Secondary Emission Yield Research Articles

An extension of first principle combined Monte Carlo method to simulate secondary electron yield of anisotropic crystal Al2O3

An extension of a first-principle combined Monte Carlo method is proposed in this work to obtain the secondary electron emission characteristics of anisotropic crystal Al2O3. Unlike isotropic crystal Cu, density functional theory calculations reveal that the q-dependent energy loss function of Al2O3 in all directions is different. Therefore, an interpolation algorithm is introduced in the Monte Carlo method to determine the loss of energy and inelastic mean free path of electrons. The simulation results are in good agreement with experimental data. This method can be further used to simulate the secondary emission yield of other anisotropic crystal materials.

Electron cloud build-up studies for FCC-ee

The Future Circular Collider (FCC) study is developing designs for a new research infrastructure to host the next generation of higher performance particle colliders to extend the research currently being conducted at the LHC. In particular, FCC-ee is an electron-positron collider, which is the first stage towards a 100 TeV proton-proton collider FCC-hh. FCC-ee may be affected by electron cloud (e-cloud) and the strongest effects are foreseen for the Z configuration, due to the highest number of bunches, which corresponds to the smallest bunch spacing. The presence of a large electron density in the beam pipe can limit the achievable performance of the accelerator through different effects like transverse instabilities, transverse emittance growth, particle losses, vacuum degradation and additional heat loads of the inner surface of the vacuum chambers. In the design phase, the goal is to suppress the e-cloud effects in FCC-ee and, therefore, a preliminary study to identify the parameters, which play a significant role in the e-cloud formation has been performed. In this paper, an extensive e-cloud simulation study is presented. In particular, the impact of the e-cloud is studied for different configurations, for example: for the electron and the positron beam; in the different elements of the particle accelerator; changing the beam chamber geometry; for different values of the Secondary Emission Yield (SEY); and for different beam parameters.

Effect analysis of spatial discrepancy of secondary emission yield on multipactor formation

Spatial discrepancy of secondary emission yield (SEY) is probably exacerbated by unexpected surface contamination or imperfect surface treatments for SEY suppression, which accordingly provokes increased multipactor risk in microwave devices. In this paper, an improved 2D2V nonstationary statistical modeling for multipactor of parallel plates capable of regarding all electron impacts and electron exchange at the periodic boundaries is developed to investigate the effect of this spatial SEY discrepancy on multipactor formation in microwave devices. The comparison with the 1D2V statistical modeling results, which is valid for the parallel-plate multipactor, proves the accuracy of this improved 2D2V statistical modeling and the necessity of appropriate boundary setting in multipactor analysis with spatial SEY variation. The modeling results also reveal that the multipactor establishment is dominated by the electron multiplication and the electron overflow in the high-SEY region, thus making the multipactor threshold strongly dependent on both the SEY property and the dimension of the region. Electron multiplication can be fully sustained in the high-SEY region when its size exceeds a critical dimension, which satisfies a scaling law (the sustaining dimension is proportional to the gap with the product of the RF frequency and the gap kept constant) and varies with the involved multipactor mode. This research will help in evaluating accidental multipactor risk caused by the surface contamination and the efficiency of multipactor prevention via applying surface treatments for SEY suppression to accessible regions.

Quantitative analysis of multipactor threshold sensitivity to secondary emission yield of microwave devices

Multipactor occurrence essentially depends on the secondary emission property of the surface material, which is, thus, the requisite input for multipactor threshold prediction using the numerical and theoretical approaches. However, secondary emission yield (SEY) deviation in experimental measurements inevitably leads to uncertainty error in multipactor threshold prediction. Therefore, this paper presents a thorough quantitative analysis of multipactor threshold sensitivity to SEY including the effect of the device geometry, the multipactor mode, and the material type. Based on the statistical modeling, multipactor threshold voltages with respect to the SEY variation in critical SEY regions are calculated for both the parallel plates and coaxial lines with different multipactor orders and typical materials. Furthermore, the distribution of electron impact energy is also obtained to elucidate the underlying mechanism for the relevant sensitivity discrepancy. The result reveals that multipactor threshold is generally most sensitive to the energy region below the first crossover energy (E1), and this is changed to higher energies below the corresponding energy to the SEY maximum (Em) with a change in the device geometry, multipactor mode, or coating material. It is also found that the magnitude relation of the threshold sensitivity between different regions is radically determined with the distribution of electron impact energy, and the SEY variation close to Em merely affects the threshold result with a high multipactor order. This research provides useful reference for properly determining the threshold margin from the measurement error of SEY, thus promoting the performance optimization with multipactor prevention in the practical application of microwave devices.

Role of surface chemistry in conditioning of materials in particle accelerators

For the vacuum scientists and the accelerator community, finding solutions to mitigate pressure rises induced by electron, photon and ion desorption, and also beam instabilities induced by ion and electron clouds is a major issue. Along the time, changes in the surface chemistry of vacuum chambers are observed during beam operations, leading to modifications of: outgassing rates, stimulated desorption processes and secondary emission yields (SEY). To understand the role of the surface chemistry of air exposed materials in the electron conditioning process, typical air exposed materials used in particle accelerators : thin film coatings (NEG, i.e a Ti-Zr-V alloy, and carbon), copper (and its oxides Cu2O and CuO) and niobium were conditioned by low energy electron irradiation for a better understanding of electron-cloud effect. First, SEY was measured to understand the changes of surface conditioning upon particle irradiation; then, surface chemistry evolution after electron irradiation was investigated by both XPS and TOF-SIMS analyses using the ANDROMEDE facility at IJCLab. Finally, the relationship between the surface chemistry and the conditioning phenomenon will be discussed.

Mitigation of electron cloud effects in the FCC-ee collider

Electron clouds forming inside the beam vacuum chamber due to photoemission and secondary emission may limit the accelerator performance. Specifically, the electron clouds can blow up the vertical emittance of a positron beam, through a head-tail-type single-bunch instability, if the central electron density exceeds a certain threshold value, that can be estimated analytically. Using the codes PyECLOUD and VSim, we carried out detailed simulations of the electron-cloud build up for the main arcs and the damping ring of the FCC-ee collider, in order to identify the effective photoemission rate and secondary emission yield required for achieving and maintaining the design emittance. To this end, we present the simulated electron density at the centre of the beam pipe for various bunch spacings, secondary emission yields, and photoemission parameters, in the damping ring and in the arcs of the collider positron ring. To gain further insight into the underlying dynamics, the obtained spatial and energy distributions of the cloud electrons are illustrated as a function of time. In addition, we compare results obtained for two different secondary emission models (“Furman–Pivi” and “ECLOUD”), thereby indicating the uncertainty inherent in this type of study, without any prototype vacuum chambers yet available. We also point out a few situations where the two secondary-emission models yield similar density values. Finally, based on our simulation results for two different design variants, we conclude that the new parameter baseline of the FCC-ee will facilitate electron-cloud mitigation.

Modeling the saturation of the multipactor effect in a dielectric-loaded parallel-plate waveguide

The aim of this paper is to establish a numerical simulation model for the multipactor effect in a partially dielectric-loaded parallel-plate waveguide, with a focus on the investigation of multipactor saturation mechanisms for different dielectric materials with different secondary emission yield (SEY) properties. An electrostatic method involving the radio-frequency fields, space charge fields, and the dynamics of charge accumulation on the dielectric surface and solutions for electrostatic fields are proposed. The evolution of the electron number, accumulated charge, and secondary electron multiplication rate for different input voltages and SEY properties of the dielectric materials are studied using numerical calculations. The results show that two physical multipactor phenomena occur in a dielectric-loaded parallel-plate waveguide: a self-sustaining phenomenon, which means that the electron population reaches a saturation level, and a self-extinguishing phenomenon. The latter can be divided into two cases: in one, the number of electrons undergoes a process of multiplication, saturation, and reduction, and in the other, the number of electrons disappears after their population reaches a maximum. Furthermore, a multipactor susceptibility diagram for SEY curves of different dielectric materials is constructed. The results show that the multipactor effect is suppressed when the maximum of the SEY curve is less than 1.3.

Electron emission properties of silver oxide and its impact on the secondary emission yield of air-exposed silver

Surface oxidation, contamination and adsorption are commonly considered as three leading factors of secondary electron yield (SEY) increase for metals exposed to various environments. Whereas, the influence of these three factors on SEY is different for diverse metal materials. To discover the SEY dependence on surface oxidation for silver, a group of silver oxide (Ag2O) and silver coatings are therefore prepared. Experimental results show the SEY of Ag2O is just about 0.1 higher than that of silver coating whether before or after ion cleaning, and SEY difference becomes smaller under oblique incidence, which indicates the surface oxidation may not be the major causes of SEY increase for air-exposed silver. To further verify the speculation, various surface conditions are achieved on silver surface. Microscopic analysis reveals the adsorptions and contaminations can be effectively removed by heating treatment, meanwhile, SEY is remarkably decreased. Furthermore, the treatment of ion cleaning eliminates all the surface additions and achieves a superlative cleanliness for silver sample, the SEY peak value of air-exposed silver decreases from 2.61 to 1.73 after ion cleaning for 10 min, and to 1.70 after ion cleaning for 30 min, which indicates the surface oxidation contributes little to the SEY raise of air-exposed silver. The SEY dependence on the thickness of contamination layer is also theoretically explained. This work does a systematic research on how the surface oxidation and modification influence SEY of silver, which makes significant sense for comprehending the SEE characteristics of silver with various surface conditions.

SECONDARY ELECTRON EMISSION FROM THIN ALUMINIUM FOILS PRODUCED BY HIGH ENERGY ELECTRON BEAMS

The description of the experimental equipment and technique for measuring the secondary emission of elec-trons (SEE) with application of accelerated electrons at the linear accelerator of the IHEPNP NSC KIPT with ener-gies up to 30 MeV and a standard secondary emission monitor [1] are presented. Experimental data of secondary electron emission yields from thin aluminum targets (8 and 50 μm) for primary electron beam energies of 16 and 25 MeV have been experimentally measured. The analysis of the experimental data and their comparison with the theory are carried out. It is shown that the proposed technique for measuring the yields of secondary electron emis-sion is useful and applied for study of low-energy and δ-electrons yields from thin foils, as well as to research the effect of the density effect depending on the energy of the primary electron beam.

Experimental Investigation of Material and Geometry Effects on Microwave Breakdown of Evanescent-Mode Cavity Resonators

This article presents an experimental investigation of microwave breakdown in evanescent-mode cavity resonators with different surface materials and post geometries, aiming at unveiling the role of secondary electron emission (SEE), attachment, and diffusion on microwave breakdown in inhomogeneous electric fields. The breakdown power of six cavities resonating at 2.6 GHz was measured at pressures ranging from 1 to 100 mbar in ambient air. The cavity with lower secondary emission yield surface material has a higher breakdown power on the left side of the breakdown curves, indicating the importance of SEE in microwave breakdown, especially at low pressures. Meanwhile, except for the cavity with hemisphere post, it was found that the quality factor has little effect on the breakdown power since electric field distributions inside these cavities are not significantly linked to post geometry. Two distinct regimes are observed in the breakdown curves with a transition pressure of 10 mbar. Namely, the breakdown curves present an emission-free branch above the transition, where microwave breakdown occurs in the gap, and a diffusion branch below the transition, where the microwave plasma expands out of the gap. We performed a theoretical calculation and found that the transition occurs when the attachment length equals the post radius.

A Combined Susceptibility Diagram Including the Average Secondary Emission Yield on the Dielectric Surface

This letter proposed a combined susceptibility diagram, applicable to investigating and differentiating the inherent mechanism of multipactor in dielectric-loaded microwave devices. This susceptibility diagram consists of average secondary emission yield on the dielectric surface and both the surfaces. Using this diagram, we predict the electron multiplication mechanism in the partially dielectric-loaded parallel-plate waveguide. It is found that the multipactor extinguishing only occurs for a low multipaction rate, while the saturation occurs for a higher multipaction rate. To verify its validity, the dynamic evolution of multipactor is investigated by electrostatic particle-in-cell. A good agreement between simulations and prediction results has been achieved. This susceptibility diagram thus constructed can provide a guidance to clearly identify the condition for multipactor extinguishing or saturation.

Analysis of multipactor in a rectangular waveguide using Spark3D software

Multipactor is a resonant nonlinear electron multiplication effect that may occur in high power microwave devices at very low pressures, such as those operating in particle accelerators and satellite subsystems. In this research, multipactor of a rectangular waveguide was analyzed using the commercially available, numerical simulation software “Spark3D.” The electromagnetic wave in the simulation was a TE10 mode-2.85 GHz wave of varying power, fed into the impedance transformer waveguide. At the lowest threshold, multipactor is generated in the minimum height region in the impedance transformer and nowhere else. More precisely, the multipacting electrons have a continuous energy distribution since the emitted secondary electrons carry a random initial velocity distribution. We observed that there are cases where the impact electron energy decreases despite an increase in power due to growing non-resonance of the microwave field and electron oscillations, resulting in not only two threshold points where secondary emission yield (SEY) = 1 but several more. As a consequence, it was uncovered that when the average SEY in the highest field region is close to or less than one, multipactor may be caused in a lower field region where the SEY is effectively higher than one. The numerical results are compared with data from the experiment. While there is some deviation between the thresholds obtained from Spark3D and the experiment, the results at higher power levels are consistent with the experiment in the view of the SEY for each power level.

Numerical investigation of secondary electron emission effect on the dusty plasma sheath with superextensive electrons

We have investigated the structure of a magnetized sheath of dusty plasma in the presence of secondary electrons emitted by the micro-size dust particles in the context of the Tsallis statistics. The fluid model is used to analyze numerically the effects of the nonextensivity parameter q on the emission of secondary electrons and therefore, on the sheath structure as well as the dust dynamics. The results show that the secondary emission yield increases with the decrease of the parameter of nonextensivity q and consequently, the dust charge becomes less negative with its range of values playing a primordial role in the secondary electron emission rate. The quantities characterizing the sheath are significantly affected by the secondary electron emission (SEE) from the dust. It is seen that as the SEE rises at a given value of q(q<0.91), the sheath potential decreases as well as its absolute value at the wall. In addition, the dynamics of the dust particles is also affected by the emission of secondary electrons.

Investigation of TiN film on an RF ceramic window by atomic layer deposition

To reduce the secondary emission yield of an RF window ceramic so as to suppress the occurrence of a multipactor, we propose coating the window with a titanium nitride (TiN) film by atomic layer deposition. We investigate two groups of samples with film thicknesses of 56 and 8 nm, respectively, to analyze the composition and optimize the coating thickness of the TiN film on the ceramic. For the first group of samples (56 nm films), x-ray photoelectron spectroscopy (XPS) results show that the film can be divided into three mixed layers: a top layer composed of TiOxNy and TiO2; a middle layer consisting of TiN, TiOxNy, TiO2, and TiC; and a bottom layer called a diffusion layer, formed by decreasing TiN, TiOxNy, TiO2, TiC, and increasing Al2O3 as the ceramic is approached. The depth of this bottom layer is ∼8 nm. Two more films (8 nm films) in the second group of samples were grown on a 96 ceramic and silicon to determine the sheet resistance, those on silicon is ∼1 kΩ/□ as measured by spectroscopic ellipsometry. According to the XPS results of the two 8 nm films, the content of TiOxNy and TiO2 increased while TiN content decreased in the film on 96 ceramic, compared to the film on silicon. Therefore, the 8 nm film is suitable for use as a coating for the RF window to weaken the multipactor effect and lower conductivity. To test the film performance, an RF ceramic window is coated with an 8 nm TiN film. Low-power measurements show that, within a frequency of 100 MHz, the 8 nm film on the RF ceramic window has a negligible effect on its transmission characteristics. 8 kW RF power tests indicate that the film coating can significantly improve the power transmission, anti-multipaction, and stability of the RF window.

Silver nanopillar coatings grown by glancing angle magnetron sputtering for reducing multipactor effect in spacecrafts

We have studied nanometric high aspect ratio Ag nanopillar coatings exhibiting reduced secondary electron emission for the mitigation of multipactor effect in radio-frequency space devices of high frequency and high power. The Ag nanopillars have been grown by glancing angle deposition with DC magnetron sputtering. Some samples have been covered by a gold capping layer to reduce oxidation and aging effects. The secondary emission yield of the surfaces of these samples has been measured and compared to those of flat Ag and Au reference samples. The results show that high aspect ratio surface roughness at the nanometer scale significantly reduce the secondary emission yield of the surface. This reduction is more important for low electron energies, which is the most influencing energy range of electrons for multipactor. The multipactor region for the nanopillar coating presenting the best secondary emission yield properties has been simulated, finding practical suppression of multipactor effect. The high-frequency surface resistance of these samples has also been estimated from published computations for similar surface roughness patterns. It was found that such nanopillar coatings are compatible with the best accomplishments of present space industry.

Simulation of magnetic field effects on the MCP gain

A multi-anode microchannel plate photomultiplier (Ma MCP-PMT) is an appropriate photo sensor for particle identification in high energy physics experiments such as PANDA, Belle II, etc. Since these detectors usually work in a strong magnetic field, the sensors must be immune to the field. In this article, the effects of the magnetic field on the conventional MCP and the atomic layer deposited MCP (ALD-MCP) are studied by simulating the electron multiplication in the microchannel of the MCP . Simulation results show that the gain of conventional MCP-PMTs increases a little before it starts decreasing while the magnetic field is further increasing. We found that the shortening of electronic trajectories and the reduction of secondary electrons mainly contribute to gain variations of a conventional MCP . For the ALD-MCP that usually has higher secondary emission yields, it is found that high re-diffusion secondary electron yields could make it more vulnerable to magnetic fields.

Modelling vacuum flashover mitigation with complex surface microstructure: mechanism and application

The vacuum discharge along the dielectric surface, also called surface flashover, brings significant damages to the vacuum-solid insulation system. Here the authors implement shape-flexible, complex surface groove microstructures on the dielectric to mitigate the initiation of vacuum flashover. A particle-in-cell simulation is employed to reveal the real-time discharge development considering the blockage of the multipactor propagation as well as the space field distortion in the presence of specific surface microstructures. Electron trajectories within barriers are theoretically analysed to present how the barrier shape affects the discharge process, showing consistent results with the simulation. By analysing three parameters, namely the anode current, the surface average charge density and the flashover threshold, it is found that the effect of suppressing surface flashover remarkably augments when the groove number and depth rise up, while such effect gradually saturates when the groove depth reaches a critical value. Theoretical analyses are also provided for the decay factor as well as the optimal shape and trapezoid grooves are proved as the optimal shape distribution of microstructures. Furthermore, different secondary emission yield (SEY) distributions are considered with two kinds of structures, including the material with low-SEY inside and on the top surface of grooves.

First-principles evaluation of the secondary electron yield (γN) from polyethylene surface

We have performed first-principles calculations (DFT) to estimate the secondary emission yield (SEY) through Auger neutralization mechanism () related to the impact of a series of ions on a polyethylene surface. We have considered many relevant ionic species, such as , which is often used as a benchmark. Our main goal is to study dielectric surfaces but, to check the validity of our approach, we have also considered some metallic surfaces (, , Cu:N, Cu:O and ) on which some more experimental data can be found. On the contrary, very few references are available regarding the Auger neutralization on insulating materials and, in particular, on polyethylene.The SEY outcomes for metals have basically reproduced the experimental references. In particular, the same decrease in , which has been associated to a ‘dirty’ (gas-exposed) metal surface, was confirmed by calculations. Thus, the applicability of the method to plastic material was considered to give realistic results. The computed values associated to polyethylene are in the order of 10−1 for most of the ionic species here considered.Moreover, we have observed that a few percentage points of variations of the surface energy levels predicted by DFT calculations, may cause, depending on the ion type, a substantial change of . Therefore a detailed sensitivity analysis has been included to address this problem. The results associated to metals have shown that variations are, for some types of ions, very sharp, while this variability is milder for a polyethylene surface.Our calculations are fully compatible with the previous relevant literature and suggest that plastic materials are characterized by coefficients similar to those of metals only slightly smaller.

Effect of Secondary Emission Yield and Initial Charge of Dielectric Material on Multipactor in Parallel-Plate Dielectric-Loaded Waveguide

In this study, the charge polarity on a dielectric surface and the effect of initial charge on the multipactor threshold in a parallel-plate dielectric-loaded waveguide are investigated through 1-D particle-in-cell (PIC) simulation. The numerical results show that owing to different dielectric materials, two kinds of multipactor mechanisms exist on the dielectric multipactor. For analyzing the effect of charged dielectrics on the multipactor threshold, the single-sided (SS) and double-sided (DS) impacts on metal and dielectric surfaces are considered. As a result, the multipactor threshold could be suppressed by adding some initial charges on the dielectric surface, satisfying a certain condition.

Theory of High-Voltage Glow Discharge with the Generation of a Monoenergetic Electron Beam

A kinetic theory of high-voltage glow discharge (HVGD) is developed. The Poisson equation in the space charge layer is solved taking into account the flow of ions coming from the plasma into the layer, the ionization of gas in the layer via electrons, ions, and fast atoms. On the cathode there is a potential and kinetic extraction of electrons from the surface. For different values of the gas density and secondary emission yield, the current–voltage characteristic is calculated, the dimensions of the space charge layer are determined, and electric field distributions in the layer are present, as well as other HVGD characteristics. The proposed mathematical model can be used to calculate the characteristics of electron accelerators based upon HVGD.