Surface Electron Emission Research Articles

Investigation into the mechanism of surface atom emission from an individual cathode spot using molecular dynamics simulation

Contact erosion on the cathode of a vacuum arc is determined by the behaviours of the cathode spots, where the plasma–surface interactions take place. A comprehensive model of a single cathode spot is developed in this work based on the molecular dynamics method, where an atomic copper substrate in the size of nanometres is built and the contributions to the development of cathode spot from leftover plasma ions, surface electron emission, surface atom emission, back ions, Nottingham heating and Joule heating are integrated. Defined based on the surface temperature distribution, a cathode spot is observed in the simulation results. The surface atom emission, which is the origin of mass loss, can be directly detected by the atoms being isolated from the surface. Two routes of surface atom emission are observed as the sources of mass loss, including evaporation, and atom sputtering or splashing. It is found that in the high-temperature region, atom sputtering or splashing dominates the surface atom emission, which leads to considerable mass losses. The simulation results are consistent with previous experimental and other simulation findings, providing fundamental insights into the cathode spot formation mechanism from a microscopic perspective.

First-principles study of Cs/O deposited Na<sub>2</sub>KSb photocathode surface

Na<sub>2</sub>KSb photocathodes have many applications in vacuum optoelectronic devices, such as photomultiplier tubes, image intensifiers, and streak image tubes for high-speed detection and imaging in extremely weak light environments, due to their advantages of high temperature resistance, small dark current, low vacuum requirement, low fabrication cost and high fabrication flexibility. In addition, this type of photocathode has important application prospect in high brightness accelerator photoinjectors. To guide the fabrication of high-sensitivity Na<sub>2</sub>KSb photocathodes, Na<sub>2</sub>KSb surfaces with different surface orientations and atom terminations are investigated by the first-principles calculation method based on the density functional theory to obtain the most stable and most favorable surface for electron emission. From the perspectives of surface energy, adsorption energy, and work function before and after Cs adsorption, it is revealed that the Na<sub>2</sub>KSb (111) K surface exhibits superior surface stability and electron emission capability. Furthermore, the electronic structure and optical properties of Cs adsorption and Cs/O co-adsorption on the Na<sub>2</sub>KSb (111) K surface under different Cs coverages are analyzed, and the mechanism of Cs/O deposition on Na<sub>2</sub>KSb surface is studied. The adsorption energy of Cs in the Cs/O adsorption model is much larger than that in the single Cs adsorption model, indicating that the adsorption of O atoms on the Na<sub>2</sub>KSb surface can make the adsorption of Cs atoms on the surface stronger, and thus increasing the adhesion of Cs atoms on the surface. After adsorption of Cs on the Na<sub>2</sub>KSb (111)K surface, the surface work function only decreases by 0.02 eV, while the maximum work function decrease for the Cs/O adsorbed surface is 0.16 eV, with the Cs coverage of 2/4 ML and the O coverage of 1/4 ML. The adsorption of Cs/O atoms on the surface facilitates the charge transfer above the surface and results in charge accumulation, which can form the effective surface dipole moment. The magnitude of the surface dipole moment is directly related to the change of work function. Furthermore, through the analysis of the electronic band structure and density of states, it is found that the adsorbed Cs atoms have additional contribution to the band structure near the conduction band minimum. After the introduction of O atoms, the valence band moves up, also the bottom of the conduction band and the top of the valence band become flat. The Cs/O deposition is beneficial to increasing the absorption of near-infrared light on the Na<sub>2</sub>KSb surface, but it will reduce the absorption of ultraviolet light and visible light, and the refractive index will also decrease. This work has a certain reference significance for understanding the optimal emission surface of Na<sub>2</sub>KSb photocathode and the mechanism of surface Cs/O deposition.

Subcycle surface electron emission driven by strong-field terahertz waveforms

The advent of intense terahertz (THz) sources opened a new era when the demonstration of the acceleration and manipulation of free electrons by THz pulses became within reach. THz-field-driven electron emission was predicted to be confined to a single burst due to the single-cycle waveform. Here we demonstrate the confinement of single-cycle THz-waveform-driven electron emission to one of the two half cycles from a solid surface emitter. Either the leading or the trailing half cycle was active, controlled by reversing the field polarity. THz-driven single-burst surface electron emission sources, which do not rely on field-enhancement structures, will impact the development of THz-powered electron acceleration and manipulation devices, all-THz compact electron sources, THz waveguides and telecommunication, THz-field-based measurement techniques and solid-state devices.

Simulation of a multichannel vacuum transistor with high cut-off frequency

A multichannel vertical vacuum transistor based on the Fowler–Nordheim tunneling emission mechanism was proposed and numerically investigated. The multichannel structure was demonstrated to be effective in enhancing the drain current when compared to the traditional single-channel structure with the same device size. For example, transconductance increased from 0.42 mS of the single-channel structure to 0.86 mS of the four-channel structure. In addition, when the vacuum channel number increases, the size of a single channel decreases correspondingly, leading to a reduction in electric field intensity on the electron emission surface. Thus, the off-state current dramatically reduced by two orders of magnitude reaching10−15 A according to the simulated results. In other words, the ON/OFF drain current ratio of the multichannel structure is significantly enhanced. Furthermore, the simulation results indicate that the cut-off frequency of the multichannel device is 33% higher than that of the traditional single-channel one reaching 0.19 THz.

Secondary electron emission from magnetron targets

Ion-induced secondary electron emission of surfaces occurs in all gas discharges which have contact to surfaces such as electrodes or chamber walls. These secondary electrons (SEs) play an important role, for instance, in the performance of DC discharges, RF discharges and magnetron sputtering discharges. SE generation can be separated into potential electron emission (PEE) due to the neutralization of the incident ion upon impact and kinetic electron emission (KEE) due to the electronic stopping of the penetrating ion in the solid. SE due to neutralization is usually described by Auger processes and the density of states of the electrons in the solid, whereas KEE scales with the electronic stopping of the ion in the solid, as being calculated by ion collision simulations. The measurement of the energy distribution of the SEs of three metals (Al, Ti, Cu) and their oxides reveals the occurrence of Auger peaks, which are not reflected by standard models such as the Hagstrum model. Instead, in this paper, a model is proposed describing these Auger peaks by Auger neutralization of holes created by the collision cascade of the incident ion. This shows decent agreement. The contribution of Auger peaks in the metals Al and Ti is very significant, whereas it is negligible in the case of Cu. The implication of these energy distributions to the performance of magnetron sputtering discharges is discussed.

Scandium wetting of tungsten surfaces in “scandate” thermionic cathodes

The surface structure and electron emission mechanism in high-performing “scandate” thermionic cathodes has long been a point of debate, in part due to the challenges inherent in observing emitting surfaces under operational conditions. Critically, the distribution and role of Sc in enabling high emitted current densities at low operating temperatures remains unclear. Here, using density functional theory calculations of the structure and energy of bare and Sc-covered W slabs, the wetting behavior of Sc on W has been explored.Computed surface excess energies reveal that, despite the bulk immiscibility of W and Sc, a single monolayer of Sc wets (001), (110), and (112) W surfaces at very low oxygen chemical potentials. The addition of further layers of Sc was found to be thermodynamically unfavorable. In contrast to previous studies, the present findings suggest the existence of a Sc interlayer directly atop W in scandate cathodes, representing a novel example of surface-energy-driven metal-on-metal wetting leading to self-assembly of atomically-thin layers.

Understanding Effects of Deep Traps on DC Surface Flashover Characteristics of Epoxy/MWCNTs-TiO2 Nanocomposites in a Vacuum

Surface flashover is likely to be triggered when a high electric field is applied due to its voltage being much lower than the breakdown voltage of either gas or solid, which restricts the development of advanced power electrical and electronic devices. To improve dc surface flashover performances in a vacuum and further explore the effects of deep traps on surface flashover, conductive multiwall carbon nanotubes (MWCNTs) and nonconductive nano-titania (TiO2) particles were incorporated into the epoxy (EP) matrix to tailor the surface deep trap level. Surface trap parameters, surface conductivity, surface charge density, and surface flashover voltage were calculated and measured. It indicates that surface flashover voltage improves by 23.07% and 14.45% compared to pristine EP when solely introducing MWCNTs or nano-TiO2 particles into the EP matrix, while it increases by 36.04% for C0.1T2 as simultaneously introducing both particles. When adding small amounts of particles, the surface charge transport is suppressed due to the introduction of bonded layers in the interfacial region, and the surface deep trap level is improved. As the surface deep trap level increases, charge transport and electron emission processes in the solid surface layer are hindered, which reduces the surface positive charge density and the distortion of the electric field and further results in the reduction of surface conductivity and the improvement of dc surface flashover voltage in a vacuum. The explanation for the effects of surface deep traps on surface flashover is essential for further improvements of surface flashover.

Understanding the surface structure evolution and electron emission behaviors during the activation of Ir-coated dispenser cathodes

Metal coated dispenser cathode is of great interest due to its low operation temperature, high current density and long lifetime, however, the emission mechanism is still far from being fully understood. In this work, the tungsten dispenser cathodes were coated with iridium film of 300 nm in thickness. The phase transition and surface structure evolution during cathode activation was systematically investigated. The as-deposited iridium coating shows a columnar grain morphology with ultrafine size. Upon heating for 1 h at 1150 °C, a multiple-layer of Ir/Ir3W/IrW was detected and it fully converted to IrW layer after 4 h heat treatment. Such a structure evolution can be well explained based on thermodynamics. The Ir → Ir3W → IrW phase transition induces obvious film thickening and grain coarsening. The full activation could increase the emission current density from 17.35 A/cm2 to 30.92 A/cm2 at 1050 °C. Density function theory (DFT) simulation results demonstrate the enhanced emission of IrW-coated cathode may be due to its balanced adsorption behaviors with Ba and O atoms. Our findings will provide deeper insights into the design and development of metal coated cathodes with better performance.

First-principles calculations of metal surfaces. II. Properties of low-index platinum surfaces toward understanding electron emission

The stability of low-index platinum surfaces and their electronic properties is investigated with density functional theory, toward the goal of understanding the surface structure and electron emission, and identifying precursors to electrical breakdown, on nonideal platinum surfaces. Propensity for electron emission can be related to a local work function, which, in turn, is intimately dependent on the local surface structure. The $(1\ifmmode\times\else\texttimes\fi{}N)$ missing row reconstruction of the Pt(110) surface is systematically examined. The $(1\ifmmode\times\else\texttimes\fi{}3)$ missing row reconstruction is found to be the lowest in energy, with the $(1\ifmmode\times\else\texttimes\fi{}2)$ and $(1\ifmmode\times\else\texttimes\fi{}4)$ slightly less stable. In the limit of large $(1\ifmmode\times\else\texttimes\fi{}N)$ with wider (111) nanoterraces, the energy accurately approaches the asymptotic limit of the infinite Pt(111) surface. This suggests a local energetic stability of narrow (111) nanoterraces on free Pt surfaces that could be a common structural feature in the complex surface morphologies, leading to work functions consistent with those on thermally grown Pt substrates.

Strongly temperature-dependent, anomalous secondary electron emission of liquid lithium surfaces exposed to a plasma

• First time reported SEE of a CPS wetted with liquid lithium exposed to a plasma. • Formation and dissolution of a strongly temperature dependent surface oxide film. • SEE values from ~0.5 for clean surfaces to ~2 for heavily oxidized samples. • Exposure to 3 × 10 −7 Torr during one hour increases the SEE to values of about 2. • Annealing at increasing temperatures decreases the SEE to lower values. Electron-induced secondary electron emission (SEE) on a liquid lithium surface confined in a capillary porous system (CPS) and exposed to a plasma is reported for the first time. The liquid lithium surface is exposed to the bombardment of a suprathermal electron flux with energies up 150 eV created in a low pressure He Dc-Glow discharge. Various methods of surface oxidation have been used to reproduce realistic conditions in fusion plasma experiments. Exposure to very low residual gas pressures (around 3 × 10 −7 Torr) is enough to increase the maximum of the SEE to values of about 2. The formation and dissolution of a surface oxide film is strongly temperature dependent and plays an important role in the resulting SEE yield and its time evolution: values have been found to range from ~0.8 for clean surfaces to ~2 for samples that have been heavily oxidized. In the case of O 2 molecule exposure, a clear difference with temperature was observed. The molecular exposure at 330 °C had a much stronger effect on the increase in SEE than the molecular exposure at 220 °C. These results have a direct impact on the development of lithium-based divertor targets in fusion as well as in the understanding of the SEE characteristics of contaminated liquid surfaces, which has never been reported before.

A Modified Steady-State Method for Space Charge-Limited Effect of SGEMP

Excitation of the system-generated electromagnetic pulse (SGEMP) effect by high-dose X-rays in a cavity may induce the high space charge-limited (SCL) effect. This nonlinear effect limits the photocurrent from the electron emission surface, thereby limiting the increase in the SGEMP electromagnetic field. A modified steady-state approximation method is derived, which considers the influence of electron velocity and the relativistic effect on the number and motion of electrons, respectively. It can be applied to simulate the motion of electrons with a discrete spectrum. Compared with the original method, this method obtains more accurate results for an increase in energy. The method can be applied to the simulation of a cavity SGEMP with the boundary layer treatment. The results show that the increase in the electron yield may induce a nonlinear change in the peak value of the electric field; however, the frequency spectra are less affected. The influence of the SCL effect on lower-energy electrons is higher than that on higher-energy electrons. With an increase in lower-energy electrons in the energy spectrum, the excited electric field is affected more significantly by changes in the energy spectrum, owing to the SCL effect.

Electron Emission Properties of Silicon-Rich Silicon Oxide Film Prepared by Reactive Magnetron Sputtering Deposition and Rapid Thermal Annealing

Abstract Silicon-rich silicon oxide (SRSO) film was prepared through the deposition of SiOx film by radio-frequency reactive magnetron sputtering from a Si target and the subsequent rapid thermal annealing treatment of SiOx film, and planar-type field emission cathodes based on SRSO film were fabricated. The surface morphologies and electron emission properties of the SRSO films deposited under different O2/Ar flow ratios were investigated. The experimental results show that a relatively high O2/Ar flow ratio during the deposition of SiOx film can lead to the formation of cluster structure in the SRSO film prepared, and the electron emission efficiency of SRSO film rises with the increase of electric field applied. An SRSO-film cathode fabricated under an O2/Ar flow ratio of 1:1 with a chamber pressure of 0.21 Pa has an emission current density of 65.61 μA/cm2 and a corresponding emission efficiency of 0.53% at a bias voltage of 18 V, and it exhibits relatively stable emission and fine emission uniformity.

Breakdown Oscillations of the ISIS H− Penning Ion-Source Hydrogen–Cesium Discharge

The ISIS Penning ion source is a pulsed dc discharge surface plasma source delivering a 55-mA beam of negative hydrogen ions (H−) in 250- ${ mu }\text{s}$ pulses to the ISIS accelerator for neutron and muon physics at 50-Hz pulse repetition rate. The H− ions are extracted from the hydrogen–cesium discharge sustained by a current-regulated pulsed power supply delivering a current of 50–60 A. The discharge exhibits an ignition transient that requires the pulselength to be prolonged to 700 ${ mu }\text{s}$ to extract a clean H− beam pulse. The current and voltage fluctuations during the breakdown oscillation enhance the sputtering damage of the ion-source cathodes, thus contributing to the lifetime limitation of the device. It has been hypothesized that the breakdown oscillations are due to cesium dynamics causing the surface work function and electron emission from the cathode surfaces to fluctuate periodically. Experiments with the ISIS H− Penning ion source, reported here, reveal that the breakdown oscillations are primarily driven by the discharge power supply and its auxiliary circuits, whereas cesium dynamics have only a minor effect on the ignition transient. The conclusion is derived from careful characterization of the discharge current–voltage characteristics, optical emission measurements combined with a 0-D model, and comparison of different discharge geometries. Engineering solutions to mitigate the breakdown oscillations are presented and the potential effect of the applied modifications on the molybdenum cathode erosion by cesium sputtering is estimated.

Modeling of thermalization phenomena in coaxial plasma accelerators

Coaxial plasma accelerators are electromagnetic acceleration devices that employ a self-induced Lorentz force to produce collimated plasma jets with velocities ~50 km s−1. The accelerator operation is characterized by the formation of an ionization/thermalization zone near gas inlet of the device that continually processes the incoming neutral gas into a highly ionized thermal plasma. In this paper, we present a 1D non-equilibrium plasma model to resolve the plasma formation and the electron-heavy species thermalization phenomena that take place in the thermalization zone. The non-equilibrium model is based on a self-consistent multi-species continuum description of the plasma with finite-rate chemistry. The thermalization zone is modelled by tracking a 1D gas-bit as it convects down the device with an initial gas pressure of 1 atm. The thermalization process occurs in two stages. The first is a plasma production stage, associated with a rapid increase in the charged species number densities facilitated by cathode surface electron emission and volumetric production processes. The production stage results in the formation of a two-temperature plasma with electron energies of ~2.5 eV in a low temperature background gas of ~300 K. The second, a temperature equilibration stage, is characterized by the energy transfer between the electrons and heavy species. The characteristic length scale for thermalization is found to be comparable to axial length of the accelerator thus putting into question the equilibrium magnetohydrodynamics assumption used in modeling coaxial accelerators.

High voltage threshold for stable operation in a dc electron gun

We report clear observation of a high voltage (HV) threshold for stable operation in a dc electron gun. The HV hold-off time without any discharge is longer than many hours for operation below the threshold, while it is roughly 10 min above the threshold. The HV threshold corresponds to the minimum voltage where discharge ceases. The threshold increases with the number of discharges during HV conditioning of the gun. Above the threshold, the amount of gas desorption per discharge increases linearly with the voltage difference from the threshold. The present experimental observations can be explained by an avalanche discharge model based on the interplay between electron stimulated desorption (ESD) from the anode surface and subsequent secondary electron emission from the cathode by the impact of ionic components of the ESD molecules or atoms.

Electron beam recording of microdomains on the nonpolar LiNbO3 crystal surface at different SEM accelerating voltages

The effect of the accelerating voltage U of the SEM electron beam on the characteristics of microdomains recorded by the electron beam method on the nonpolar Y-surface of LiNbO3 crystals has been studied. The thickness Td of domains in the Y direction is determined by the depth range Re of primary electrons, which depends on U. This makes it possible to define Td in the range of 0.2–4 μm at U = 5–25 kV, respectively. The electron emission coefficient σ is estimated for different values of U exceeding the second equilibrium point U2 (σ = 1) in the σ(U) diagram. These data are used to construct the dependence σ(U) for LiNbO3. Based on the exposure characteristics of the length Ld of domains growing along the polar Z axis, the dependence of the space charge field controlling the domain planar growth along Z on the surface electron emission σ is found.

Modeling electron emission and surface effects from diamond cathodes

We developed modeling capabilities, within the Vorpal particle-in-cell code, for three-dimensional simulations of surface effects and electron emission from semiconductor photocathodes. They include calculation of emission probabilities using general, piece-wise continuous, space-time dependent surface potentials, effective mass, and band bending field effects. We applied these models, in combination with previously implemented capabilities for modeling charge generation and transport in diamond, to investigate the emission dependence on applied electric field in the range from approximately 2 MV/m to 17 MV/m along the [100] direction. The simulation results were compared to experimental data. For the considered parameter regime, conservation of transverse electron momentum (in the plane of the emission surface) allows direct emission from only two (parallel to [100]) of the six equivalent lowest conduction band valleys. When the electron affinity χ is the only parameter varied in the simulations, the value χ = 0.31 eV leads to overall qualitative agreement with the probability of emission deduced from experiments. Including band bending in the simulations improves the agreement with the experimental data, particularly at low applied fields, but not significantly. Using surface potentials with different profiles further allows us to investigate the emission as a function of potential barrier height, width, and vacuum level position. However, adding surface patches with different levels of hydrogenation, modeled with position-dependent electron affinity, leads to the closest agreement with the experimental data.

Electron emission and exciton production of LiF surfaces by grazing impact of hydrogen neutral atoms

We put forward an alternative mechanism to understand the physics underlying the electron emission and exciton formation occurring during grazing collision of neutral hydrogen on LiF(001) surfaces. Instead of visualizing the negative ion formation as the precursor, we propose the formation of excited H(n=2) as a catalyzer to produce excitons and electrons.

Stabilized Operation of a Microwave Compressor Driven by Relativistic S-Band Magnetron

The stabilized operation of a microwave compressor based on a travelling wave resonator, producing at its output microwave pulses with a power of 1.15 ± 0.05 GW and duration of 12 ± 2 ns, is described. The compressor is pumped by a relativistic S-band magnetron generating microwave pulses with a power up to 250 MW and duration of ~100 ns. To stabilize the frequency of the microwaves generated by the magnetron, a transparent cathode having a limited electron emission surface and double ring-type straps between the anode resonators were applied. It was shown that the application of a phase-shifter between the magnetron and resonator results in the influence of the positive feedback on the magnetron operation beginning within the first 10 ns of the microwaves generation. Finally, the incorporation of symmetrically placed phase-shifters in the compressor's resonator allows the travelling wave phase to be adjusted, and accordingly, reliable ignition of the microwave gas discharge switch.

17.3: First‐Principles Study on Secondary Electron Emission of MgO (200) and (111) Surfaces

AbstractThe electronic structures of flat (200), flat (111) and hydrogenated (111) surfaces are investigated based on first‐principles calculations. The results reveal that SEE coefficients γ increase in the following order: flat(111)< flat(200)< hydrogenated(111). It is also found that hydrogenation on the MgO (111) surface can reduce band gap and increase the γ value.