Low Secondary Electron Yield Research Articles

Suppression of Secondary Electron Emission by Vertical Graphene Coating on Ni Microcavity Substrate.

Suppression of secondary electron emission (SEE) from metal surfaces is crucial for enhancing the performance of particle accelerators, spacecraft, and vacuum electronic devices. Earlier research has demonstrated that either etching the metal surface to create undulating structures or coating it with materials having low secondary electron yield (SEY) can markedly decrease SEE. However, the effectiveness of growing vertical graphene (VG) on laser-etched metal surfaces in suppressing SEE remains uncertain. This study examined the collective impact of these methods by applying nanoscale arrays of VG coating using plasma-enhanced chemical vapor deposition on Ni substrates, along with the formation of micrometer-sized microcavity array through laser etching. Comparative tests conducted revealed that the SEY of the samples subjected to VG coating on a microcavity array was lower compared to samples with either only a microcavity array or VG coating alone. Additionally, the crystallinity of VG grown on substrates of varying shapes exhibited variations. This study presents a new method for investigating the suppression of SEE on metal surfaces, contributing to the existing body of knowledge in this field.

Sensitivity analysis of multipactor susceptibility zone to variations in secondary electron yield values

We present a sensitivity analysis of the multipactor susceptibility zones to variations in the secondary electron yield (SEY) of materials, specifically focusing on the first and second unity crossover points of SEYs. In conducting this research, we leveraged our semi-analytic approach, which allows for the rapid prediction of the full multipactor zones with enhanced accuracy. Using this approach, we unveil several unique features of multipactor susceptibility zones, including the infinite extension of different-order multipactor zones and the overlap between them. Building upon this prediction capability, our results complement previous findings on the same topic and reveal that the multipactor zones depend not only on the first crossover point but also on the second crossover point of SEY, which this latter predominantly impacts multipactor susceptibility zones for low SEY materials. To validate our predictions, we present two distinct sets of multipactor experiments, providing empirical support for our results.

Operational experience of a low beam coupling impedance injection kicker magnet for the CERN SPS ring

The CERN SPS injection kicker magnets (MKP) were developed in the 1970’s, before beam power deposition was considered an issue. There are two types of these magnets in the SPS: MKP-S (small aperture) and MKP-L (large aperture) versions. The MKP-L magnets are very lossy from a beam impedance perspective: this would be an issue during SPS operation with the higher intensity beams needed in the future for HL-LHC. Hence, a beam screen has been developed, which is inserted in the aperture of each MKP-L module. The screen consists of silver fingers applied to alumina U-shaped chambers: the fingers have been optimized to achieve both adequately low beam induced power deposition and good high voltage (HV) behaviour. A surface coating, with a low secondary electron yield, is applied to the inner surface of the alumina chambers to reduce dynamic vacuum. The low-impedance MKP-L has been extensively HV tested in the lab before installation in the SPS. This paper briefly presents the design and focuses on the operational experience in the SPS, including heating and vacuum.

FELICIA - A probe to survey the RHIC magnet beampipe diameter for EIC beam screen insertion

The Electron Ion Collider (EIC) Hadron Storage Ring (HSR) will reuse many of the existing superconducting (SC) magnets of the RHIC storage rings. To comply with the beamline vacuum requirements in more demanding operational scenarios, the beampipe of the RHIC SC magnets will be equipped with low surface impedance, low secondary electron yield (SEY) beam screens. The installation of these beam screens will be done with the SC magnets as installed today, thus making it a critical operation for a timely EIC installation. The beam screen inner dimensions must be maximized to retain enough aperture to the beam. On the other hand, keeping enough clearance between the screen and the beampipe is critical to ensure a smooth beam screen installation. A survey probe was designed and built to measure the inner diameter of several RHIC SC magnets in-situ and provide critical data for the beam screen design optimization. This paper reports on the design of the probe and the results from the survey campaign.

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.

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.

The Role of Hydrogen Incorporation into Amorphous Carbon Films in the Change of the Secondary Electron Yield.

Over the last few years, there has been increasing interest in the use of amorphous carbon thin films with low secondary electron yield (SEY) to mitigate electron multipacting in particle accelerators and RF devices. Previous works found that the SEY increases with the amount of incorporated hydrogen and correlates with the Tauc gap. In this work, we analyse films produced by magnetron sputtering with different contents of hydrogen and deuterium incorporated via the target poisoning and sputtering of CxDy molecules. XPS was implemented to estimate the phase composition of the films. The maximal SEY was found to decrease linearly with the fraction of the graphitic phase in the films. These results are supported by Raman scattering and UPS measurements. The graphitic phase decreases almost linearly for hydrogen and deuterium concentrations between 12% and 46% (at.), but abruptly decreases when the concentration reaches 53%. This vanishing of the graphitic phase is accompanied by a strong increase of SEY and the Tauc gap. These results suggest that the SEY is not dictated directly by the concentration of H/D, but by the fraction of the graphitic phase in the film. The results are supported by an original model used to calculate the SEY of films consisting of a mixture of graphitic and polymeric phases.

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.

Electron Beam Characterization of REBCO-Coated Conductors at Cryogenic Conditions

Particle accelerators with superconducting magnets operating at cryogenic temperatures use a beam screen (BS) liner that extracts heat generated by the circulating bunched charge particle beam before it can reach the magnets. The BS surface, commonly made of high–conductivity copper, provides a low impedance for beam stability reasons, low secondary electron yield (SEY) to mitigate the electron–cloud (EC) effect, and low electron–stimulated desorption yield (ESD) to limit the dynamic pressure rise due to EC. Rare–earth barium copper oxide (REBCO) high–temperature superconductors (HTSs) recently reached technical maturity, are produced as coated conductor tapes (REBCO–CCs), and will be considered for application in future colliders to decrease the BS impedance and enable operation at around 50 K, consequently relaxing the cryogenic requirements. Aside from HTS properties, industry–grade REBCO–CCs also need qualification for EC and dynamic vacuum compatibility under accelerator–like conditions. Hence, we report the SEY and ESD measured at cryogenic temperatures of 12 K under low–energy electron irradiation of 0–1.4 keV. We also verify the sample compositions and morphologies using the XPS, SEM, and EDS methods. The energy and dose dependencies of ESD are comparable to those of technical–grade metals and one sample reached SEYMAX = 1.2 after electron conditioning.

Influence of wavelength and accumulated fluence at picosecond laser-induced surface roughening of copper on secondary electron yield

Ultrashort-pulse laser processing of copper is performed in air to reduce the secondary electron yield (SEY). By UV (355 nm), green (532 nm), and IR (1064 nm) laser-light induced surface modification, this study investigates the influence of the most relevant experimental parameters, such as laser power, scanning speed, and scanning line distance (represented as accumulated fluence) on the ablation depth, surface oxidation, topography, and ultimately on the SEY. Increasing the accumulated laser fluence results in a gradual change from a Cu2O to a CuO-dominated surface with deeper micrometer trenches, higher density of redeposited surface particles from the plasma phase, and a reduced SEY. While the surface modifications are less pronounced for IR radiation at low accumulated fluence (<1000 J/cm2), analogous results are obtained for all wavelengths when reaching the nonlinear absorption regime, for which the SEY maximum converges to 0.7. Furthermore, independent of the extent of the structural transformations, an electron-induced surface conditioning at 250 eV allows a reduction of the SEY maximum below unity at doses of 5×10-4 C/mm2. Consequently, optimization of processing parameters for application in particle accelerators can be obtained for a sufficiently low SEY at controlled ablation depth and surface particle density, which are factors that limit the surface impedance and the applicability of the material processing for ultrahigh vacuum systems. The relations between processing parameters and surface features will provide guidance in treating the surface of vacuum components, especially beam screens of selected magnets of the Large Hadron Collider or of future colliders.

Efficient Combination of Surface Texturing and Functional Coating for Very Low Secondary Electron Yield Surfaces and Rough Nonevaporable Getter Films

AbstractThe formation of a fissured copper surface by picosecond pulsed laser irradiation is combined with functional coatings consisting of Ti and amorphous carbon layers or a Ti–Zr–V compound film to fabricate surfaces with the maximum of the secondary electron yield being as low as 0.4. By structural and spectroscopic analysis of the formed surfaces it is demonstrated that both coatings enclose the nanostructures generated by redeposition of metal structures from the laser‐induced plasma plume, keeping the initial topography intact. This allows an efficient elimination of secondary electron emission by combining the benefits from structural surface modification and adaption of electronic surface properties to efficiently dissipate the energy of impinging electrons. Thermal activation tests of the Ti–Zr–V nonevaporable getter films revealed that for films on nanostructured substrates, which have a much higher effective surface, a slight diminution of surface activation occurs at 160 and 200 °C, while this effect is completely compensated when heating up to 250 °C indicating promising pumping capabilities. Both examples highlight the benefits from combining 3D substrate patterning with classical 2D deposition technologies.

Electron conditioning of technical surfaces at cryogenic and room temperature in the 0–1 keV energy range

In the superconducting magnets of the Large Hadron Collider (LHC) at CERN, most of the beam-induced heat load is intercepted by a beam-screen (BS) cryogenically cooled to 5–20 K. When circulating the bunched proton beam, an electron cloud (EC) can form and bombard the BS copper surface with high doses of predominantly low-energy electrons, which desorb gas and consequently increase the pressure. The beam-induced pressure rise decreases during operation as the electron irradiation diminishes the secondary electron yield (SEY) and the electron-stimulated desorption (ESD) yield, a phenomenon referred to as ‘beam conditioning’. Low ESD and SEY values achieved rapidly are requisite to mitigate EC and maintain UHV in storage rings. We report data on ESD and SEY electron conditioning completed at cryogenic temperature with 0–1 keV electrons up to an electron dose of 5.10−3 C mm−2. Our results show that SEY conditioning depends on the primary electron energy and also that ESD yield significantly decreases with temperature. At 15 K, the amorphous-carbon coating and laser-treated copper present SEY below 1.1 and have initial ESD yields 3–6 times lower than OFE copper. Our results conform to the SEY and ESD's general understanding and extend it towards cryogenic 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.

Ultralow electron emission yield achieved on alumina ceramic surfaces and its application in multipactor suppression

Alumina ceramics used in microwave systems are susceptible to the multiplication of secondary electron emission on the surface due to the influence of resonance between electrons and the radiofrequency electric field, and a detrimental multipactor effect may therefore be triggered. For the alumina-loaded microwave components, it is essential to achieve low secondary electron yield (SEY) on the inserted alumina surfaces to mitigate multipactor. In this work, to achieve an ultralow SEY surface of alumina, two recognized low-SEY treatments were combined. For the primary SEY suppression, a series of microstructures were fabricated on the alumina surfaces with varied porosity and aspect ratio at the hundred-micrometer scale by infrared laser etching. The microstructure with 52.14% porosity and 1.78 aspect ratio showed an excellent low-SEY property, which could suppress the SEY peak value ( δ m ) of alumina from 2.46 to 1.00. For the secondary SEY suppression, the SEY dependence of TiN coating on sputtering parameters was studied, and the lowest δ m of 1.19 was achieved when the gas flow ratio of Ar:N2 was 15:7.5. Thereafter, by depositing TiN ceramic coating onto the laser-etched porous samples, an ultralow SEY, with δ m of 0.69, was achieved on the alumina surfaces. The simulation work revealed the impact of dielectric surface charge on electron multiplication and revealed a mechanism of using low-SEY surfaces to inhibit multipactor. Some coaxial filters filled with alumina were fabricated for verification; the results revealed that the multipactor threshold increased from 125 W to 425 W after applying the TiN-coated porous alumina, and to 650 W after treating another multipactor-sensitive area with the same low-SEY process. This work developed an advisable method to sharply reduce SEY, which is of great significance for the multipactor mitigation of alumina-loaded microwave components.

Secondary electron yield characterization of high porosity surfaces for multipactor-free microwave components

We present secondary electron yield (SEY) characterization of high porosity surfaces for multipactor-free microwave components. We first calculate the SEYs of through porosity surfaces using Monte Carlo simulations. We demonstrate that these high porosity surfaces can be treated as homogeneous materials with low effective SEYs. We prove that a significant SEY reduction is attainable by high porosities, and above a certain porosity level, the entire effective SEY of the surface falls below unity, offering a multipactor free capability. We import the resultant SEYs into our semi-analytic approach to obtain multipactor susceptibility charts corresponding to different surface porosities. We predict the reduction of the multipactor-susceptible zone as the porosity increases and a total multipactor suppression with 0.66 through porosity level. The theoretical results were validated with multipactor experiments, and relatively good agreement was observed. Finally, we propose an alternative blind porosity approach that can offer the same multipactor suppression capabilities. The approach discussed herein can be adopted to design high-power multipactor-free microwave components.

Effect of the Surface Morphology of Porous Coatings on Secondary Electron Yield of Metal Surface.

Surface roughening is an important material surface treatment technique, and it is particularly useful for use in secondary electron yield (SEY) suppression on metal surfaces. Porous structures produced via roughening on coatings have been confirmed to reduce SEY, but the regulation strategy and the influence of process parameters both remain unclear in the practical fabrication of effective porous structures. In this paper, the effect of the surface morphology of porous coatings on the SEY of aluminum alloy substrates was studied. Surface characterization and SEY measurements were carried out for samples with a specific process technique on their surfaces. An exponential fitting model of the correlation between surface roughness and the peak values of SEY curves, , was summarized. Furthermore, an implementation strategy to enable low surface SEY was achieved from the analysis of the effect of process parameters on surface morphology formation. This work will aid our understanding of the effect of the irregular surface morphology of porous coatings on SEY, thereby revealing low-cost access to the realization of an easy-to-scale process that enables low SEY.

Influence of Film Coating Thickness on Secondary Electron Emission Characteristics of Non-Evaporable Getter Ti-Hf-V-Zr Coated Open-Cell Copper Foam Substrates.

The application of vacuum materials with low secondary electron yield (SEY) is one of the effective methods to mitigate the electron cloud (EC). In this study, the Ti-Hf-V-Zr non-evaporable getter (NEG) film was deposited on open-cell copper foams with different pore sizes for the suppression of electron multipacting effects. Besides, the influence of the film thickness on the secondary electron emission (SEE) characteristics of Ti-Hf-V-Zr NEG film-coated open-cell copper foam substrates was investigated for the first time. The results highlighted that all uncoated and NEG-coated foamed porous copper substrates achieved a low SEY (<1.2), which reduced at least 40% compared to the traditional copper plates, and the foamed porous coppers with 1.34-μm-thick NEG coating had the lowest SEY. Moreover, the surface chemistry and the morphological and structural properties of foamed porous coppers of different pore sizes with and without Ti-Hf-V-Zr NEG films were systematically analyzed.

Secondary electron yield engineering of copper surfaces by 532 nm ultrashort laser pulses

Nanostructured surfaces exhibit outstanding properties and enable manifold industrial applications. In this study the laser surface processing of polycrystalline, flat copper surfaces by 532 nm picosecond laser irradiation for secondary electron yield (SEY) reduction is reported. The laser beam was scanned in parallel lines across the sample surface in order to modify large surface areas. Morphology and SEY are characterized in dependence of the process parameters to derive correlations and mechanisms of the laser-based SEY engineering process. The nano- and microstructure morphology of the laser-modified surface was characterized by scanning electron microscopy and the secondary electron yield was measured. In general, an SEY reduction with increasing accumulated laser fluence was found. In particular, at low scanning speed (1 mm/s – 10 mm/s) and “high” laser power (∼ 1 W) compact nanostructures with a very low SEY maximum of 0.7 are formed.

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.

Electron flux and pressure dynamic in the LHC vacuum pilot sector as a function of beam parameters and beam pipe properties

The Large Hadron Collider (LHC) is affected by the electron cloud (EC) phenomenon that can provoke beam instabilities, detrimental heat loads and pressure increases in the vacuum system. An innovative dedicated system called vacuum pilot sector (VPS) provides a continuous monitoring of the electron flux and of the pressure signals thanks to electron pickup and vacuum gauges. The VPS system is installed in a room temperature, field-free part of the LHC storage ring. Several technical surfaces, such as ex situ nonevaporable getter (NEG), amorphous carbon coating and copper, are simultaneously tested. The main outcomes of this study show that the EC signals have: (1) a linear dependence upon the number of bunches and upon the bunch population in the multipacting regime, (2) a multipacting threshold at a given bunch population, (3) a reduction under beam conditioning, (4) a strong dependence on the filling pattern and beam energy. The comparison between different surfaces shows that amorphous carbon coating reduces drastically the EC buildup, thanks to its low secondary electron yield (SEY) and photoelectron yield (PY), while copper and ex situ NEG coated surfaces suffer of EC multipacting, even after several months of operation. The multipacting rate coefficients are higher for copper than for ex situ NEG, as predicted from the SEY estimation. Other detailed experimental observations are discussed in this paper.