Low Secondary Electron Emission Research Articles

Low secondary electron emission characteristics of carbon nano-onion coating via plasma enhanced chemical vapor deposition

Suppression of secondary electron emission (SEE) by surface coating technology on metal materials has important applications in many fields, for which the investigation on new coating materials with low secondary electron yield (SEY) is desired significantly. In this study, the maximum SEY of nickel (Ni) substrate was reduced from 2.30 to 0.75 (67 % reduction) after a carbon nano-onion (CNO) coating prepared by plasma enhanced chemical vapor deposition (PECVD), and the SEY less than 1 means that some problems caused by secondary electron (SE) multiplication on metal materials could be solved completely. Various measurements, material characterization and CASINO Monte Carlo model simulation indicated that the low SEY characteristic of CNO coating could be attributed to two main mechanisms: Firstly, true SEs emitted from the surface of CNO could be reabsorbed by the adjacent CNO attributed to the micrometer scale undulating structure formed by the accumulation of CNO on the coating surface; Secondly, the defects degree of CNO directly affects the emission of true SE, which plays the role of low-energy electron capturing trap. This discovery can provide a reference for future coating production.

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.

Multifunctional-high resolution imaging plate based on hydrophilic graphene for digital pathology

In the present study, we showed that hydrophilic graphene can serve as an ideal imaging plate for biological specimens. Graphene being a single-atom-thick semi-metal with low secondary electron emission, array tomography analysis of serial sections of biological specimens on a graphene substrate showed excellent image quality with improved z-axis resolution, without including any conductive surface coatings. However, the hydrophobic nature of graphene makes the placement of biological specimens difficult; graphene functionalized with polydimethylsiloxane oligomer was fabricated using a simple soft lithography technique and then processed with oxygen plasma to provide hydrophilic graphene with minimal damage to graphene. High-quality scanning electron microscopy images of biological specimens free from charging effects or distortion were obtained, and the optical transparency of graphene enabled fluorescence imaging of the specimen; high-resolution correlated electron and light microscopy analysis of the specimen became possible with the hydrophilic graphene plate.

Ultralow secondary electron emission and improved vacuum surface insulation of polyimide with scalable nanocomposite coating

Polymers with high surface electric insulation capability are greatly demanded in various electrical devices operating in vacuum ambient. Low secondary electron emission (SEE) is favorable to mitigate vacuum flashover by inhibiting the SEE avalanche across the surface. Although numerous approaches, esp. the surface modifications, promote the flashover strength of the insulator, a facile yet cost-effective modification strategy of a high-quality and scalable flexible insulator remains a formidable challenge. Herein, a facile ion-exchange technique is implemented to fabricate a nanoscale polyimide/chromic oxide (PI/Cr2O3) nanocomposite coating comprising hundreds of organic/inorganic interfaces securely adhered to a flexible PI substrate, which features ultralow secondary electron emission yields (SEEY). The low emissivity is achieved by introducing weakly emissive Cr2O3 nanoparticles as well as interfacial traps within the nanocomposite layer, which outperforms the existing and modified polymers. The PI/Cr2O3 shows a remarkable increase in flashover strength while keeping sufficient mechanical durability. The proposed surface engineering strategy is convenient and scalable, bringing up new insight into industrial applications.

Prospects for microwave plasma synthesized N-graphene in secondary electron emission mitigation applications

The ability to change the secondary electron emission properties of nitrogen-doped graphene (N-graphene) has been demonstrated. To this end, a novel microwave plasma-enabled scalable route for continuous and controllable fabrication of free-standing N-graphene sheets was developed. High-quality N-graphene with prescribed structural qualities was produced at a rate of 0.5 mg/min by tailoring the high energy density plasma environment. Up to 8% of nitrogen doping levels were achieved while keeping the oxygen content at residual amounts (~ 1%). The synthesis is accomplished via a single step, at atmospheric conditions, using ethanol/methane and ammonia/methylamine as carbon and nitrogen precursors. The type and level of doping is affected by the position where the N-precursor is injected in the plasma environment and by the type of precursors used. Importantly, N atoms incorporated predominantly in pyridinic/pyrrolic functional groups alter the performance of the collective electronic oscillations, i.e. plasmons, of graphene. For the first time it has been demonstrated that the synergistic effect between the electronic structure changes and the reduction of graphene π-plasmons caused by N doping, along with the peculiar “crumpled” morphology, leads to sub-unitary (< 1) secondary electron yields. N-graphene can be considered as a prospective low secondary electron emission and plasmonic material.

First Experimental Data on a New SR Beamline of the VEPP-2000 Complex for the HL-LHC Vacuum System

In the framework of the large hadron collider (LHC) upgrade project, new materials are proposed for the vacuum. Amorphous carbon deposited onto the vacuum-chamber wall is examined as a coating with a low secondary electron emission rate for superconducting systems designed to upgrade the LHC, i.e., to increase the luminosity of the machine (HL-LHC). Since protons will generate synchrotron radiation with a critical energy of ~10 eV and a flux of 1016 photon m–1 s–1, it is important to study the effect of photons on a surface covered with amorphous carbon at room and cryogenic temperatures. The construction and parameters of the setup on the new synchrotron beamline of the VEPP-2000 booster are described. The first results of measuring the photodesorption coefficient are also presented.

Low total electron yield graphene coatings produced by electrophoretic deposition

Stainless steel and copper technical substrates have been coated by free standing graphene using electrophoretic deposition technique, with the final goal to obtain chemically inert low secondary electron emission surfaces. This class of materials is of utmost interest in future accelerators in order to increase further the flux of accelerated charged particles in the beam. The measured maximum total electron yield of pristine graphene, which has been previously characterized by electron microscopies and X-ray photoelectron spectroscopy, appears to be about 1.0. The deposition parameters have been optimized in order to obtain surfaces with lowest total electron yield, but also composition and morphology close to that of pristine graphene. When applying these optimized deposition parameters graphene coatings on two substrates, i.e. stainless steel and copper, the maximum total electron yield of about 1.04 was obtained. The composition of coatings and the relative amounts of sp2 bonds are slightly worse than those of the pristine graphene, while the surface morphology appears to be the same. Annealing at 150 °C for 64 h in high vacuum, showed that vacuum baking did not affect considerably the electron emission properties of the coatings.

Dynamic secondary electron emission in rough composite materials

The interaction of ionizing radiation with matter is of critical importance in numerous areas of science and technology like space and vacuum technology and even medicine and biotechnology. Secondary electron emission is a consequence of electron irradiation on materials. We achieve extremely low secondary electron emission yield values smaller than 0.2, even up to incident electron energies ~1 keV, due to an undocumented synergy between neighbouring metal and dielectric domains in composite samples. To investigate this experimental discovery, we propose a simple 3D model where the dielectric and metallic domains are arranged in parallel and interleaved. The proposed surface profile has a triangular shape to model the surface roughness. We obtain a continuous equation to describe the electric field that arises between grounded conductors and charged dielectrics domains. The calculated trajectories of secondary electrons in this 3D geometry are used to predict dynamic secondary emission yield, which strongly depends on the charge accumulated in the dielectric domains. This research paves the way to design new materials of low secondary emission yield, addressing the technological problem not yet resolved to inhibit the electron avalanche in RF equipment that limit their maximum working power.

Low-secondary electron emission yield under electron bombardment of microstructured surfaces, looking for multipactor effect suppression

The reduction of the total secondary electron emission during electron-matter interactions is required to improve the maximum power limit of the vacuum RF equipments prone to multipactor discharge. In this work, a chemical deposition technique was developed to create the appropriate micron or submicron surface roughness to significantly improve the typical characteristics of the flat silver coatings used for high-power microwave devices for space applications. Rough silver coated RF filters with a high surface aspect ratio, >3, was achieved. It is found that the secondary electron emission yield (SEY) remains lower than 1 throughout the primary electron energy range and therefore multipactor electron avalanche is avoided. The influence of aging on the multipactor discharge is also drastically reduced in rough silver coatings. An experimental linear correlation was found between the multipactor power level and the minimum interaction energy for which the number of incident and emitted electrons is equal.

Breakdown of the Spectator Concept in Low-Electron-Energy Resonant Decay Processes.

We suggest that low-energy electrons, released by resonant decay processes, experience substantial scattering on the electron density of excited electrons, which remain a spectator during the decay. As a result, the angular emission distribution is altered significantly. This effect is expected to be a common feature of low-energy secondary electron emission. In this Letter, we exemplify our idea by examining the spectator resonant interatomic Coulombic decay of Ne dimers. Our theoretical predictions are confirmed by a corresponding coincidence experiment.

Carbon Nanotube Film Gate in Vacuum Electronic Devices.

A superaligned carbon nanotube (SACNT) film can act as an ideal gate electrode in vacuum electronics due to its low secondary electron emission, high electron transparency, ultrasmall thickness, highly uniform electric field, high melting point, and high mechanical strength. We used a SACNT film as the gate electrode in a thermionic emission electron tube and field emission display prototype. The SACNT film gate in a thermionic emission electron tube shows a larger amplification factor. A triode tube with the SACNT film gate is used in an audio amplification circuit. The SACNT film gate electrode in field emission devices shows better field uniformity. The field emission display prototype is demonstrated to dynamically display Chinese characters.

A novel Y2O3-Gd2O3-HfO2 impregnated W base direct-heated cathode in magnetron tube

As the heart of a magnetron, cathode plays an important role in the device. At present, the pure W cathode is mainly used in high-power continuous wave magnetron tube. However, the pure W cathode has low thermionic emission capability and secondary electron emission yield (1.25-1.50), which result in the cathode operating at a high temperature (2450-2700 K). The higher the operating temperature of the cathode, the faster the evaporation of its surface is, which can shorten the cathode lifetime. In order to enhance the emission current, reduce the operation temperature and prolong the lifetime of the pure W cathode, a novel refractory Y2O3-Gd2O3-HfO2 impregnated W base direct-heated cathode (Y-Gd-Hf-O impregnated cathode) is developed in this paper. The present investigation mainly focuses on the thermionic emission, work function, lifetime, emission mechanism, and anti-bombing property. The direct current (dc) emission properties of the Y-Gd-Hf-O impregnated cathode are investigated, showing that it can provide more than 0.4, 1, 4.0, 7.74, 10.5 A/cm2 current density for the space charge limitation (SCL) at 1300, 1400, 1500, 1600, 1700 ℃ respectively. Absolute zero work function for the cathode is only 1.68 eV obtained by the Richardson line method. The effective work function for the cathode is in a range of 2.6-3.1 eV obtained by the Richardson-Dushman formula. The lifetime for the cathode is more than 3600 h with an initial load of 1.5 A/cm2 at 1600 ℃. The surface microstructure, element composition and content of the Y-Gd-Hf-O impregnated cathode are analyzed by the scanning electron microscope (SEM), Auger electron spectroscopy (AES), and energy disperse spectroscopy (EDS). The analysis results show that the surface of the cathode contains the Y2O3-x semiconductor layer, which causes an improvement of the electro-conductivity during the activation. The work function of the cathode can also be reduced due to the improvement of the electro-conductivity. Besides, the addition of the transition-metal oxide HfO2 changes the internal lattice energy level, which can further reduce the work function. Therefore, the Y-Gd-Hf-O impregnated cathode has good thermionic emission capability. In addition, the anti-bombing performance of the cathode is also studied, which shows that the dc emission current density decreases linearly from the initial current density of 1.5 A/cm2 to 0.4 A/cm2 after 150-h continuous electron bombing at 10 W/cm2. In the future research, we will focus on enhancing the anti-bombing property for the Y-Gd-Hf-O impregnated cathode by using Y-Gd-Hf-O doped W base direct-heated cathode.

Study on surface electrical performance of machinable ceramics for high voltage electro-vacuum insulation

There are some special requests for the insulating materials for high voltage electro-vacuum devices. Traditionally alumina ceramics and polymers are widely used, but both of them show some disadvantages such as difficulty of being manufactured to some complex shape due to the restriction of die arrangement, or lower mechanical and heat-resistant properties. The paper reports a newly invented low-melting temperature machinable glass ceramic (MC) for remedy these problems. The influence of crystallization preparation parameters (temperature and time) on its basic mechanical and electrical properties is investigated, such as bending strength, dielectric parameters, and desorbed gases, revealing its advantages suitable for vacuum utilization. Moreover, different metal oxides (Cu2O and Cr2O3) with low secondary electron emission (SEE) yield are doped into MC, and a better surface electrical withstanding capability is achieved.

Secondary electron emission under electron bombardment from graphene nanoplatelets

We present novel strategies for obtaining materials with low secondary electron emission under electron beam irradiation for advanced technological applications. Apart from the promising low secondary electron emission yield (SEY) of amorphous carbon coatings deposited on rough surfaces, graphene nanoplatelets are shown to be very effective in reducing the SEY even over flat substrates. Here, we report very low SEY from coatings of graphene platelets exposed to the air, indicating a 60% reduction of the SEY respect to the uncoated flat surfaces. The shape of the SEY curve as a function of primary energy looks like the inverted image of a typical SEY curve.

Numerical study on the electron—wall interaction in a Hall thruster with segmented electrodes placed at the channel exit

Electron—wall interaction is always recognized as an important physical problem because of its remarkable influences on thruster discharge and performance. Based on existing theories, an electrode is predicted to weaken electron—wall interaction due to its low secondary electron emission characteristic. In this paper, the electron—wall interaction in an Aton-type Hall thruster with low-emissive electrodes placed near the exit of discharge channel is studied by a fully kinetic particle-in-cell method. The results show that the electron—wall interaction in the region of segmented electrode is indeed weakened, but it is significantly enhanced in the remaining region of discharge channel. It is mainly caused by electrode conductive property which makes equipotential lines convex toward channel exit and even parallel to wall surface in near-wall region; this convex equipotential configuration results in significant physical effects such as repelling electrons, which causes the electrons to move toward the channel center, and the electrons emitted from electrodes to be remarkably accelerated, thereby increasing electron temperature in the discharge channel, etc. Furthermore, the results also indicate that the discharge current in the segmented electrode case is larger than in the non-segmented electrode case, which is qualitatively in accordance with previous experimental results.

Ultralow Secondary Electron Emission of Graphene

In order to ensure that vacuum electronic devices work with high overall efficiency, it is required to use materials with low secondary electron emission to fabricate or coat collectors, grids, and envelope walls of the devices. We report that the secondary electron yields of monolayer graphenes are ultralow, comparable with the lowest yields of the materials currently used in this practical application. This offers a pathway for the application of light graphene with only one-atom thickness and good electronic and thermal conductivities in vacuum electronic devices.

Experimental Observation of Discharge Characteristics Under Variable Ambient Temperature in AC Plasma Display Panel

The effect of an ambient temperature ranging from -5degC to +65degC on the discharge characteristics was examined. The change in the firing voltage according to the ambient temperature was much higher under MgO cathode conditions with a high secondary electron emission coefficient than under phosphor cathode conditions with a low secondary electron emission coefficient. Through the weak reset discharge, the wall voltages were increased with an increase in the ambient temperature, as the wall charges were less erased at a higher ambient temperature during the ramp-down period. An increase in the ambient temperature was also found to have a more significant influence on the statistical delay time than on the formative delay time. Since the statistical delay time is strongly related to the priming condition, a higher ambient temperature may contribute to a better priming condition for reducing the statistical delay time during the address period.

Feasibility of electrostatic microparticle propulsion

This paper discusses the feasibility of electrostatic space propulsion which uses microparticles as propellant. It is shown that particle charging in a plasma is not sufficient for electrostatic acceleration. Moreover, it appears technically difficult to extract charged particles out of a plasma for subsequent acceleration without them being discharged. Two novel thruster concepts are proposed. In the first one, particles with low secondary electron emission are charged using energetic electrons in the order of magnitude of 100 eV. The second concept charges the particles by contact with needle electrodes at high electrostatic potential (∼20 kV). Both methods allow the maximum possible charges on microparticles.

Design and development of high efficiency 140W space TWT with graphite collector

4-stage graphite collector assembly has been designed and developed for a 140W Ku-band space TWT to achieve the collector efficiency more than 80%. The UHV compatible, high density, copper impregnated POCO graphite (DFP-1C) was used to fabricate the four collector electrodes of the 4-stage depressed collector. Copper impregnated graphite material is used for the collector electrodes because of its low secondary electron emission coefficient, high thermal and electrical conductivities, easy machining and brazing, low thermal expansion coefficient and low weight. The graphite material was characterized for the UHV compatibility. The collector electrodes were precisely fabricated by careful machining, and technology was developed for brazing of graphite electrodes with high voltage alumina insulators. Complete TWT with four-stage graphite collector was developed and 140W output power at gain more than 55 dB was achieved. The TWT was pumped from both the gun and the collector ends.

Transmission and secondary electron emission of 20–50 keV electrons through aluminium foils

Experimental data and Monte Carlo results of 20–50 keV electron penetration through thin aluminium foils are presented. Transmission ratios as well as energy characteristics of secondary electrons are measured for 3–20 µm thick foils. Implications of these results for the requirements of a radioisotope micro-power sources energy conversion device are analyzed. It is shown that for a given foil thickness, there is a discrete peak in the low energy secondary electron emission. In addition, it is noted that the conversion factor for high energy primaries to low energy secondaries is less than one.