Secondary Electron Emission Measurements Research Articles

Simulation studies of secondary electron yield with electron transport from Cu (110) surfaces containing C2, N2, CO2, or NO2 adsorbates

Secondary electron yields of (110) copper surfaces, covered with either carbon, nitrogen, or their dioxides, have been studied by employing combined first principles methods for the material properties and Monte Carlo simulations for electron transport. Furthermore, by studying electron transport inside the Cu system and modeling the power loss taking account of the inelastic electron scattering within the material, changes in the thermal energy of the system have been modeled. The physical reasons behind the increase and decrease of the yield for each system from an electronic perspective are discussed. In agreement with results observed in studies of secondary electron emission, it is shown that the formation of C2 and N2 monolayers reduce the secondary electron yields, while CO2 and NO2 increase the yield significantly. It is demonstrated that in the case of C2 and N2 formation, changes in the surface electronic barrier reduce the probability of electron escape from the Cu surface, resulting in lower secondary electron emission. Formation of CO2 and NO2, on the other hand, reduce the electronic barrier effects. In addition, due to weak bonding of the CO2 layer with the Cu host, the surface provides an additional source of secondary electrons resulting in higher electronic emission yield. Moreover, the NO2 adsorbate creates a surface electric field that changes the surface electron energy and increases the electron escape probability. Additionally, it is verified that thermal change in the system is negligible and so during secondary electron emission measurements, negligible (if any) surface adsorption or desorption could occur.

Collector-based measurement of gas desorption and secondary electron emission induced by 0–1.4 keV electrons from LHC-grade copper at 15 K

CERN’s Large Hadron Collider cryomagnets embed a 1.9 K UHV chamber lined with a 5–20 K beam-screen (BS) that intercepts the synchrotron radiation and electron cloud (EC). The low–energy EC irradiates the BS and desorbs gas, creating a dynamic vacuum effect. A novel setup controllably reproduces this by irradiating an unbaked as-received BS copper sample at 15 K with 0–1.4 keV electrons, representing a slice of the EC spectrum. This collector-based setup is qualified using a HOPG reference for secondary electron yield (SEY) and 15N2 as a tracer in low-energy electron stimulated desorption (ESD) measurements. Measurement at 15 K revealed sub-10 eV ESD thresholds and a maximum around 300 eV of 0.18 H2/e− and 0.13 CO/e−. Irradiation with 300 eV and 1 keV electrons at ∼ 8.10–4 C.mm−2 conditioned ESD and SEY alike. Similar dose at 17 eV only caused minor SEY reduction and no ESD decline. The as-received H2 and CO2 yields at 300 eV decreased 5-150x between 15 and 265 K, respectively..

Secondary electron emission characteristics of the Al2O3/MgO double-layer structure prepared by atomic layer deposition

As a secondary electron emission layer, an Al2O3/MgO double-layer structure is fabricated by atomic layer deposition (ALD) technology. The thickness range from 1 nm to 4 nm of the top Al2O3 layer deposited on 20 nm MgO creates a double-layer. The morphology of the cross section, element distribution, surface roughness, X-ray diffraction, and secondary electron yield (SEY) values were measured by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM) and secondary electron emission (SEE) measurement systems. The SEE characteristics of the MgO single layer, Al2O3 single layer and MgO/Al2O3 double-layer were measured. The maximum SEY of a single MgO layer reached 6.2@600 eV, and the maximum SEY of a single Al2O3 layer reached 3.92@400 eV. The SEY of the Al2O3/MgO double-layer decreased when the Al2O3 thickness ranged from 1 nm to 4 nm, and the SEY reduction value of the double-layer decreased as the Al2O3 thickness increased. Finally, Dionne's semiempirical SEE model was employed to explain the SEE yield of the prepared composite film structures. These results are useful for depositing a secondary electron emission layer in the channel of microchannel plates.

Comparison of Carbon Thin Films with Low Secondary Electron Yield Deposited in Neon and Argon

Modification of vacuum chamber surface properties by introducing a layer of material with low secondary electron yield (SEY) is one of the most useful solutions to suppress the electron-cloud in high-energy particle accelerators. In the present work, amorphous carbon thin films have been produced by DC magnetron sputtering with Neon and Argon sputtering gases. Microstructures of the thin films have been characterized by using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The sp2 and sp3 hybridized carbon atoms are evaluated using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The amorphous carbon coatings comprise tiny granularities of tens of nanometers. The amorphous carbon films show more graphite-like properties as revealed by XPS and Raman spectroscopy. The secondary electron emission measurement results indicate that amorphous carbon coatings present SEY of <1.2. The thin film deposited by Ne exhibits a higher sp2 hybridization content, leading to a slightly lower SEY compared with the film produced with Ar.

Secondary electron emission measurement from Cr and Cu bombarded by an Ne10+ beam at 6 MeV/n

Abstract Backward secondary-electron-emission yield ($\delta$) from plates of 0.1 mm-thick Cu and 1 mm Cr and Al have been measured by irradiation of a fully stripped Ne$^{10+}$ beam at 6 MeV/$n$ stopped within each plate. A difference between $\delta$s from Cr and Cu larger than the ambiguity of this measurement ($\pm$3%) has been observed, with the discrepancy of the predicted small difference by kinetic emission based on stopping power and work function of each metal. The measured $\delta$ from Cr is larger than that from Cu, and also than previous measurements of other transition metals of the 3$d$ series, not only for the process of potential emission but also for kinetic. The conduction-electron density of states around the Fermi level calculated for the metals shows that the number of electrons just below the Fermi level and excited over it to an empty level by many kinds of reaction with the beam irradiation has to be considered. The number involved in this emission from Cr thus seems to be much larger than from Cu, which explains the relation of magnitudes for $\delta$. The measured $\delta$ from Al is larger than previous results, but is still consistent due to the existence of Al$_2$O$_3$ on the Al.

Design of a mechatronic system for accurate measurement of secondary electron emission

Accurate measurement of Secondary Electron Emission (SEE) in scientific experiments provides better insight and understanding of field emission and multipacting properties in accelerator cavities. SEE properties of Niobium (Nb) are of significant importance to the performance and operation of accelerator cavities. Additionally, accurate Secondary Electron Yield (SEY) measurements from primary electrons generated from samples measured under different incident angles represent a technical challenge. For that purpose, the sample positioning system presented in this article is a design that allows for precise sample orientation and handling during single pump-down of the vacuum system in the experimental setup. Measurement techniques supported by a mechatronic system provide the means for acquisition of large quantities of data and at the same time reduce the experimental errors related to sample positioning. The authors describe a sample manipulation system and method for controlling the position of samples used for the measurement of SEY.

Secondary electron emission from plasma-generated nanostructured tungsten fuzz

Recently, several researchers [e.g., Yang et al., Sci. Rep. 5, 10959 (2015)] have shown that tungsten fuzz can grow on a hot tungsten surface under bombardment by energetic helium ions in different plasma discharges and applications, including magnetic fusion devices with plasma facing tungsten components. This work reports the direct measurements of the total effective secondary electron emission (SEE) from tungsten fuzz. Using dedicated material surface diagnostics and in-situ characterization, we find two important results: (1) SEE values for tungsten fuzz are 40%–63% lower than for smooth tungsten and (2) the SEE values for tungsten fuzz are independent of the angle of the incident electron. The reduction in SEE from tungsten fuzz is most pronounced at high incident angles, which has important implications for many plasma devices since in a negative-going sheath the potential structure leads to relatively high incident angles for the electrons at the plasma confining walls. Overall, low SEE will create a relatively higher sheath potential difference that reduces plasma electron energy loss to the confining wall. Thus, the presence or self-generation in a plasma of a low SEE surface such as tungsten fuzz can be desirable for improved performance of many plasma devices.

Secondary electron emission from lithium and lithium compounds

In this work, measurements of electron-induced secondary electron emission (SEE) yields of lithium as a function of composition are presented. The results are particularly relevant for magnetic fusion devices such as tokamaks, field-reversed configurations, and stellarators that consider Li as a plasma-facing material for improved plasma confinement. SEE can reduce the sheath potential at the wall and cool electrons at the plasma edge, resulting in large power losses. These effects become significant as the SEE coefficient, γe, approaches one, making it imperative to maintain a low yield surface. This work demonstrates that the yield from Li strongly depends on chemical composition and substantially increases after exposure to oxygen and water vapor. The total yield was measured using a retarding field analyzer in ultrahigh vacuum for primary electron energies of 20–600 eV. The effect of Li composition was determined by introducing controlled amounts of O2 and H2O vapor while monitoring film composition with Auger electron spectroscopy and temperature programmed desorption. The results show that the energy at which γe = 1 decreases with oxygen content and is 145 eV for a Li film that is 17% oxidized and drops to less than 25 eV for a fully oxidized film. This work has important implications for laboratory plasmas operating under realistic vacuum conditions in which oxidation significantly alters the electron emission properties of Li walls.

Research on the secondary electron yield of TiZrV-Pd thin film coatings

In particle accelerators, the build-up of electron cloud has an important influence on beam quality. Especially for the positron and proton accelerators, massive electrons lead to electron cloud, which affects the stability, energy, emittance and beam life adversely. A secondary electron emission (SEE) measurement system has been designed and used to study the SEE of palladium (Pd), TiZrV and TiZrV-Pd with an independently adjustable energy from 50 eV to 5 keV. Here, we obtained the characteristics of the SEE from Pd, TiZrV and TiZrV-Pd film coatings with different thickness under ultrahigh-vacuum (UHV) conditions. Moreover, the minimum maximum secondary electron yield (SEY), δmax, of the Pd, TiZrV and TiZrV-Pd film coatings under different primary electron doses were 1.38, 1.51, and 1.52, respectively. Finally, the variation of the secondary electron yield with the incident electron energy will be discussed for Pd, TiZrV and TiZrV-Pd thin film coatings.

Surface flashover characteristics in polyimide/ZnO nanocomposite under DC voltage in vacuum

It is well known that the solid materials flashover phenomenon is very complicated and influenced by many factors, such as surface conditions, electrodes and traps, etc. In this paper, dc flashover experiments of polyimide (PI) and PI/ZnO nano-composite are studied in vacuum to reveal the surface flashover mechanism. The trap levels and space charge distributions were obtained by thermally stimulated current (TSC) and pulsed electro-acoustic (PEA) methods respectively. Further researches on surface states of flashover tested sample, such as observation by scanning electron microscopy (SEM), and measurement of secondary electron emission (SEE) coefficient, were carried out as well. Experimental results showed that the dc surface flashover voltage of PI/ZnO nano-composite is significantly enhanced compared to that of pure PI. The experiments results can be explained as that the doped ZnO nano-particles introduced deeper traps, which can reduce the level of charge carrier mobility, secondary electron emission and space charge accumulation. Meanwhile, the gas desorption efficiency of nano-composite is much lower than that of pure PI. It suggests that the doped ZnO nano-particles plays a critical role in affecting the surface flashover performance under dc voltage in vacuum.

劣化照射が可能な広エネルギー帯域二次電子放出測定システムの開発

劣化照射が可能な広エネルギー帯域二次電子放出測定システムの開発

Investigation of the secondary electron emission characteristics of alternative dynode materials for imaging photomultipliers

The requirement to accurately detect and image very high speed photon events is a necessity for many applications across a range of disciplines. Photon imaging detectors often make use of the planar geometry of MCPs for high resolution imaging. However, the inherent rate limitations of MCPs provide opportunities for discrete dynode devices where rapid recharge times and high throughput are required.Novel dynode materials and designs offer the possibility to further improve event time resolution and pulse height distribution. Significant performance advantages can be achieved by increasing the gain of each dynode stage by utilizing materials such as CVD diamond. Secondary electron emission (SEE) coefficients for polycrystalline CVD diamond of 84 have been reported for Hydrogen terminated surfaces and similar values for Cs terminated surfaces [1, 2]. These values are far in excess of currently used materials such as AgMgO(Cs) and CuBeO(Cs) which have SEE coefficients < 15 [3].Polycrystalline diamond is a widely available, relatively inexpensive material that can be deposited over large areas, making the manufacture of large format imaging detectors a practical possibility. The application of new dynode designs for imaging photomultipliers requires knowledge of the secondary electron emission characteristics of the material. We describe a facility to measure the relevant characteristics of novel dynode materials and designs with 2D imaging capability, and discuss the challenges and complexities involved with SEE coefficient measurements. We present secondary electron emission measurements for a variety of dynode materials and designs and discuss their application in imaging photon-counting devices.

Vacuum Surface Flashover Characteristics and Secondary Electron Emission Characteristics of Epoxy Resin and FRP Insulator

Neutral beam injectors (NBI) used for JT-60 are required to generate negative ions of 500 keV energies. To produce such high-energy ions, the electrostatic accelerators consisting of 3-stage of electrodes and three insulator rings are applied. The insulators are made of Fiberglass Reinforced Plastic (FRP) which is composed of epoxy resin and glass fibers. The surface discharges along the insulators are one of the most serious problems in the development of NBI. To increase the hold-off voltage against surface flashover events, it is necessary to investigate the FRP and epoxy resin insulator properties related to surface discharges in vacuum. This paper describes surface flashover characteristics for epoxy resin, FRP and Alumina samples under vacuum condition. In addition, the measurements of secondary electron emission (SEE) characteristics are also reported. These are important parameters to analyze surface discharge characteristics of insulators in vacuum.

Reversible Ion Induced Modification of Consequent Secondary Electron Emission in Porous Silicon

We report measurements of secondary electron emission (SEE) induced by electron and ion bombardment on porous silicon (PS). We found that electron induced emission is strongly reduced by ion bombardment, and that this reduction is reversible. The reduction effect is large even for ion fluxes much lower compared to that of the electron beam. We attribute this effect to changes in the charge distribution of the surface dipole originated in the difference between ion and electron charge deposition depths. The nanostructure of PS plays an important role in this effect as well as in the reversibility of the process. We think that this effect could be useful in the dynamic centering and monitoring of ion and electron beams in electron spectroscopy.

Lunar Prospector measurements of secondary electron emission from lunar regolith

We present the first in situ measurements of the secondary electron emission efficiency of lunar regolith, utilizing Lunar Prospector measurements of secondary electrons emitted from the negatively charged night side and accelerated upward by surface electric fields. By comparing measurements of secondary currents emitted from the surface and incident primary electron currents, we find that the secondary yield of lunar regolith is a factor of ∼3 lower than that measured for samples in the laboratory. This lower yield significantly affects current balance at the lunar surface and the resulting equilibrium surface potentials. This information must be folded into models of the near-surface plasma sheath, in order to predict the effects on dust and other components of the lunar environment, and ultimately determine the importance for surface exploration and scientific investigations on the Moon.

Ion induced high energy electron emission from copper

We present measurements of secondary electron emission from Cu induced by low energy bombardment (1–5 keV) of noble gas (He +, Ne + and Ar +) and Li + ions. We identify different potential and kinetic mechanisms and find the presence of high energetic secondary electrons for a couple of ion-target combinations. In order to understand the presence of these fast electrons we need to consider the Fermi shuttle mechanism and the different ion neutralization efficiencies.

A new method of measuring electron emission induced by low energy ions from solids

A newly developed experimental system for measurements of electron emission induced by low energy ions at below low ion flux (105 ions s−1 cm−2) avoiding the fast emission degradation caused by high ion fluxes (usually applied 1010–1013 ions s−1 cm−2) has been established. The method, based on an event-by-event recording of the emitted cluster of electrons for each impinging ion event, overcomes the difficulty of measurement of reliable ion-induced secondary electron emission (IIEE) yield, associated with fast degradation of the electron yield. This method provides not only the average number (γ) of IIEE under a given ionic impact, but actually higher moments of their distribution, which contains more information on the physical processes involved in the IIEE phenomenon. The secondary electron emission coefficients showed a dependence on the kinetic energy of incident argon ions and protons ranging from 1 keV to 10 keV on hydrogenated diamond film surfaces.

Secondary electron emission from alumina RF windows

The development of alumina RF windows for use at the output section of high-power RF sources is an important issue for accelerators. The breakdown of RF windows is caused by 1. the multipactor effect (electron multiplication on the surface) and/or 2. the discharge of accumulated charges (due to the multipactor effect). Measurements of secondary electron emission (SEE) and surface charging were carried out to determine the durability of various alumina ceramics in this application. Since excessive surface heating takes place during surface discharge, SEE yields at high temperature were also measured. The results indicate that SEE yields become lower at high temperature, and that higher purity alumina ceramics show higher SEE yields. High-power tests using a resonant ring were conducted in order to observe surface charging and electron accumulation. Higher purity alumina ceramics showed superior performance in the high-power tests, probably due to lower surface charging.

UHV reactive evaporation growth of titanium nitride thin films, looking for multipactor effect suppression in space applications

In this work, an ultra-high vacuum reactive evaporation procedure has been tested and optimized for the growth of TiN thin films, throwing very good results both in composition of the TiN layers and secondary emission yield values and performance. Stoichiometry and composition are analyzed by means of X-ray photoemission spectroscopy which, together with the secondary electron emission measurements, allow us determine the appropriate growth parameters in order to obtain in a reproducible manner optimum results towards the multipactor effect suppression. After refining the growth on test layers, several waveguide pieces, specially designed and constructed for testing multipactor level (maximum RF power attainable without multipactor discharge), have been coated with TiN films following those optimized growth conditions.

Effect of double layer coating on carbon nanotubes for field emission and secondary electron emission measurement

Field emissions of wide-band-gap-material (WBGM)-coated single-walled carbon nanotubes (SWCNTs) were studied. Field emission current was gradually increased after monolayer coating (MgO-coated SWCNTs) and successive double layer coating (CsI∕MgO-coated SWCNTs). The turn-on field decreased from 3.26 to 2.43 and 1.61μAcm−2, respectively. The calculated work functions were found to be 3.34 and 3.01eV, and the maximum secondary electron emission yields were 115 and 40 for MgO- and CsI∕MgO-coated SWCNTs, respectively. These results would be due to a higher electric field generation around the SWCNT surface after double layer WBGM coating rather than single layer WBGM coating. To reveal the influence of WBGM coating on carbon nanotubes, x-ray photoelectron spectroscopy and energy dispersive analysis of x-ray spectra were analyzed.