Effect Of Electron Emission Research Articles

(Invited) Hot Electron Photodetectors, Sensors and Solar-Cells

Plasmonic nanostructures can generate energetic “hot” electrons from light in a broad band fashion depending on their shape, size, and arrangement. These nanostructures have a promising use in photodetectors, allowing high speed, broad band, multicolor and polarized photodetection. Because they function without a band-to-band transition, photon detection at any energy is possible through engineering of the plasmonic nanostructure. Compact hot-electron-based photodetectors that combine polarization sensitivity and light detection in the near-infrared region were fabricated using an indium tin oxide (ITO)−gold hybrid layer.[1] Sensitivity of the device was significantly increased by use of self-assembled monolayers in a capacitor configuration. The resulting devices are capable of detecting light below the ITO band gap at ambient temperature without any bias voltage. The devices can also function as photovoltaic solar cells using an electrolyte which provides charge transport to the electrodes (Fig 1).[2] Interestingly the surface sensitivity of the hot electron emission effect can be used for femtomole sensitivity. Figure 1: current voltage characteristic under 1 sun AM 1.5 solar simulation of an array of gold nano-antenna cathode hot-electron emission dye-sensitized-like solar cell.

Simulated temperature of a tungsten spot facing large plasma heat loads

In fusion devices like ITER, plasma-wall interactions are a significant concern due to the high heat fluxes, often tens of MW/m2, impacting the first wall. These intense heat fluxes can lead to the formation of hot spots on components facing the plasma, such as tungsten, used in divertor plates and antennas. This results in material erosion and plasma core contamination. Our study investigates the thermal behavior of tungsten surfaces under these conditions using fluid modeling and Particle-In-Cell (PIC) simulations. We examine the effects of thermionic electron emission on the sheath potential and heat transmission. The simulations reveal that thermionic emission can decrease the sheath voltage, increasing the surface temperature due to enhanced heat flux due to electrons. Additionally, we explore how the ratio between the spot size (S) and the surrounding surface length (Ly) influences the surface temperature. We find that a higher Ly/S ratio allows the surface to reach higher temperatures before the system enters the space-charge-limited regime, where thermionic current is maximized and considerably larger than the case where the entire surface is emissive (Ly=S).

Thermoradiative coupling graphene-based thermionic solar conversion

Thermionic conversion, due to its simple solid-state structure capable of converting heat to electricity directly, is promising for concentrated solar power. However, because of the extremely high cathode temperature, a large portion of the heat is lost to the environment. The paper introduces a novel concept of selective thermoradiative-graphene thermionic conversion (STR-GTI) that involving a combined control of photon and electron emission to modify the radiation dissipation. A detailed thermodynamic model is developed to evaluate the energy transfer irreversibility of STR-GTI solar conversion. The results demonstrate that the selective themoradiative photon emission and graphene thermionic electron emission effects synergistically reduce internal irreversible losses in the STR-GTI system, leading to a maximum energy efficiency of 34.34 % at a concentration ratio of 425, cathode work function of 1.8 eV and thermoradiative bandgap of 0.2 eV. The STR-GTI system outperforms both individual GTI and STR converters in terms of exergy efficiency and entropy production minimization. It exhibits a remarkable 102.03 % increase in exergy efficiency compared to the STR & GTI system at a thermoradiative voltage of −0.14 eV, accompanied by a 28.56 % reduction in exergy loss and a 37.83 % decrease in entropy production. The combination of narrowing the spectral radiation bandwidth and significant electron emission capabilities of graphene contribute to the system’s resilience against solar radiation fluctuations.

Fully discrete model of kinetic ion-induced electron emission from metal surfaces

Ion-induced electron emission (IIEE) is an important process whereby ions impinging on a material surface lead to net emission of electrons into the vacuum. While relevant for multiple applications, IIEE is a critical process of electric thruster (ET) operation and testing for space propulsion, and, as such, it must be carefully quantified for safe and reliable ET performance. IIEE is a complex physical phenomenon, which involves a number of ion-material and ion-electron processes, and is a complex function of ion mass, energy, and angle, as well as host material properties, such as mass and electronic structure. In this paper, we develop a discrete model of kinetic IIEE to gain a more accurate picture of the electric thruster chamber and facility material degradation processes. The model is based on three main developments: (i) the use of modern electronic and nuclear stopping databases, (ii) the use of the stopping and range of ions in matter to track all ion and recoil trajectories inside the target material, and (iii) the use of a scattering Monte Carlo approach to track the trajectories of all mobilized electrons from the point of first energy transfer until full thermalization or escape. This represents a substantial advantage in terms of physical accuracy over existing semi-analytical models commonly used to calculate kinetic IIEE. We apply the model to Ar, Kr, and Xe irradiation of W and Fe surfaces and calculate excitation spectra as a function of ion depth, energy, and angle of incidence. We also obtain minimum threshold ion energies for net nonzero yield for each ion species in both Fe and W and calculate full IIEE yields as a function of ion energy and incidence angle. Our results can be used to assess the effect of kinetic electron emission in models of full ET facility testing and operation.

Simulation of dust grain charging under hypervelocity impact plasma environment

Dust plasma readily forms during hypervelocity impact, which serves as a source of plasma macroscopic charge separation and strong electromagnetic fields. In this study, we examine the dynamic evolution of surface charging of aluminum dust grains with micrometer or submicrometer sizes in a hypervelocity impact plasma environment based on the theory of orbital motion limited. As dust grains traverse the expanding plasma, plasma density and temperature decrease with increasing distance from the impact point. This leads to longer relaxation times for charging equilibrium (ranging from picoseconds to microseconds) and reduced equilibrium charges. The model incorporates thermionic and secondary electron emission effects on dust grain charging processes while also examining the impacts of five heating and cooling mechanisms on the thermal equilibrium temperatures of dust grains. Near the impact point, thermal equilibrium temperatures exceed aluminum's boiling point, which results in phase transition ablation processes. As dust grain temperatures increase, thermionic emission currents may dominate charging dynamics and influence final equilibrium charge numbers. High-temperature dust grains tend to acquire positive charges. Moreover, we observe that the radius of dust grains considerably affects charging processes, and smaller grain radii correspond to low equilibrium charges and longer relaxation times.

Effect of low current cold atmospheric plasma on grains surface structure and water absorption capacity

The use of preparatory electrophysical methods of influencing food raw materials is one of the main trends in the development of innovative processes and technologies in the food and processing industry. Based on the physical effect of electron emission from a thermal emission source, a cold atmospheric plasma (CAP) was obtained, which was successfully applied to the grain material. Physical characteristics and evolution of low-temperature atmospheric plasma were considered as the main methods of analysis of electrophysical effects. To assess the effect of low-temperature plasma on grain material, measurements of water absorption capacity and analysis of surface modification by electron scanning microscopy were carried out. It has been experimentally established that CAP treatment contributes to a more intensive process of water absorption due to changes in the surface structure of the grain material. The total duration of the process of water absorption of grain material after processing of CAP decreased by more than three times until the equilibrium moisture content was reached. Scanning electron microscopy has shown that the processing of CAP leads to the appearance of a fine-mesh structure of the surface of the grain material. The effect of CAP treatment leads to modification of the seed surface, which consists in the manifestation of a fine-meshed structure on the surface of the seeds. Taking into account the advantages of CAP technology, namely the absence of the need for vacuuming and short processing time, the technology has a high practical potential.

Effect of cage bias and electron emission on the two-electron temperature groups in a hot cathode discharge

In the target multi-pole magnetic cage of the double plasma device, where the plasma diffuses through the magnetic filter after its production in the source region, the effect of the cage biasing, introduction of auxiliary filament and accelerating voltage on the control of the temperature and density of the two-electron groups in a hydrogen bi-Maxwellian plasma is carried out. This control of the electron groups in turn will help to enhance the negative ion density. In the absence of biasing voltage, the density of low-energy electrons and high-energy electrons were of the orders ∼1014 m−3 and ∼1012 m−3 respectively. As the cage is biased negatively, these values increased to an order of ∼1016 m−3 and ∼1013 m−3 respectively. With negative cage biasing, the temperature of the high energy electrons was ∼5 eV and low energy electrons ∼0.6 eV. The introduction of an auxiliary electron source in the target region together with the application of an accelerating voltage along with biasing voltage further increased the density of low-energy electrons to ∼4 × 1016 m−3.

Secondary electron emission and collisional effects in a two‐electron temperature plasma sheath

AbstractThe effects of secondary electron emission from the surface of a material that is in direct contact with a plasma containing two different electron groups have been investigated based on the hydro‐dynamic sheath model. The two‐electron groups present in the system have distinct densities and temperatures. They are described by the non‐extensive distribution function, whereas cold ions are assumed to be fluid elements. In addition, the effect of ion‐neutral collision has also been taken into account. The study reveals that the electron non‐extensivity has a significant effect on the secondary electron population as well as space charge deposition near the wall. The results obtained from the study will provide a better understanding of the plasma‐wall transition region under an electron‐emitting condition.

A novel thermionic crystal electron emission effect similar to Kikuchi lines

Kikuchi lines, known since 1928 [S. Kikuchi, Jpn. J. Phys. 5, 83 (1928)], are generated by irradiating a crystal with high-energy e-beam in SEM or TEM and observing backscattered electrons diffraction on crystalline planes. The Kikuchi line effect gave rise to several useful tools in electron microscopy of crystalline and nanocrystalline materials [K. Saruwatari, et al., J. Mineral. Petrol. Sci. 103, 16 (2007)]. We have discovered a similar diffraction effect but of the crystal’ own thermally emitted electrons.

Standing wave instability in large area capacitive discharges operated within or near the gamma mode

Large-area capacitive discharges used for plasma deposition operate in a regime where both electromagnetic and secondary electron emission effects are important. The standing wave shortened wavelength in the presence of plasma depends on the sheath size, and in the γ mode, the secondary electron multiplication controls the sheath physics. Near the α-to-γ transition, and within the γ mode, the sheath width typically varies inversely with the discharge voltage, and large center-to-edge voltage (standing wave) ratios may exist. This can give rise to a standing wave instability, in which the central voltage of the discharge grows uncontrollably, for a given voltage excitation at the discharge edge. Using a simple model, we determine the discharge equilibrium properties, the linearized stability condition, and the nonlinear time evolution. For sufficiently large areas, we show that a discharge equilibrium no longer exists above a critical edge voltage at marginal stability.

Low-threshold field electron emission from graphene nanostructures

This paper presents the results of a study of field electron emission from two-dimensional graphene-like nanostructures synthesized by the method of self-propagating high-temperature synthesis. It is shown that this technology makes it possible to create efficient large-area field emitters. It is found that such structures exhibit the effect of low-threshold field electron emission. The possibility of obtaining high-current electron beams with currents up to several hundred amperes has been confirmed in pulsed electric fields.

Thermal effects in field electron emission from idealized arrangements of independent and interacting micro-protrusions

Modelling studies of thermo-field electron emission (TFE) from protrusions at a cathode surface usually use simulations in 2D axial symmetry. Indeed, time-dependent simulations in 3D are very demanding in computation time. Often, 3D simulations have been restricted to stationary pure field electron emission to account for the drastic current decrease caused by electric field screening when the emitters are close. Little interest has therefore been granted to the heat exchanges occurring between nearby emitters. Although the temperature is a second-order parameter in TFE compared to the electric field, thermal effects become non-negligible in high current density regimes, where self-heating is well established. The present study focuses on the thermal effects occurring during the TFE from micro-protrusions. Our model considers a DC voltage but solves in time the temperature evolution coupling the heat equation and the current continuity equation. The protrusions are modelled as hemiellipsoids with 2D axial symmetry. Emission enhancement due to the increase of the temperature in the thermo-field regime compared to the pure field regime is detailed as a test case for isolated protrusions. Then, full 3D simulations are used to investigate the thermal coupling between multiple neighbouring protrusions via their outwards heat fluxes inside the cathode. The results show a higher current increase due to thermal coupling for dome-like protrusions with a low field enhancement factor. The current increases up to 13% of the total current for aspect ratios of 1, but this value is reached for an extreme applied electric field, hardly reachable in experiments. For sharper protrusions with higher field enhancement, the interaction range through the cathode being shorter, the thermal coupling is suppressed by electrostatic screening. Nevertheless, in arrangements of densely distributed field emitter, when the screening is compensated by a higher voltage, our model predicts the possibility of a moderate but noticeable thermal coupling even for sharp protrusions: a parametric study indicates up to 14.5% of the emitted current being caused by a thermal coupling through the cathode bulk, for protrusions with an aspect ratio of 10 under a fixed applied electric field of 0.4 GV m−1 in DC mode.

Plasma simulation for dual-frequency capacitively coupled plasma incorporating gas flow simulation

We aimed to improve the calculation accuracy by focusing on the neutral radical Ar* and electron density in a dual-frequency excited Ar plasma. The neutral radical Ar* has a long lifetime and becomes exhausted without actually disappearing. The coupled calculation of plasma and gas flow simulation was performed, and confirmed that Ar* in the coupled calculation spreads around, which was not shown by plasma simulation only. Regarding electron density, the effect of the secondary electron emission (SEE) factor was evaluated on basis of the measured electron density. The errors of electron densities between the experimental and simulation results were decreased from 36% to 17% by adding the effect of secondary electrons from the electrodes to the plasma simulation. It was shown that the coupled calculation and the addition of SEE are effective for calculating the dual-frequency excited Ar plasma.

Simulations of divertor plasmas with inverse sheaths

The effect of strong electron emission from material surfaces has been proposed to form an “inverse sheath”: a region with a positive potential relative to the near-wall plasma which prevents the flow of ions to the wall [M. D. Campanell, “Negative plasma potential relative to electron-emitting surfaces,” Phys. Rev. E. 88, 033103 (2013); M. D. Campanell and M. V. Umansky, “Strongly emitting surfaces unable to float below plasma potential,” Phys. Rev. Lett. 116, 1–5 (2016); M. D. Campanell and G. R. Johnson, “Thermionic cooling of the target plasma to a sub-ev temperature,” Phys. Rev. Lett. 122, 1–5 (2019)]. We assess the viability of this regime in a tokamak device using the 2D edge plasma transport code UEDGE [T. Rognlien et al., “A fully implicit, time dependent 2-D fluid code for modeling tokamak edge plasmas,” J. Nucl. Mater. 196–198, 347–351 (1992)]. Since the UEDGE code does not consider the sheath region directly, we apply boundary conditions at the divertor targets which emulate the physics of both “standard” and “inverse” sheath regimes [R. Masline et al., “Influence of the inverse sheath on divertor plasma performance in tokamak edge plasma simulations,” Contrib. Plasma Phys. 60, e201900097 (2020)]. Using these boundary conditions, we perform scoping studies to assess plasma parameters near the target by varying the density at the core-edge interface. We observe a smooth transition in the resultant profiles of plasma parameters for the standard sheath, and a bifurcation across the simulation set for plasmas with an inverse sheath. The cause of this bifurcation is assessed by performing the parameter scan both with and without impurity radiation; we observe that the bifurcation persists in both cases, indicating that this bifurcation is caused by plasma recombination.

Numerical investigation of secondary electron emission effect on the dusty plasma sheath with superextensive electrons

We have investigated the structure of a magnetized sheath of dusty plasma in the presence of secondary electrons emitted by the micro-size dust particles in the context of the Tsallis statistics. The fluid model is used to analyze numerically the effects of the nonextensivity parameter q on the emission of secondary electrons and therefore, on the sheath structure as well as the dust dynamics. The results show that the secondary emission yield increases with the decrease of the parameter of nonextensivity q and consequently, the dust charge becomes less negative with its range of values playing a primordial role in the secondary electron emission rate. The quantities characterizing the sheath are significantly affected by the secondary electron emission (SEE) from the dust. It is seen that as the SEE rises at a given value of q(q<0.91), the sheath potential decreases as well as its absolute value at the wall. In addition, the dynamics of the dust particles is also affected by the emission of secondary electrons.

Plasma-material boundary conditions for discontinuous Galerkin continuum-kinetic simulations, with a focus on secondary electron emission

Continuum kinetic simulations of plasmas, where particle distribution functions are directly discretized in phase-space, permit fully kinetic simulations without the statistical noise of particle-in-cell methods. Recent advances in numerical algorithms have made continuum kinetic simulations computationally competitive. This work presents a continuum kinetic description of high-fidelity wall boundary conditions that utilize the readily available particle distribution function without coupling to additional physical models. The boundary condition is realized through a reflection function that can capture a wide range of cases from simple specular reflection to more involved first principles models. While the framework is usable for various numerical methods and boundary conditions, this work focuses on the discontinuous Galerkin implementation of electron emission using a first-principles quantum-mechanical model. Presented results demonstrate effects of electron emission from a dielectric material on formation of a classical plasma sheath.

Low-Threshold Field Emission from Carbon Structures

The surface of graphene-like structures placed in an electric field with a strength of 1−10 V/μm is found to undergo considerable deformation. This effect is observed in both a direct field (emission current is present) and a reverse field (emission current is absent). The deformation of the surface of cathodes facilitates low-threshold electron emission. The qualitative model proposed here to explain the effect of low-energy electron emission is based on the idea of surface reconstruction and the formation of centers with negative correlation energies (negative-U centers).

The effects of electron surface interactions in geometrically symmetric capacitive RF plasmas in the presence of different electrode surface materials

Particle-in-cell/Monte Carlo collision (PIC/MCC) simulations are performed to investigate the asymmetric secondary electron emission (SEE) effects when electrons strike two different material electrodes in low pressure capacitively coupled plasmas (CCPs). To describe the electron-surface interactions, a realistic model, considering the primary electron impact energy and angle, as well as the corresponding surface property-dependent secondary electron yields, is employed in PIC/MCC simulations. In this model, three kinds of electrons emitted from the surface are considered: (i) elastically reflected electrons, (ii) inelastically backscattered electrons, and (iii) electron induced secondary electrons (SEs, i.e., δ-electrons). Here, we examined the effects of electron-surface interactions on the ionization dynamics and plasma characteristics of an argon discharge. The discharge is driven by a voltage source of 13.56 MHz with amplitudes in the range of 200–2000 V. The grounded electrode material is copper (Cu) for all cases, while the powered electrode material is either Cu or silicon dioxide (SiO2). The simulations reveal that the electron impact-induced SEE is an essential process at low pressures, especially at high voltages. Different electrode materials result in an asymmetric response of SEE. Depending on the instantaneous local sheath potential and the phase of the SEE, these SEs either are reflected by the opposite sheath or strike the electrode surface, where they can induce δ-electrons upon their residual energies. It is shown that highly energetic δ-electrons contribute significantly to the ionization rate and a self-bias forms when the powered electrode material is assumed to be made of SiO2. Complex dynamics is observed due to the multiple electron-surface interaction processes and asymmetric yields of SEs in CCPs.

Low-Threshold Field Electron Emission from Two-Dimensional Carbon Structures

Particles of multilayer graphene obtained by the method of self-propagating high-temperature synthesis are proposed as an active cathode component for field electron emission. It is shown that this material makes it possible to implement a new technology for creating efficient field emitters with a developed surface. It has been established that the effect of low-threshold field electron emission is observed in this material. In pulsed electric fields, the possibility of obtaining high-current electron beams with currents up to hundreds of amperes has been confirmed.

Низкопороговая полевая электронная эмиссия из двумерных углеродных структур

Particles of multilayer graphene, obtained by the method of self-propagating high-temperature synthesis, are proposed as an active component of the cathode for field electron emission. It is shown that this material allows realizing a new technology for creating efficient field emitters with a developed surface. It has been established that the effect of low-threshold field electron emission is observed on this material. In pulsed electric fields, the possibility of obtaining high-current electron beams with currents up to hundreds of amperes has been confirmed.