Maximum Secondary Electron Yield Research Articles

Effect of amorphous carbon film on secondary electron emission of metal

Amorphous carbon films have attracted much attention in the field of abnormal discharge of vacuum microwave devices and equipment due to their extremely low secondary electron yields (SEYs). However, the dynamic process and microscopic mechanism of the effect of amorphous carbon film on secondary electron emission are still poorly understood. In this work, a numerical simulation model of the secondary electron emission of amorphous carbon film on copper surface is developed by the Monte Carlo method, which can accurately simulate the dynamic processes of electron scattering and emission of the film and the substrate. The results show that the maximum SEY decreases by about 20% when the film thickness increases from 0 to 1.5 nm. Further increasing the thickness, the SEY no longer decreases. However, when the film is thicker than 0.9 nm, the SEY curve exhibits a double-hump form, but with the thickness increasing to 3 nm, the second peak gradually weakens or even disappears. The electron scattering trajectories and energy distribution of secondary electrons indicate that this double-hump phenomenon is caused by electron scattering in two different materials. Compared with previous models, the proposed model takes into account the change of work function and the effect of interfacial barrier on electron scattering path. Our model can explain the formation of the double-hump of SEY curve and provides theoretical predictions for suppressing the SEY by amorphous carbon film.

Performance evaluation of a single microchannel plate with large length-to-diameter ratio

The conventional microchannel plate (MCP) detector has found widespread application in various fields. This article focuses on investigating the performance of a single MCP with a large length-to-diameter ratio (L/D) of 80:1, particularly for single electron counting. To enhance MCP performance, alumina with a high secondary electron yield (SEY) is coated onto the MCP using atomic layer deposition (ALD). The SEYs of alumina thin films with different thicknesses are measured using a pulsing electron beam method. The 80:1 L/D MCPs operate in electron counting mode, and the optimal alumina thickness is determined through a comparative study of MCP performance before and after coating. The relationships between maximum SEY, primary electron energy, gain, peak-to-valley ratio (P/V), and pulse height resolution (PHR) are analyzed. After alumina coating, the single 80:1 MCP exhibits improved gain, P/V and PHR. The optimal P/V and PHR of a single MCP as functions of the primary electron energy align with the relationship between the SEY of the alumina coating and the primary electron energy. Additionally, the variation of DC gain with extracted charge is investigated. This article provides valuable insights for parameter selection in achieving the optimal working state of MCP and explores the potential application of single electron counting using a single MCP.

Secondary electron emission enhancement of MgO-Au composite film by SiO2 doping in MgO surface layer

The secondary electron emission (SEE) properties of MgO-Au composite film deposited by magnetron sputtering was improved by SiO2 doping with a Si/Mg ratio of 14 % in its MgO surface layer. Compared to the conventional electronic material MgO/(MgO-Au) film, the SiO2-doped MgO/(MgO-Au) film achieved a secondary electron yield (SEY) of 7.69 with an increase of 9.2 % at the primary electron energy (Ep) of 300 eV and the maximum SEY of 11.4 eV with an increase of 12.9 % at the Ep of 700 eV, and it kept relatively high SEY consistently under the continuous bombardment of the incident electrons with the Ep of 200 eV. This SEE enhancement is attributed to the reductions of both the band gap and work function of MgO crystal as well as the grain enlargement and surface roughness decrease of MgO surface layer induced by SiO2 doping. This scheme of SiO2 doping in the MgO surface layer of MgO-Au film provides a new idea for the preparation of high performance MgO-based SEE material.

Picosecond pulsed 532nm laser system for roughening and secondary electron yield reduction of inner surfaces of up to 15m long tubes.

Laser-induced surface structuring is a promising method to suppress electron mulitpacting in the vacuum pipes of particle accelerators. Electrons are scattered inside the rough surface structure, resulting in a low Secondary Electron Yield (SEY) of the material. However, laser processing of internal pipe surfaces with a large aspect ratio is technologically challenging in terms of laser beam guidance and focusing. We present a 532nm ultrashort-pulse laser setup to process the inner parts of 15m long beam vacuum tubes of the Large Hadron Collider (LHC). Picosecond pulses at a repetition rate of 200kHz are guided through an optical fiber toward an inchworm robot traveling inside the beam pipe. The system was installed, characterized, and tested for reliability. First surface treatments achieved the required scan precision. Cu2O-dominated nano-features were observed when processing at high average laser power (5 W) and slow scanning speed (5mms-1) in nitrogen flow, and the maximum SEY of copper was decreased from 2.1 to 0.7.

Amorphous carbon thin films: Mechanisms of hydrogen incorporation during magnetron sputtering and consequences for the secondary electron emission

Amorphous carbon (a-C) films, having low secondary electron yield (SEY), are used at CERN to suppress electron multipacting in the beam pipes of particle accelerators. It was already demonstrated that hydrogen impurities increase the SEY of a-C films. In this work, a systematic characterization of a set of a-C coatings, deliberately contaminated by deuterium during the magnetron sputtering deposition, by scanning electron microscopy, ion beam analysis, secondary ion mass spectrometry, and optical absorption spectroscopy was performed to establish a correlation between the hydrogen content and the secondary electron emission properties. In parallel, the mechanisms of contamination were also investigated. Adding deuterium allows resolving the contributions of intentional and natural contamination. The results enabled us to quantify the relative deuterium/hydrogen (D/H) amounts and relate them with the maximum SEY (SEYmax). The first step of incorporation appears to be formation of D/H atoms in the discharge. An increase in both the flux of deposited carbon atoms and the discharge current with a D2 fraction in the gas discharge can be explained by target poisoning with deuterium species followed by etching of CxDy clusters, mainly by physical sputtering. For overall relative D/H amounts between 11% and 47% in the discharge gas, the SEYmax increases almost linearly from 0.99 to 1.38. An abrupt growth of SEYmax from 1.38 to 2.12 takes place in the narrow range of D/H relative content of 47%–54%, for which the nature of the deposited films changes to a polymer-like layer.

Secondary electron emission characteristics of alumina coating on metallic substrate prepared by atomic layer deposition

The main objective of this study is to research the secondary electron emission characteristic of different thickness alumina coatings which are prepared by atomic layer deposition (ALD) technique on several metallic secondary-emitting materials. The experimental results reveal that secondary electron yield (SEY) property concerned with the metal work function and the surface topography of alumina coatings when coating thickness is in the nanometer level. In addition, after utilizing SEY measurement setup and pulsed electron gun, the SEY curves of metallic substrates with different thickness alumina coatings and primary energy (PE) of incident electron could be measured. From an analysis of the SEY curves, some key parameters related to secondary electron emission characteristics such as the maximum SEY value and its corresponding PE of incident electron, and the penetration depth of incident electron can be obtained. Furthermore, basing on the ALD technique and on the SEY results of stainless steel substrate coated with 80-cycle alumina thin film, a novel 18-stage discrete dynode electron multiplier (DDEM) was fabricated and tested. The gain of DDEM 1.6× 106 was obtained.

Enhanced secondary electron emission properties of Zn doped MgO thin films prepared by aerosol assisted chemical vapor deposition

In this study Zn doping was applied to MgO thin films in order to improve the secondary electron emission (SEE) property via relieving the surface charging and modifying the electron structure of MgO. MgO and Mg1-xZnxO thin films (the Zn doping proportion x is within 11 at.%) were prepared on Si substrates by a simple, cost effective aerosol assisted chemical vapor deposition method followed by annealing at elevated temperature. The surface morphology and microstructure of the Mg1-xZnxO films were characterized. Secondary electron yield (δ) as a function of primary electron energy the films was examined. First principles calculation based on the density functional theory was introduced to evaluate the electronic structure and work function of Mg1-xZnxO crystals. SEE testing results revealed that δ of the MgO film is effectively improved by Zn doping of proper concentration. The undoped MgO film with relatively smooth surface exhibits a maximum secondary electron yield (δm) of 5.1 at Ep of 750 eV. The δm of the Mg1-xZnxO film (x = 0.09) is 7.0 at Ep = 1500 eV, which is 34.6% higher than that of the MgO film prepared under identical method. Improvement of δ of the Mg1-xZnxO films with x < 0.1 is probably due to the unique surface morphology with protrusions and sharp edges and the increase of electrical conductivities of the films. Another factor contributing to the δ increase is the decrease of work function due to Zn doping, which is confirmed by first-principles calculation of Mg1-xZnxO crystals.

Suppression of secondary electron emission of oxygen free copper by two-step electrodeposition of rGO/Cu composite coating

We prepared a reduced-graphene-oxide/Cu (rGO/Cu) composite coating to suppress the total secondary electron yield by two-step electrodeposition. We found that driven by the applied electric field, copper ions can enter the interior of graphene oxide (GO) film and nucleate, while the surface is still GO. The rGO/Cu composite coating was obtained by heat treatment with a combination of hydrogen and argon. After heat treatment, XPS analysis shows that the oxygen content of rGO on the surface of the composite coating is reduced from 31.29 at.% to 11.00 at.%, and the carbon content is 85.25 at.%. Raman analysis shows that rGO is still mainly sp2 hybrid structure. The deposited copper particles sintered in the composite coating and formed Cu-O-C chemical bond. The maximum secondary electron yield (δmax) is 1.10, which is close to the δmax of rGO coating (1.09). Compared with the rGO coating, the presence of copper in the rGO/Cu composite coating slightly increases the TEY, but significantly improves the bonding strength between the coating and the substrate at high temperatures.

Suppression of electron cloud using copper thermal spray coating in SuperKEKB

The electron cloud effect (ECE) has posed serious challenges in recent high-intensity proton and positron rings. An aluminum beam pipe coated with copper thermal spray film (T.S. coating), which had a low secondary electron yield (SEY), was developed to suppress electron clouds in accelerators. After 72 h conditioning, the lowest δmax (the maximum SEY within scanning) reached ∼0.7. In addition, to check the applicability of T.S. coating in accelerators, its outgassing rate and surface resistance were measured as a reference. Finally, an aluminum beam pipe with a T.S. coating was produced and installed in the positron ring of SuperKEKB to measure the electron density. The results show that the electron density in the finished beam pipe was comparable with that of the TiN-coated beam pipe; however, the relatively high surface resistance of T.S. coating must be considered when applied to accelerators.

Suppression of secondary electron emission on oxygen-free copper surface of reduced graphene oxide coatings prepared by electrophoretic deposition

We proposed a new carbon coating to inhibit secondary electron emission. Graphene oxide(GO) coatings were prepared on the surface of oxygen-free copper by DC electrophoretic deposition, and reduced graphene oxide(rGO) coatings were obtained by hydrogen thermal reduction. The X-ray photoelectron spectroscopy (XPS) analysis shows that the oxygen content of the coating reduced from 26.37 at.% to 9.89 at.% after thermal reduction, The XPS and Raman analysis shows that rGO still maintained the sp2 hybrid structure. The inhibitory effect of rGO coatings on secondary electron emission was tested. It was found that the maximum total secondary electron yield (δmax) of the oxygen-free copper surface reduced from 1.38 to 1.09 after the preparation of rGO coating. The total secondary electron yield did not decrease further with the film thickness increasing when the rGO coating reached a certain thickness. Our research provides a new perspective on the preparation of carbon coatings to reduce the secondary electron yield.

Monte Carlo simulation of the secondary electron yield of silicon rich silicon nitride

The effect of doping in Si3N4 membranes on the secondary electron yield is investigated using Monte Carlo simulations of the electron-matter interactions. The effect of the concentration and the distribution of the doping in silicon rich silicon nitride membranes is studied by using the energy loss function as obtained from ab initio density functional theory calculations in the electron scattering models of the Monte Carlo simulations. An increasing doping concentration leads to a decreasing maximum secondary electron yield. The distribution of the doped silicon atoms can be optimised in order to minimize the decrease in yield.

The secondary electron yields and related surface properties evolution of laser treated stainless steel induced by different ultrasonic cleaning duration

As a novel method, laser treatment was proposed to be applied on the surface modification of stainless steel 316L commonly used in accelerator vacuum system to mitigate the secondary electron emission. However, the final property of the material surfaces, especially the secondary electron yield (SEY) property depends on a rather large number of factors, among them are the surface cleaning scenarios needed before installation. In particular, the surface morphologies and surface chemical states may be changed during the ultrasonic cleaning used for laser treated stainless steel surfaces. Therefore, the influence of ultrasonic cleaning duration on the surface composition, surface morphology and secondary electron characteristics of stainless steel were studied for the first time. The secondary electron yield test results demonstrated that the maximum SEY (δ max) of laser treated stainless steel samples increased with the increase of ultrasonic cleaning time. After ultrasonic cleaning in acetone and absolute ethyl alcohol each for 3 minutes, 10 minutes and 15 minutes, respectively, the δ max of laser treated stainless steel samples increased about 0.7%–1.7%, 1.3%–2.4% and 4.9%–15.3%, respectively, compared with that of uncleaned laser treated ones. The surface morphologies, surface roughness, surface chemical states related to the SEY evolution were also evaluated and analyzed.

Influence of primary electron incident angle and electron bombardment on the secondary electron yield of laser-treated copper

Electron cloud is a persistent problem in operating modern accelerators. It might be eliminated by reducing the secondary electron yield (SEY), which is a property of the material of vacuum chambers. In the present study, the SEYs of oxygen-free copper samples are dramatically mitigated by grooving their surfaces with a laser-etching technique. Such mitigation is realized by trapping incident primary electrons and their induced secondary electrons in the grooves. The SEYs of the laser-etched samples are dependent on the geometrical characteristics of the grooves and the incident angles of the primary electrons, i.e., reducing the incident angle can lead to a reduction in the SEY. Electron bombardment of the grooved surface with an electron dose of 2 × 10−2 C mm−2 will further reduce its maximum SEY from 1.15 to 0.98, which might be attributed to the formation of Cu2O and graphite-like C—C bonds and the removal of surface contaminants.

Structural and Secondary Electron Yield Properties of Titanium-Palladium Films with Laser-Treated Copper Substrate for Application in Neutron Generators.

Secondary electron emission (SEE) of the oxygen-free high-conductivity copper (OFHC) target surface in neutron generators limits the stability and improvement of the neutron yield. A novel-type target of titanium–palladium films coated on laser-treated OFHC target substrate was proposed and explored in this work to obtain low secondary electron yield (SEY) without introducing any components. The combination of Ti–Pd films and laser-treated OFHC substrate can effectively suppress secondary electron emission and enhance the adsorption ability to hydrogen isotopes with the existence of Pd film. The surface morphologies, surface chemical states, and SEYs of Ti–Pd films with laser-treated OFHC substrate were studied systematically for the first time. The XPS results showed that the laser-treated OFHC substrate surface was basically covered by Pd film. However, the Pd film surface was partially oxidized, with percentages of 21.31 and 10.02% for PdO and PdO2, respectively. The SEYs of Ti–Pd films with laser-treated OFHC substrate were all below 1 within the investigated primary energy range of 100–3000 eV, which would be sufficient for application in neutron generators. Specifically, the maximum SEY (δmax) of laser-treated OFHC substrate coated by Ti–Pd films was 0.87 with corresponding incident electron energy of 400 eV.

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.

The Activation of Ti-Zr-V-Hf Non-Evaporable Getter Films with Open-Cell Copper Metal Foam Substrates.

Secondary electron emission (SEE) inhibition and vacuum instability are two important issues in accelerators that may induce multiple effects in accelerators, such as power loss and beam lifetime reduction. In order to mitigate SEE and maintain high vacuum simultaneously, open-cell copper metal foam (OCMF) substrates with Ti-Zr-V-Hf non-evaporable getter (NEG) coatings are first proposed, and the properties of surface morphology, surface chemistry and secondary electron yield (SEY) were analyzed for the first time. According to the experimental results tested at 25 °C, the maximum SEY () of OCMF before and after Ti-Zr-V-Hf NEG film deposition were 1.25 and 1.22, respectively. The XPS spectra indicated chemical state changes of the metal elements (Ti, Zr, V and Hf) of the Ti-Zr-V-Hf NEG films after heating, suggesting that the NEG films can be activated after heating and used as getter pumps.

Impact of deconditioning on the secondary electron yield of Cu surfaces in particle accelerators

Electron cloud is a critical phenomenon in particle accelerators operating with high intensity and positively charged beams, as it is responsible for beam instabilities, vacuum degradation, and heat load on cryogenic sections. Electron clouds provoke a conditioning of the beam pipe that is reflected on the reduction of its secondary electron yield (SEY). However, such a benefit is partially lost when vacuum sectors are vented for maintenance of accelerators; this phenomenon is called deconditioning. Samples removed from accelerators are also vented before surface analysis. Deconditioning amplifies the electron cloud at the resuming of beam operation and, on the other hand, hinders the understanding of the electron multipacting mechanism from surface analysis data. In this paper, copper deconditioning was studied for samples stored in a desiccator over months. Immediately after air exposure, an increase of the SEY is observed. This increase is driven by carbon recontamination and copper hydroxide growth on the conditioned surface as observed by x-ray photoelectron spectroscopy. After deconditioning, the differences of SEY present on the tested samples partially vanish, in particular, for surfaces conditioned to a maximum SEY below 1.45, limiting the level of accessible information when analyzing components extracted from accelerators. However, for a maximum SEY above 1.45, the differences remain visible for at least 8 weeks of storage. Among different storage conditions, vacuum efficiently stops the SEY increase over time. Besides, the memory effect of the conditioning is preserved over at least 4 months when closing the vacuum system on itself after venting with a clean and dry gas.

Theoretical and experimental investigation of secondary electron emission characteristics of ALD-ZnO conductive films

Microchannel plates (MCPs) are widely utilized as key device components in various photomultipliers; however, the performance of MCPs cannot be further improved by traditional processing. Atomic layer deposition (ALD) is a promising route to prepare a composite conductive layer and secondary electron emission (SEE) layer structure on the inner wall of the MCP. Moreover, ZnO is an essential component of a composite conductive layer, which is located at the bottom of the SEE layer and significantly influences the SEE coefficient, which, in turn, affects the gain performance of MCPs. Herein, ALD is used to deposit different thicknesses of ZnO films (1–50 nm) on an Si substrate, resulting in an ZnO/Si double-layer film structure. The relationship between the SEE coefficient and the primary electron energy of ZnO films with different thicknesses was established. The maximum secondary electron yield value of 2.04 is achieved at a film thickness of 30 nm. Moreover, Dionne's SEE model and theory of semiconductors are used to simulate and verify the experimental results. These results provide useful guidelines for the development of ALD-MCPs.

Enhanced surface insulating performance for polystyrene by atmospheric pressure plasma jet deposition

Surface flashover at the solid dielectric/vacuum interface is a fundamental physical phenomenon, which restricts the development of pulsed power system and high voltage electrical equipment. The initiation and developing processes of surface flashover at the solid dielectric are associated with electrode-dielectric interface, dielectric surface and other external factors. Especially, the surface charges transport properties that related with surface flashover developing process are determined by the physicochemical of the dielectric surface. Therefore, the surface insulating performance can be enhanced by deposited with appropriate functional films. TiN film has a low secondary electron yield (SEY), which can help to suppress the secondary electron multipactor effect and suppress the surface flashover. In this paper, a preparation method of film contained TiCxOyNz using atmospheric pressure plasma jet (APPJ) is proposed. The experimental devices include AC power supply, plasma jet chamber and gas/precursor system. The AC power supply has a power of 1000 VA and frequency of 20 kHz. The precursor tetraethyl titanate (TTEO) is pumped into plasma jet chamber by argon gas and the working gas is nitrogen. The experimental results show that the maximum SEY of sample with TiCxOyNz film is approximately decreased to 1.5. The flashover voltage of sample with TiCxOyNz film is increased obviously. The morphology of the dielectric surface indicates that the rough surface is introduced after modification. Also, the physical processes of film deposition are investigated by theemissionspectroscopicanalysis.

Non-Evaporable Getter Ti-V-Hf-Zr Film Coating on Laser-Treated Aluminum Alloy Substrate for Electron Cloud Mitigation

For improving the vacuum and mitigating the electron clouds in ultra-high vacuum chamber systems of high-energy accelerators, the deposition of Ti-V-Hf-Zr getter film on a laser-treated aluminum alloy substrate was proposed and exploited for the first time in this study. The laser-treated aluminum surface exhibits a low secondary electron yield (SEY), which is even lower than 1 for some selected laser parameters. Non-evaporable getter (NEG) Ti-V-Hf-Zr film coatings were prepared using the direct current (DC) sputtering method. The surface morphology, surface roughness and composition of Ti-V-Hf-Zr getter films were characterized and analyzed. The maximum SEY of unactivated Ti-V-Hf-Zr getter film on laser-treated aluminum alloy substrates ranged from 1.10 to 1.48. The X-ray photoelectron spectroscopy (XPS) spectra demonstrate that the Ti-V-Hf-Zr coated laser-treated aluminum alloy could be partially activated after being heated at 100 and 150 °C, respectively, for 1 h in a vacuum and also used as a pump. The results were demonstrated initially and the potential application should be considered in future particle accelerators.