ANSYS Electromagnetic’ software) are presented to confirm the experimental data and to analyze the effect of patterning on the electric field distribution for high-density CNT arrays. It is shown that selective-area removal of nanotubes in the arrays leads to the formation of additional edges characterized by the high field enhancement factor and enhanced emission from the CNT cathodes. In addition, scanning probe microscopy techniques are employed to examine surface properties of the high-density CNT arrays. For CNT arrays of ‘short’ nanotubes, the work function distribution over the sample surface is detected using a scanning Kelvin microscopy method.     

Critical (burst) electron emission from dielectrics, induced by injection of a dense electron beam

A dense pulsed electron beam and nanosecond pulse length has been used to inject negative electric charge into various dielectric materials (single crystals, glasses, composites, plastics) for initiation of electron field emission from the dielectric into a vacuum. It has been shown that upon reaching a critical electric field in the bulk and at the dielectric surface there is intense critical electron emission. The local current density from the emission centers reaches a record value (for dielectrics) of the order of 10⁶ A/cm². The emission occurs in the form of a single gigantic pulse. The measured amplitude of the emission current averaged over the emitting surface is the same order of magnitude as the injected electron current: 10–1000 A. the emission current pulse lages behind the current pulse of the primary electron beam injected into the sample. The delay time is in the range 1–20 nsec and decreases with increasing current density of the injected beam. Direct experimental evidence is found for intense generation of carriers (band or quasifree electrons) in the near-surface layer of the dielectric in a strong electric field due to the Frenkel-Poole effect and collisional ionization of traps, usually various donor levels. This process greatly strengthens the field emission from the dielectric. It has been shown experimentally that the emission is nonuniform and is accompanied by “point bursts” at the surface of the dielectric and ionized plasma spikes in the vacuum interval. These spikes are the main reason that the transition of the field emission into “bursts” is critical, similar to the current which has been previously observed in metals and semiconductors. However there are a number of substantial differences. For example the critical field emission current density needed for the transition into “bursts” is three orders of magnitude less than for metals. If we provide sufficient electron current at the surface or from the bulk of the dielectric to the emission centers, then the critical emission is always accompanied by a vacuum discharge between the surface of the dielectric and a metallic collector. A detailed computer model of the processes in the dielectric during injection of a high-density electron beam has been developed which allows one to understand the complex physical pattern of the phenomenon. Tomsk Polytechnic University. Institute of High-Current Electronics, Siberian Section, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 45–67, November, 1997.     

Liquid-crystal diode generator of low-frequency oscillations

The excitation of rhythmic current oscillations in a diode cell containing a nematic liquid crystal is studied. The external electric field in the interelectrode gap is directed parallel to the surfaces which orient the liquid crystal molecules. The current oscillations are accompanied by the formation of an autosoliton at the cathode, which propagates and disappears at the anode. A hypothetical model is proposed to explain this current instability. Zh. Tekh. Fiz. 68, 125–127 (January 1998)     

Modeling the development of the stepped leader of a lightning discharge

A stochastic-deterministic model is presented for the propagation of a downward-moving leader. Lightning formation is described by a stochastic growth of branching discharge channels which is determined by the electrostatic field. The dynamics of the electric field and of the charge distribution over the lightning structure are calculated deterministically. The model includes the initiation of lightning, a preliminary discharge in a cloud, the propagation of a downwardmoving stepped leader toward the earth, and the initiation and upward motion of a return stroke from the earth’s surface. Numerical execution of the model yields a dynamic picture of the development of the downward-moving leader and of the intracloud discharge structure. The effect of the charge density in the cloud and the parameters of the developing channels on the spatial-temporal, current, and charge characteristics of the stepped leader’s propagation are studied. The effect of free-standing structures on the distribution of points on the earth’s surface where lightning strikes is examined. Zh. Tekh. Fiz. 69, 48–53 (April 1999)