Electron Emission Yield Research Articles

Electric signals measured during plasma thin-film etching and their connection to the electron concentration and the properties of the treated surface

The electric characteristics of a discharge are usually changed when a thin film is deposited on or etched from a discharge electrode or a substrate. The electric characteristics include the plasma potential, discharge voltage and discharge current, including higher harmonic frequencies of these quantities. This fact can be used for the monitoring of various plasma processes, but the mechanism by which the thin film influences the electric characteristics of the discharge has not been fully clarified. Our study of diamond-like carbon (DLC) film etching verified that variations of electric discharge parameters are caused by variations of electron concentration, which is caused by a difference in the electron emission yield between the DLC film and its substrate.

High-heat flux ball-pen probe head in ASDEX-Upgrade.

A new high heat flux ball-pen probe head installed on the midplane manipulator is currently being used in ASDEX-Upgrade (AUG). The probe was designed to withstand high heat fluxes making possible the investigation of the plasma edge under harsh conditions, such as low power H-mode. Composed of seven pins (four Langmuir probes, mounted in two Mach probe pairs, and three ball-pen probes), the new probe head allows us to measure several plasma parameters simultaneously and with high temporal resolution. A novel method to correct the sheath potential dynamically accounting for the total secondary electron emission is introduced together with applications to obtain the electron temperature and plasma potential profiles. The total secondary electron emission yield is obtained from particle in cell simulations in AUG condition and probe realistic impact angle with respect to the magnetic field. Finally, the probe capability to investigate turbulence around the separatrix of AUG is discussed.

Metal-wire-embedded alumina insulating material using micro- and nanoscale 3D printing for surface flashover mitigation in a vacuum

A micro- and nanoscale 3D printing technique is applied to fabricate a functional insulating material that mitigates surface discharge in a vacuum based on microscopic electron multipactor suppression. The proposed alumina ceramic insulator design consists of surface-embedded thin metal wires that introduce a local gradient of secondary electron emission (SEE) yield, such that the trajectories of multipactor electrons are distorted by accumulated negative surface charge and the SEE avalanche across the insulator surface becomes intermittent. Considerable increases of surface flashover threshold and surface charging reduction are verified by the experiment. Also, additional efforts are made to determine the optimal size and spatial distribution of the metal wire. A convex-shape flashover voltage trace is observed when increasing the width of the wire, suggesting a trade-off between the multipactor mitigation and the insulator strength. The wire’s position between the adjacent cathode triple junction and the middle of the insulator is proved to be favorable for flashover mitigation. The physical details of surface flashover mitigation by the proposed insulator design are revealed by an ab initio particle-in-cell simulation code, corroborating the experiment from a microscopic aspect.

Mechanism on improved surface flashover performances in vacuum of epoxy resin using fluorocarbon plasma treatment

To improve the insulation, strength of materials is a permanent topic in the fields of high voltage and insulation, and surface insulation is one of the difficulties. Research shows that the surface fluorination by a non-equilibrium plasma is a promising method to enhance the surface flashover characteristics of insulating materials. However, the determinants of flashover characteristics are so many that the mechanism is not well known. This paper aims to find out the reasons for the surface insulation improvement and optimise the plasma process parameters. The epoxy resin surfaces are modified by a fluorocarbon plasma, and the flashover voltages in vacuum and secondary electron emission yield are tested experimentally. Combined with the state of the art and the authors' previous work, the mechanism of improved surface flashover performances is investigated in depth from three aspects: the plasma process parameters, the surface physical and chemical properties and the surface electrical performances. A comprehensive understanding of the mechanism is concluded, of which the whole processes from the plasma surface modification of materials to the surface insulation performances are taken into account. It is hopeful to promote the practical applications of plasma fluorination in electrical engineering through the findings of this paper.

Numerical Simulation Study on Gain Nonlinearity of Microchannel Plate in Photomultiplier Tube

Microchannel plate-photomultiplier tubes (MCP-PMTs) with high dynamic ranges and strong outputs are still challenges for the future inertial confinement fusion (ICF) studies, aiming at detecting the large-scale intensities of the radiation pulses. In this article, to investigate the influence factors of the gain nonlinearity causing high-linearity limits of the MCP-PMTs, 3-D microchannel plate (MCP) channel models were built in computer simulation technology (CST) Particle Studio. The Monte Carlo and particle-in-cell methods were carried out to simulate the electron cascade processes in the channels of the MCPs. The dependences of MCP gain nonlinearity on the number of incident electrons, operating voltage, and secondary electron emission (SEE) yield properties were studied. The gains obtained by the simulations for the conventional one-stage and two-stage MCPs are in good agreement with the available experimental data, which verifies the reliabilities of the 3-D MCP models. The simulation results show that the gain of the MCP single channel decreases as the number of incident electrons increases due to the space charge effects. The higher the operating voltage and SEE yield of the MCP, the faster the gain deteriorates. To mitigate the gain saturation effect of MCP single channels, a novel structural design of MCP-PMTs has been proposed by adjusting the design of the MCP chevron pair. A significant improvement in the output pulse peak can be obtained.

Surface effects in a capacitive argon discharge in the intermediate pressure regime

One-dimensional particle-in-cell/Monte Carlo collisional simulations are performed on a capacitive 2.54 cm gap, 1.6 Torr argon discharge driven by a sinusoidal rf current density amplitude of 50 A m−2 at 13.56 MHz. The excited argon states (metastable levels, resonance levels, and the 4p manifold) are modeled self-consistently with the particle dynamics as space- and time-varying fluids. Four cases are examined, including and neglecting excited states, and using either a fixed or energy-dependent secondary electron emission yield due to ion and/or neutral impact on the electrodes. The results for all cases show that most of the ionization occurs near the plasma-sheath interfaces, with little ionization within the plasma bulk region. Without excited states, secondary electrons emitted from the electrodes are found to play a strong role in the ionization process. When the excited states, and secondary electron emission due to neutral and ion impact on the electrodes are included in the discharge model, the discharge operation transitions from α-mode to γ-mode, in which nearly all the ionization is due to secondary electrons. Secondary electron production due to the bombardment of excited argon atoms was approximately14.7 times greater than that due to ion bombardment. Electron impact of ground state argon atoms by secondary electrons contributes about 76% of the total ionization; primary electrons, about 11%; metastable Penning ionization, about 13%; and multi-step ionization, about 0.3%.

Secondary electron emission yield measurements of dielectrics based on a novel collector-only method

The total electron-induced Secondary Electron Yield (SEY) of alkali-free glass has been determined by a novel technique which is solely based on the use of a collector electrode and of two interchangeable dielectric samples. The current from a charge-saturated sample provides a direct measure of the primary beam current. The SEY from the (uncharged) sample under investigation can then be determined simply as the ratio between the collector currents measured at the same electron energy from both samples. We further apply a low-current, short-pulse procedure to limit sample charging, needed to mitigate distortion of the SEY by charging effects. The potential build-up during the experiment could be reconstructed based on the primary electron currents and the measured SEY. This strategy permitted us to measure a 6-point SEY curve, without the need for intermediate discharging of the sample, as the total induced surface potential rise could be kept below +2.55 V.

Simulation of [formula omitted]I Auger emission spectrum with new atomic parameters from MCDHF calculations

New 125I atomic decay emission data of medical interest are presented. The calculations are based on two atomic structure codes that implement the multi-configuration Dirac-Hartree-Fock method. Radiative and non-radiative ransition rates are calculated in this method and then used to generate the atomic deexcitation cascade. Subshell transition rates, level widths and fluorescence yields are compared to the Evaluated Atomic Data Library. Coster-Kronig and Auger electron emission yields are also compared with results from other authors. The comparison with the experimental electron emission spectrum shows that the new calculations can reproduce very well the structure of the K-LL Auger electron peaks and improve the description of the M Auger peaks below 300 eV. The 125I dose-point kernel is also simulated using the new data, resulting in higher values below 10 nm when compared those obtained with the Evaluated Atomic Data Library.

Atypical secondary electron emission yield curves of very thin SiO2 layers: Experiments and modeling

The secondary electron emission phenomenon often refers to the emission of electrons as a result of the interaction of impinging energetic electrons with the surface of a material. Although it is fairly well described for metals, with a typical shape of the total electron emission yield (TEEY) first increasing to reach a maximum and then decreasing along with the energy increase in the primary electrons, there is still a lack of data and detailed analysis for dielectrics, in particular thin layers. The present work proposes a new insight into the electron emission phenomenon from very thin dielectric layers. It reports on the TEEY from very thin SiO2 layers, less than 100 nm. It is found that a departure from the typical shape of the TEEY curve occurs for primary electrons with energy of around 1 keV. The TEEY curve presents a dip, a local minimum that might be as deep as below 1. This atypical shape depends substantially on the layer thickness. The measured TEEY is compared to an electron emission 1D-model in which we consider the combined effect of the space-charge electric field induced by trapped charges in the dielectric layer and of the processes of field-dependent conductivity and radiation-induced conductivity on the fate of secondary electrons. Those mechanisms govern the charge transport in the dielectric and, consequently, the electron emission. The effects of the SiO2 layer thickness, an incidence angle of the primary electrons, and an applied external electric field on the TEEY curves are reported.

Effects of secondary electron emission yield properties on gain and timing performance of ALD-coated MCP

The technology of atomic layer deposition has been used to improve the lifetime of the microchannel plate-photomultiplier tube (MCP-PMT) effectively and makes MCP possible to choose to coat different potential emissive materials on the internal surface of the MCP channels in the future. However, it is still an open question to what extent the secondary electron emission (SEE) yield properties of the emissive materials influence the behavior of the ALD-coated MCP. In this work, the dependences of the gain and timing performance on the SEE yield properties were assessed by using the Monte Carlo and particle-in-cell methods. We established the three-dimensional MCP single channel model in Computer Simulation Technology (CST) Particle Studio. Three important secondary electron emissions, the backscattered, rediffused and true SEEs, were discussed numerically based on the probabilistic model. The secondary electron cascade processes in the MCP single channel were simulated. The simulation results indicate that the opportunities for improving the gain of the ALD-coated MCP by improving the SEE yields corresponding to the incident energies of 0 eV–100 eV. The backscattered and rediffused electrons are found to have strong effects on the gain and timing performance of the MCP. Although the higher the SEE yield the higher the MCP gain, the drawback is the extremely high SEE yield will make the MCP saturated prematurely and degrade the time resolution. The simulation results will be used to guide the design and selection of emissive material for ALD-coated MCP development.

Low-energy electron transport in gold: mesoscopic potential calculation and its impact on electron emission yields

In a previous work, we extended and benchmarked the MDM Monte Carlo code with available data for gold metallic media irradiated by electron beams. In this paper, we worked with the aim of improving the cross sections on which our Monte Carlo simulation is based, and which are essential for an accurate description of the transport of electrons in gold. The mesoscopic potential of solid gold has been predicted, and its sensitivity toward electron emission has been evaluated. This potential was derived from the calculation of the electrostatic and atomistic potential by density functional theory and used to calculate inelastic inverse mean free path for electron transport. After integrating these results into our Monte Carlo code, we evaluated the impact of these new cross sections on yields of electron emission from solid gold irradiated by monoenergetic electron beams. We obtained a mesoscopic potential value of − 12.77 eV for our model of bulk metal gold, 27% lower than the one commonly estimated from the Fermi energy. This result impacted on the inverse mean free path for plasmon excitations with a 10% decrease for electrons in the range of 6–30 eV. Regarding electron emission yields, there was no impact of the new mesoscopic potential on the primary electron yields, but for secondary electrons, the emission yields were increased by a factor of up to two depending on the primary beam energy and thickness of the gold foil.

In situ measurement of electron emission yield at silicon dioxide surfaces exposed to argon plasmas

Plasma simulations require accurate yield data to predict the electron flux that is emitted when plasma-exposed surfaces are bombarded by energetic particles. One can measure yields directly using particle beams, but it is impractical to create a separate beam of each particle produced by typical plasmas. In contrast, measurements made in situ, during plasma exposure, provide useful values for the total emitted flux produced by all incident particles. Here, in situ measurements were performed in a radio-frequency (rf) biased, inductively coupled plasma (icp) system in 0.67 Pa and 1.33 Pa (5 mTorr and 10 mTorr) of argon gas. The rf current and voltage across the sheath adjacent to the rf-biased electrode were measured, along with Langmuir probe measurements of ion current density and electron temperature. The measurements are input into a numerical sheath model, which allows the emitted electron flux to be distinguished from other current mechanisms. The effective yield, i.e., the ratio of the total emitted electron flux to the incident ion flux, is also determined, as a function of incident ion energy. Results for the effective yield of a sputter-deposited SiO2 film are reported and compared with previous work. The measured effective yield can be considered the yield from Ar+ kinetic emission, which, in these experiments, dominates other emission processes. From additional literature data, emission yields for other processes—incident photons, Ar metastables, fast Ar atoms, and Ar+ potential emission—are recommended and are shown to be consistent.

Secondary electron emission yield from vertical graphene nanosheets by helicon plasma deposition

The secondary electron emission yields of materials depend on the geometries of their surface structures. In this paper, a method of depositing vertical graphene nanosheet (VGN) on the surface of the material is proposed, and the secondary electron emission (SEE) characteristics for the VGN structure are studied. The COMSOL simulation and the scanning electron microscope (SEM) image analysis are carried out to study the secondary electron yield (SEY). The effect of aspect ratio and packing density of VGN on SEY under normal incident condition are studied. The results show that the VGN structure has a good effect on suppressing SEE.

Surface characterization determined from the secondary electron emission coefficient upon ion bombardment

The ion induced electron emission yield upon Ar+ ion impact at 420 eV was measured for a stainless-steel sample that was partially covered with a 50 nm thick gold layer. The ion induced electron emission yield for the two target materials differs strongly and enables to reproduce the shape of the gold film with a spatial resolution that corresponds to the width of the ion beam, which is 240 µm in the present case. Details in the two-dimensional map of the ion induced electron emission yield are explained by comparison with optical and scanning electron microscopy as well as with X-ray photoelectron spectroscopy measurements of the sample surface. In contrast to the sputtering efficiency, the ion induced electron emission yield does not depend on the orientation of the crystal structure.

A new method for measuring total electron emission yield of insulators.

For insulators, the deposited charge on the surface after electron bombardment interferes with the total electron emission yield (TEEY) measurement. The key to measuring the TEEY of insulators is to eliminate the surface deposited charge. Being different from traditional measurement methods, we develop and demonstrate a new method for measuring the TEEY of insulators in a scanning electron microscope by preparing the micro-patterned metal on the surface of the insulators as a conducting channel. The TEEY of the insulators is measured based on the TEEY of the metal part and the metal-insulator periodic structure. By theoretical analysis, we get a criterion for judging whether the deposited charge is eliminated. The result shows that this method is reliable by measuring the TEEY of Si. Then, we obtained the TEEY of SiO2 by this method with an energy of up to 3000 eV. The measured TEEY characteristics of SiO2 are consistent with the data from the references. Moreover, the surface charge diffusion distance of the insulators can also be estimated under this measuring condition. We get that the maximum value of the TEEY of SiO2 is ∼4.2, and the surface charge diffusion distance of SiO2 is ∼1 µm. In this work, the criterion for judging whether the deposited charge is eliminated is obtained, which has not been proposed in traditional measurement methods.

On the limits of multipactor in rectangular waveguides

Multipactor in WR-284-like geometries is measured utilizing local and global detection techniques. To emulate conditions one may find in a waveguide filter structure while maintaining the fundamental microwave mode, a standard rectangular waveguide geometry with the reduced waveguide height set to 2.1 or 5.5 mm was adopted. Two high power RF sources were used to investigate a large range of input power (few kWs to MWs): a solid state source using GaN HEMTs allowing for larger pulse widths than standard magnetrons (100 μs as opposed to ∼4 μs) and a MW level S-band coaxial magnetron for the high power end. Particular interest was taken in capturing the lower and upper limits of multipactor threshold. Lower multipactor thresholds for finite pulse duration are governed by the appearance of one or more electrons in the multipactor gap during the applied pulse as well as a minimum power (electric field) level that affects a secondary electron emission yield above unity. As shown, such initial electrons(s) may easily be seeded via an external UV source illuminating the gap. However, wall collisions of excited metastable molecules may be another source of electrons, an observation based on the experiment and prior research. A multipactor upper threshold was non-existent in the experiment, even at powers over 200 kW within a 2.1 mm test gap, which numerically yielded a gap transit time significantly shorter than one half-period of the GHz wave. This is attributed to the electric field distribution within the waveguide structure, which results in the multipactor's spatial position moving to more favorable locations within the test gap.

Erratum: “Multipactor threshold sensitivity to total electron emission yield in small gap waveguide structure and TEEY models accuracy” [Phys. Plasmas 23, 123118 (2016)

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Dynamic annealing effects under high-fluence ion irradiation of glassy carbon

The ion induced modification of the structure and morphology of low-temperature (SU-1300) and high-temperature (SU-2500) glassy carbons after high-fluence (≥1018 cm−2) irradiation with 30 keV Ar+ ion in the temperature range from 60 to 600 °C are presented and discussed. Raman spectra show an amorphized state of surface both for the low and high-temperature glassy carbons at an irradiation temperature below the temperature of the dynamic annealing of radiation damage, determined by the temperature dependence of the ion-induced electron emission yield; a graphite-like structure at 140 ≤ Т ≤ 350°С for SU-1300 and at 140 ≤ Т ≤ 250°С for SU-2500; a structure similar to non-irradiated low-temperature glassy carbon at 350 <Т ≤ 600°С and 250 <Т ≤ 600°С for low- and high-temperature glassy carbons, respectively. In both cases, ion irradiation leads to the temperature-dependent changes in the topography of the modified layer. Ion irradiation under conditions of the dynamic annealing of radiation damage leads to a network topography of the nanowalls.

A three-dimensional Dionne model for multipactor simulations

The multipactor phenomenon is characterized by a very fast growth of the electronic population in radio-frequency (RF) devices under vacuum. As this effect limits the transmissible RF power and can harm RF systems, it has been widely studied during the last few decades. Due to the high cost of experimental tests, simulation tools are heavily used to predict the threshold of multipactor growth. However, their reliability is limited for complex configurations, e.g., when dielectrics or magnetic fields are present. A crucial element of these multipactor simulations is the secondary-emission model. Dionne's model is able to model both metals and dielectrics secondary emission but is one-dimension only. As the three-dimensional aspect is essential for complex configurations, the Dionne model is extended to three-dimensions. Measurements of the total electron emission yield have been carried out at the ONERA and show a good agreement for low-impact energy and low-impact angle electrons, which is relevant in multipactor simulations.

Plasma sheath material induced dependence due to secondary electron emission

Plasma sheaths in front of six different material samples (BN, BNSiO2, Al2O3, SiO2, stainless steel, and silicon) used in various experiments and devices (Hall thrusters, plasma discharge, and microelectronics) are studied using the laser induced fluorescence diagnostic. The specific secondary electron emission (SEE) yield of each material is expected to induce differences in the sheath structure from one sample to another. The experiments are carried out in two different plasma discharges (multipolar device and ECR device), exhibiting distinct electron distribution functions: bi-Maxwellian and Maxwellian. The agreement between the two experiments is good and allows us to classify the materials in a consistent way regarding their SEE yields. The multipolar experiment results are compared to a 1D kinetic sheath model and a 1D-1 V kinetic sheath simulation code. The predictions of the model are discussed and are in good agreement with previous theory. The influence of the low energy impinging electrons on the SEE yield and emissive sheaths is investigated with the code.