Total Electron Emission Yield Research Articles

Mechanism of total electron emission yield reduction using a micro-porous surface

Suppression of the total secondary electron yield (TEY) of metal surfaces is important in many areas such as accelerator, satellite, and Hall thruster. Among TEY suppression techniques, micro-porous surfaces have been demonstrated as an effective method. In this work, we developed an analytical model that is able to obtain the contributions of TEY from both the 1st and 2nd generation secondary electrons (SEs). Calculation results show that the TEY contributed by the bottom of the hole dominates the TEY of the micro-porous surface with the aspect ratio we have chosen. Thus, we developed the following design guidance for the improvement of the TEY suppression efficiency of the micro-porous surface: either lower the TEY of the bottom or guide its SEs to the lateral side of the hole. To verify this idea, we performed the following numerical simulations: a micro-hole with its inner surfaces coated with a low TEY material and a micro-hole with nano-triangular grooves or nano-truncated cone pillars embedded at its bottom. Compared with a usual micro-hole, the proposed hybrid micro/nano structures show improved TEY suppression efficiency as expected from the analytical model. The percentage ratios of the 1st and 2nd generation SEs obtained from the simulation agree well with the predictions of the analytical model. What is more, we also present the results of the emitting angle distribution of SEs which represent remarkable deviation from the usual cosine distribution.

Electron emission under uniform magnetic field of materials for fusion and space applications

The power handling capacity of RF components can be limited by a resonant process known as Multipactor effect. Multipactor-induced breakdowns can be critical to microwave systems in space communication payloads or in experimental fusion devices. Multipactor simulations can be used to predict power thresholds but the results highly depend on the electron emission properties of the RF component materials. Moreover, in both space and fusion applications, the RF devices can be subjected to DC magnetic fields. This magnetic field may affect the electron emission properties. In order to improve understanding and predictability of the multipactor effect, the assessment of DC magnetic field on the electron emission properties is required; an experimental setup has been developed to measure the Total Electron Emission Yield (TEEY) under DC magnetic field. The aim of this paper is to describe the newly developed experimental setup and the associated TEEY measurements techniques. The effect of DC magnetic field on the TEEY of copper is investigated.

Multipactor threshold sensitivity to total electron emission yield in small gap waveguide structure and TEEY models accuracy

Radio-frequency (RF) systems used in vacuum can be damaged by electronic avalanches triggered by the multipactor effect. This undesirable phenomenon can appear for RF components used in space communication payloads as well as in experimental fusion devices, among others. To determine the multipactor breakdown threshold, expensive multipactor experimental tests can be made or multipactor simulations can be performed. This second method uses the Total Electron Emission Yield (TEEY) curve as input and the breakdown threshold predicted by simulation highly depends on this curve. Consequently we decided to make a sensitivity study of multipactor breakdown threshold in relation to the TEEY curve variations. We study the silver conductor material with a RF waveguide transformer. Two critical energy regions are found for a small gap waveguide structure: energies around first cross-over energy and energies between the first cross-over and the yield maximum. Electron emission data have to be accurate on these regions to get a coherent multipactor threshold. Six TEEY models are benchmarked with respect to their capability to accurately model these regions.

Experimental investigation of electron emission from dielectric surfaces due to primary electron beam: a review

When charged particles impact the surface of a solid material, they can be backscattered from that surface or cause secondary electrons to be emitted. The ratio of the number of emitted electrons (secondary and backscattered) to the incident electrons number is known as the total electron emission yield. Knowledge of this total electron emission yield (TEEY) is highly required in various modern technologies, and several experimental techniques were developed with the purpose of its determination. However, experimental data, obtained with different methods are not in good agreement and sometimes are even contradictive. The purpose of this paper is to review the existing experimental methods of investigating electron emission (EE) from dielectric surfaces, as well as to analyze their accuracy and reliability. The focus is on the processes of the emission from thick layers of insulators which is the case for modern high voltage technologies.

A novel experimental setup for the measurement electron backscattering yield

An experimental setup that permits total electron emission yield and backscattered electron yield measurements was developed. To ensure its efficiency the experimental setup was modeled on a particle in cell model. Measurements of total electron emission yield and backscattered electron yield were performed on air-exposed technical silver. The results are in agreement with the literature.

Effects of Ultraviolet Irradiation and Atomic Oxygen Erosion on Total Electron Emission Yield of Polyimide

Polymers used on low earth orbit (LEO) spacecraft surface suffer from an ultraviolet (UV) irradiation and orbital atomic oxygen (AO) erosion. These degradations may change the total electron emission yield (TEEY) of the materials and ultimately result in unexpected surface charging. In this paper, we chose polyimide (PI) film, a thermal control material, and carried out two types of ground-based degradation. The degradation methods were UV irradiation with five different equivalent solar hours, and AO erosion with two fluences equivalent to 0.5 and 1 year LEO flight time, respectively. Using an Auger microscope-based TEEY measurement system with a scanning measuring method, the TEEY of virgin and degraded PI films were tested and analyzed comparatively. The X-ray photoelectron spectrum, field-emission scanning electron microscope, and computational simulations were also used for element bonds analysis, roughness imaging, and electron depth calculation to discover the mechanisms of the degradation and their effects on the TEEY of the material.

Investigation on Space Environmental Degradation Effects of Solar Cell Coverglass

Based on the ground-based simulation for space irradiation environment, the degradation of CMG100-AR solar cell coverglasses produced by Qioptiq was conducted by thermal cycling at the range -160°C to +110°C, 50-keV protons irradiation, 500-keV electrons irradiation, ~ 4000 equivalent solar hours vacuum ultraviolet (UV) irradiation, and their multifactors radiation in sequence. The key parameter related to surface charging of virgin and degraded coverglasses, total electron emission yield (TEEY), was measured and discussed. The Monte Carlo simulation, infrared spectroscopy, UV-visible-near-infrared spectroscopy, and X-ray photoelectron spectroscopy spectrum were used to disclose the damage mechanism and its influence on TEEY.

Total Electron Emission Yield of Solar Cell Coverglass and Optical Solar Reflector

Electron-induced secondary electron (SE) emission from space insulating materials is a very important factor in understanding spacecraft charging behavior. Because of the injection of primary electrons (PEs) and the emission of SEs in the surface layer of insulating materials, the target surface can be either negatively or positively charged. In the conventional measurement methods, a single short, low-density pulsed e-beam, charge neutralization using low energy flood e-gun and UV light, and charge dissipation by heating is used. In this paper, a pulsed scanning method for the total electron emission yield measurement of insulating, semiconducting or conducting solid materials is proposed. A total electron detector with large hollow space and insulating spacer was designed, and a pulsed yield scanning measurement system was developed. This method can avoid influence from the charged surface of the insulating material. Using the system, the total SE emission yields of 200 nm thick Au coating on glass, and 11 kinds of solar cell coverglasses and optical solar reflectors produced by Qioptiq have been reported, as induced by a 30 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm s}$</tex></formula> pulse of PEs with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\sim}{\rm 20}~{\rm nA}$</tex></formula> incident beam current and energies up to 2500 eV.

The study of electron emission from aluminum in the very low primary energy range using Monte Carlo simulations

The total electron emission yield (TEEY), at very low energy, is investigated with a Monte Carlo (MC) technique and compared with experimental results. The important points of an MC tool for the simulation of electron emission are presented. The tool is based on a combination of properly selected models taking into account plasmon excitations at the surface and the effect of the crossing surface-barrier potential. The reflectivity is calculated as a function of the primary energy using an analytical expression found in the literature. By comparing MC results with the measurements, a good correspondence between obtained curve shapes is observed. Irregularities in measured and simulated yields are identified and explained by considering plasmon excitations. This comparison allows one, among other things, to remove the ambiguity on the variation of the yield obeying the change in reflectivity as a function of primary energy. In fact, some results reported in the literature claim that the TEEY coefficient approaches unity when the primary electron energy tends to zero. This occurs due to nearly total electron reflection from the surface, for electron energy less than an electron volt. In this paper, we summarize comprehensive studies rendering this claim inaccurate. Aluminum, a free-electron-like metal, will be considered here as a representative illustration.

Total electron emission yield measurement of insulator by a scanning small detector

Due to the injection of primary electrons and the emission of secondary electrons in the surface layer of insulating materials, the target surface will be negatively or positively charged. A method by injecting a single pulse beam and using a small current detector for the total electron emission yield measurement of insulating material is proposed, which can avoid the influence from charged surface. Using the developed system, the total electron emission yield of 25 μm thick polyimide film has been studied, as induced by a single 50 μs pulse of primary electrons with energy up to 2500 eV.

Methods for measurement of electron emission yield under low energy electron-irradiation by collector method and Kelvin probe method

Secondary electron emission yield of gold under electron impact at normal incidence below 50 eV was investigated by the classical collector method and by the Kelvin probe method. The authors show that biasing a collector to ensure secondary electron collection while keeping the target grounded can lead to primary electron beam perturbations. Thus reliable secondary electron emission yield at low primary electron energy cannot be obtained with a biased collector. The authors present two collector-free methods based on current measurement and on electron pulse surface potential buildup (Kelvin probe method). These methods are consistent, but at very low energy, measurements become sensitive to the earth magnetic field (below 10 eV). For gold, the authors can extrapolate total emission yield at 0 eV to 0.5, while a total electron emission yield of 1 is obtained at 40±1 eV.

Electron emission from amorphous solid water induced by passage of energetic protons and fluorine ions.

Absolute doubly differential electron emission yields were measured from thin films of amorphous solid water (ASW) after the transmission of 6 MeV protons and 19 MeV (1 MeV/nucleon) fluorine ions. The ASW films were frozen on thin (1-microm) copper foils cooled to approximately 50 K. Electrons emitted from the films were detected as a function of angle in both the forward and backward direction and as a function of the film thickness. Electron energies were determined by measuring the ejected electron time of flight, a technique that optimizes the accuracy of measuring low-energy electron yields, where the effects of molecular environment on electron transport are expected to be most evident. Relative electron emission yields were normalized to an absolute scale by comparison of the integrated total yields for proton-induced electron emission from the copper substrate to values published previously. The absolute doubly differential yields from ASW are presented along with integrated values, providing single differential and total electron emission yields. These data may provide benchmark tests of Monte Carlo track structure codes commonly used for assessing the effects of radiation quality on biological effectiveness.

The effects of incident electron current density and temperature on the total electron emission yield of polycrystalline CVD diamond

The effects of temperature and incident electron current density on the total electron emission yield (TEEY) of polycrystalline diamond deposited by the chemical vapour deposition technique (CVD) were investigated at low electron beam fluence. It was found that the TEEY reversibly increases with the temperature and reversibly decreases with the current density. This behaviour is explained on the basis of a dynamic competition between the accumulation of holes (positive space charge), which internally reduces the secondary electron emission, and the thermally activated conductivity that tends to reduce the space charge formation.

Experimental investigation of the effect of the internal space charge accumulation on the electron emission yield of insulators submitted to e-irradiation: application to polycrystalline MgO

Experiments are performed on polycrystalline MgO in order to investigate the internal effects of positive charging on the total electron emission yield (TEEY) under low incident electron fluence (a few to hundreds of fC mm−2) at 200 eV electron irradiation energy. It is experimentally shown that the Coulomb-attractive recombination of electrons undergoing emission with holes generated by the previously emitted electrons significantly affects the TEEY. Furthermore, quantitative analyses of experimental data are done to deduce the electron–hole recombination cross-section which is found to be of a few 10−11 cm−2.

A Kelvin probe based method for measuring the electron emission yield of insulators and insulated conductors subjected to electron irradiation

A technique for the determination of the total electron emission yield (TEEY) of insulators or insulated conductors, due to low energy electron irradiation is described. It is based on the measurement of the surface potential variation induced by a pulsed electron beam with the help of a high-sensitivity Kelvin probe. The results obtained on a floating copper sample are in agreement with those obtained by the well-known electron-collector method, on a grounded copper sample. We illustrate the use of the technique on insulators with measurements on Kapton-HN. The measured TEEY of Kapton-HN is compared with those given in the literature with a relatively good consistency.

Charge kinetics in spinel and yttria-stabilized zirconia

Spinel and zirconia were studied by measuring the total secondary electron emission (SEE) yield σ in a dedicated scanning electron microscope (SEM) especially equipped to study the fundamental aspects of the charge trapping in insulating materials during a 1.1 keV electron irradiation at room temperature. The variation of the total SEE yield with the injected dose for both spinel and zirconia is different. In spinel the coefficient σ starts from its intrinsic value σ 0 = 4 and reaches a plateau at σ = 1 at the end of irradiation, which corresponds to the self-regulated regime. The continuity of the curves, shot after shot, proves that the trapped charges are stable and does not spread out in the material as injection proceeds. In this case spinel is called “trapper insulator”. In contrast with the spinel, σ in zirconia, never reaches unity while the injected charge increases: it evolves from its intrinsic yield σ 0 = 2.3 to a steady value a few percent above 1. The curve shows the relaxation of the positive generated charge. In this case zirconia is called “conductive insulator”. The difference in the charging kinetics of the two materials is attributed to the difference in conductivities.

Electron Emission from Insulators Irradiated by Slow Highly Charged Ions

Total electron emission yields have been measured for the first time resulting from impact of slow highly charged Arq+ (q ≤ 17), Xeq+ (q ≤ 50) and Hgq+ (q ≤ 68) ions on clean insulating LiF(001) and CaF2(111) surfaces at various impact angles. The surprisingly large yields show that even for the highest projectile charge states, a local charge-up of the surface poses no barrier for electron emission. We demonstrate that this is due to a strong sub-surface contribution in the potential electron emission process which is considerably more efficient in insulators because of the increased inelastic mean free path and the production of secondary electrons. [DOI: 10.1380/ejssnt.2008.54]

Separation of potential and kinetic electron emission from Si and W induced by multiply charged neon and argon ions

The total secondary electron emission yields, γT, induced by impact of the fast ions Neq+ (q=2–8) and Arq+ (q=3–12) on Si and Neq+ (q=2–8) on W targets have been measured. It was observed that for a given impact energy, γT increases with the charge of projectile ion. By plotting γT as a function of the total potential energy of the respective ion, true kinetic and potential electron yields have been obtained. Potential electron yield was proportional to the total potential energy of the projectile ion. However, decrease in potential electron yield with increasing kinetic energy of Neq+ impact on Si and W was observed. This decrease in potential electron yield with kinetic energy of the ion was more pronounced for the projectile ions having higher charge states. Moreover, kinetic electron yield to energy-loss ratio for various ion-target combinations was calculated and results were in good agreement with semi-empirical model for kinetic electron emission.

The role of Al-Auger electrons in kinetic electron emission from Al surfaces by slow Ne+ and Na+ ions

This work reports measurements of energy distributions and yields of electrons emitted in the interaction of slow singly charged neon and sodium ions with Al surfaces. The measurements show the strong interplay between different emission mechanisms, such as projectile and target Auger electron emission, decay of bulk plasmons and electronic collision cascade. These results indicate that the Auger decay of Al-2p excitations occurring in binary atomic collision between recoiling target atoms, is the dominant primary excitation mechanism. Measurements performed using sodium projectile show that a significant contribution to the total electron emission yield comes also from a vacancy transfer process that produce Al-2p excitation by a vacancy transfer process in asymmetric collisions between target atoms and incoming ions that have survived neutralization at the surface.

Charging regime of PMMA studied by secondary electron emission

Foils of Polymethyl methacrylate (PMMA) 2 mm thick were studied by measuring the total Secondary Electron Emission yield (SEE yield) in a dedicated Scanning Electron Microscope especially equipped to study the fundamental aspects of the charge transport and trapping in insulating materials. The intrinsic SEE yield , (yield of the uncharged material) and the charging kinetics were studied under low current density pA/cm2. The curve of the primary beam energy variation of exhibits a maximum intrinsic yield at 370 ± 20 eV and two crossover energies ± 20 eV and ± 20 eV for which . For 1 PMMA is positively charged and negatively for 1. As electron injection is proceeding under the low current density used, the SEE yield varies from to the steady value . This value that expresses the equality between the average number of emitted and injected electrons, characterizes the steady charge regime called “Self-Regulated Regime”. The evolution of during the injection process is due to the internal field that blocks or enhances the secondary electron emission, according to the positive or negative nature of the trapped charges. A current density effect, characterized by a steady SEE yield slightly higher than unity, , instead of one, is observed at high energy (for example 4000 eV) for a strong current density $J>10^{+6 }$ pA/cm2. It is interpreted by a field ionisation (Poole-Frenkel type) that enhances the secondary electron emission.