Secondary Electron Emission Yield Research Articles

Analytical model of secondary electron emission yield in electron beam irradiated insulators

The study of secondary electron emission (SEE) yield as a function of the kinetic energy of the incident primary electron beam and its evolution with charge accumulation inside insulators is a source of valuable information (even though an indirect one) on charge transport and trapping phenomena. We will show that this evolution is essentially due, in plane geometry conditions (achieved using a defocused electron beam), to the electric field effect (due to the accumulation of trapped charges in the bulk) in the escape zone of secondary electrons and not to modifications of trapping cross sections, which only have side effects.We propose an analytical model including the main basic phenomena underlying the space charge dynamics. It will be observed that such a model makes it possible to reproduce both qualitatively and quantitatively the measurement of SEE evolution as well as to provide helpful indications concerning charge transport (more precisely, the ratios between the mobility and diffusion coefficient with the thermal velocity of the charge carrier).

Theoretical Study and Monte Carlo Simulation on the Dynamic Process of a Novel Multipacting Cathode for High-Current Diode

The prototype and configuration design of a novel multipacting cathode producing a ring-like electron beam for high-current diode are presented. The whole dynamic process of the moving secondary electron on the multipacting cathode surface is theoretically investigated and numerically simulated using the Monte Carlo method. The results indicate that the prototype of the novel multipacting cathode for high-current diode is feasible. Under the interactions of the applied electric field (~MV/m) in axial and radial directions and the guiding magnetic field (~T) in axial direction, the seed electrons from the triple junctions move with spiral advancing trajectories along axial direction. Electron number can be effectively increased by each impact on the coating surface with high secondary electron emission yield coefficient. This phenomenon will achieve electron current amplification with the ring-like electron beam output till multipacting enters into saturation. The simulated characteristic parameters of the secondary electron and the working range of the multipacting cathode agree with the theoretical results. Under the typical condition, the moving distance of the secondary electron is on the order of submillimeters, the fight time is on the order of picoseconds, and the spread speed is on the order of 106 to 107 m/s. In addition, the multipacting saturation mechanism is analyzed, and the emission current density is roughly estimated to ~kA/cm2. Enhancing the magnitude of applied electric field in radial direction can effectively improve the emission current density.

Secondary electron emission from textured surfaces

In this work, a Monte Carlo model is used to investigate electron induced secondary electron emission for varying effects of complex surfaces by using simple geometric constructs. Geometries used in the model include: vertical fibers for velvet-like surfaces, tapered pillars for carpet-like surfaces, and a cage-like configuration of interlaced horizontal and vertical fibers for nano-structured fuzz. The model accurately captures the secondary electron emission yield dependence on incidence angle. The model shows that unlike other structured surfaces previously studied, tungsten fuzz exhibits secondary electron emission yield that is independent of primary electron incidence angle, due to the prevalence of horizontally-oriented fibers in the fuzz geometry. This is confirmed with new data presented herein of the secondary electron emission yield of tungsten fuzz at incidence angles from 0–60°.

An experimental assessment of methods used to compute secondary electron emission yield for tungsten and molybdenum electrodes based on exposure to Alcator C-Mod scrape-off layer plasmas

Plasma potentials computed from Langmuir probe data rely on a method to account for secondary electron emission (SEE) from the electrodes. However, significant variations exist among published models for SEE and the reported experimental parameters used to evaluate them. As a means to critically assess SEE computation methods, two of four tungsten electrodes on a Langmuir–Mach probe head were replaced with molybdenum and exposed to Alcator C-Mod boundary plasmas where electron temperatures exceed 50 eV and SEE becomes significant. In this situation, plasma potentials computed for either material should be identical—the SEE evaluation method should properly account for the differences in SEE yields. Of the six methods used to compute SEE, two are found to produce consistent results (Sternglass model with Bronstein experimental parameters and Young–Dekker model with Bronstein experimental parameters). In contrast, the method previously used for C-Mod data analysis (Sternglass model with Kollath parameters) was found to be inconsistent. We have since adopted Young–Dekker–Bronstein as the preferred method.

Modeling of reduced secondary electron emission yield from a foam or fuzz surface

Complex structures on a material surface can significantly reduce the total secondary electron emission yield from that surface. A foam or fuzz is a solid surface above which is placed a layer of isotropically aligned whiskers. Primary electrons that penetrate into this layer produce secondary electrons that become trapped and do not escape into the bulk plasma. In this manner the secondary electron yield (SEY) may be reduced. We developed an analytic model and conducted numerical simulations of secondary electron emission from a foam to determine the extent of SEY reduction. We find that the relevant condition for SEY minimization is u¯≡AD/2≫1 while D ≪ 1, where D is the volume fill fraction and A is the aspect ratio of the whisker layer, the ratio of the thickness of the layer to the radius of the fibers. We find that foam cannot reduce the SEY from a surface to less than 0.3 of its flat value.

Optimization of surface morphology with micro meter size for suppressing secondary electron emission

Suppression of the secondary electron (SE) multipactor is a key issue for improving the performance of high power microwave devices and particle accelerators. The decrease of the SE emission yield (SEY) by using certain surface morphology is one of the effective methods. To optimize the surface morphology, we simulate the SE emissions of different surface structures by using the Monte Carlo method. The effects of geometric parameters, such as duty ratio of area, depth-to-height ratio, pattern and its arrangement on SEY are investigated. For surface morphology with patterns of square, round and triangle, and for both convex and concave structures, the corresponding values of SEY first decrease and then become steady with the increase of duty ratio of area and depth-to-height ratio. For convex structures, the values of SEY are different for different pattern shapes, in which triangle pattern has the smallest SEY. However, the value of SEY is nearly independent of arrangement of pattern. For concave structures, on the other hand, the value of SEY is scarcely different for different patterns or different arrangements. In general, a convex structure has a better suppression effect than a concave structure if other geometric parameters are identical. The shading effect from side wall of structure is found to be the intrinsic reason of the suppression effect.

Modeling of the effect of field electron emission from the cathode with a thin insulating film on its emission efficiency in gas discharge plasma

A model of field electron emission from the metal cathode with a thin insulating film under the strong electric field, generated in the film by ions bombarding its surface in gas discharge, is developed. It takes into account tunneling of electrons from the electrode metal substrate into the insulating film, their motion in the film and going out of it into the discharge volume. An analytical solution of the one-dimensional kinetic equation for the energy distribution function of emitted electrons in the film conduction band is found and an expression for the film emission efficiency equal to the fraction of emitted electrons, which escapes from the film and increases the cathode effective secondary electron emission yield, is obtained. It is demonstrated that calculated dependence of the emission efficiency on the electric field strength in the aluminum oxide film is in an agreement with experimental data for metal-insulator-metal tunneling cathodes. The proposed model can be used for investigation of an influence of the field electron emission from the cathode with a thin insulating film on its emission characteristics in gas discharge devices.

Picosecond Breakdown in High-Voltage Open Pulse Discharge With Enhanced Secondary Electron Emission

A 300-ps breakdown was registered in the experiments in the high-voltage open discharge with counter-propagating electron-beams with applying voltage pulses with rise times in 10–20 ns range. Discharge operates between two cathodes and grid-anode in the midplane at 10–35 Torr in helium at voltage 5–13 kV. It is shown in Particle-in-cell Monte Carlo collision (PIC MCC) simulations that the contributions of electrons, ions and energetic atoms in ionization and excitation processes are important for current development. The influence of different emissive cathode materials (titanium, silicon carbide, and CuAlMg-alloy) on evolution of discharge current is studied in the experiment and in kinetic PIC MCC simulations. The secondary electron emission from the cathodes is found to be a dominant process at the final stage of the breakdown. The current growth rate is larger (up to 700 A/cm2ns) for the cathode with a higher secondary electron emission yield. By choosing the cathode material, applied voltage amplitude and gas pressure, the characteristic time of the gas breakdown can be reduced.

Secondary electron emission yield from high aspect ratio carbon velvet surfaces

The plasma electrons bombarding a plasma-facing wall surface can induce secondary electron emission (SEE) from the wall. A strong SEE can enhance the power losses by reducing the wall sheath potential and thereby increasing the electron flux from the plasma to the wall. The use of the materials with surface roughness and the engineered materials with surface architecture is known to reduce the effective SEE by trapping the secondary electrons. In this work, we demonstrate a 65% reduction of SEE yield using a velvet material consisting of high aspect ratio carbon fibers. The measurements of SEE yield for different velvet samples using the electron beam in vacuum demonstrate the dependence of the SEE yield on the fiber length and the packing density, which is strongly affected by the alignment of long velvet fibers with respect to the electron beam impinging on the velvet sample. The results of SEE measurements support the previous observations of the reduced SEE measured in Hall thrusters.

Transition in discharge plasma of Hall thruster type in presence of secondary electron emissive surface

Modification of the sheath structure near the emissive plate placed in magnetized DC discharge plasma of Hall thruster type was studied in the experiment and in kinetic simulations. The plate is made from Al2O3 which has enhanced secondary electron emission yield. The energetic electrons emitted by heated cathode provide the volume ionization and the secondary electron emission from the plate. An increase of the electron beam energy leads to an increase of the secondary electron generation, which initiates the transition in sheath structure over the emissive plate.

Secondary electron emission of tin and tin-lithium under low energy helium plasma exposure

Secondary electron emission (SEE) yields of tin (Sn) and tin-lithium (SnLi) eutectic (20 at.% Li) samples were measured in He-plasma at a mean incoming electron energy up to 120 eV. SnLi shows a maximum yield of about 1.45 at 110 eV electron energy while the yield of the Sn surface was measured to be maximally 1.05 at 120 eV. X-ray photoelectron spectroscopy (XPS) analysis demonstrated the segregation effect of Li to the surface of the eutectic, both after melting in the argon atmosphere and in molten state with simultaneous He-plasma exposure. At least the top 10 nm of the SnLi samples were heavily enriched with Li, and Sn/Li ratios varied in the range 0.8–5% depending on eutectic treatment conditions. After the plasma exposure Sn3d is detected predominantly in the oxidized state while after extended atmospheric oxidation there was still a significant amount of Sn3d detected in the metallic state. The liquid surface of SnLi indicated a possible decrease of SEE yield. All measurements gave values of SEE yield close to or above unity. Such values can lead to significant plasma sheath disturbances and subsequent additional heat flux from electrons on such a plasma-facing material, thus, should be accounted for in designing fusion reactors using these components.

Temperature and energy effects on secondary electron emission from SiC ceramics induced by Xe17+ ions

Secondary electron emission yield from the surface of SiC ceramics induced by Xe17+ ions has been measured as a function of target temperature and incident energy. In the temperature range of 463–659 K, the total yield gradually decreases with increasing target temperature. The decrease is about 57% for 3.2 MeV Xe17+ impact, and about 62% for 4.0 MeV Xe17+ impact, which is much larger than the decrease observed previously for ion impact at low charged states. The yield dependence on the temperature is discussed in terms of work function, because both kinetic electron emission and potential electron emission are influenced by work function. In addition, our experimental data show that the total electron yield gradually increases with the kinetic energy of projectile, when the target is at a constant temperature higher than room temperature. This result can be explained by electronic stopping power which plays an important role in kinetic electron emission.

“Feathered” fractal surfaces to minimize secondary electron emission for a wide range of incident angles

Complex structures on a material surface can significantly reduce the total secondary electron emission from that surface. The reduction occurs due to the capture of low-energy, true secondary electrons emitted at one point of the structure and intersecting another. We performed Monte Carlo calculations to demonstrate that fractal surfaces can reduce net secondary electron emission produced by the surface as compared to the flat surface. Specifically, we describe one surface, a “feathered” surface, which reduces the secondary electron emission yield more effectively than other previously considered configurations. Specifically, feathers grown onto a surface suppress secondary electron emission from shallow angles of incidence more effectively than velvet. We find that, for the surface simulated, secondary electron emission yield remains below 20% of its un-suppressed value, even for shallow incident angles, where the velvet-only surface gives reduction factor of only 50%.

On singlet metastable states, ion flux and ion energy in single and dual frequency capacitively coupled oxygen discharges

We apply particle-in-cell simulations with Monte Carlo collisions to study the influence of the singlet metastable states on the ion energy distribution in single and dual frequency capacitively coupled oxygen discharges. For this purpose, the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 is used, in which the discharge model includes the following nine species: electrons, the neutrals O(3P) and O), the negative ions O−, the positive ions O+ and O, and the metastables O(1D), O and O2(b). Earlier, we have explored the effects of adding the species O) and O2(b), and an energy-dependent secondary electron emission yield for oxygen ions and neutrals, to the discharge model. We found that including the two molecular singlet metastable states decreases the ohmic heating and the effective electron temperature in the bulk region (the electronegative core). Here we explore how these metastable states influence dual frequency discharges consisting of a fundamental frequency and the lowest even harmonics. Including or excluding the detachment reactions of the metastables O) and O2(b) can shift the peak electron temperature from the grounded to the powered electrode or vice versa, depending on the phase difference of the two applied frequencies. These metastable states can furthermore significantly influence the peak of the ion energy distribution for O-ions bombarding the powered electrode, and hence the average ion energy upon bombardment of the electrode, and lower the ion flux.

Tutorial: Reactive high power impulse magnetron sputtering (R-HiPIMS)

High Power Impulse Magnetron Sputtering (HiPIMS) is a coating technology that combines magnetron sputtering with pulsed power concepts. By applying power in pulses of high amplitude and a relatively low duty cycle, large fractions of sputtered atoms and near-target gases are ionized. In contrast to conventional magnetron sputtering, HiPIMS is characterized by self-sputtering or repeated gas recycling for high and low sputter yield materials, respectively, and both for most intermediate materials. The dense plasma in front of the target has the dual function of sustaining the discharge and providing plasma-assistance to film growth, affecting the microstructure of growing films. Many technologically interesting thin films are compound films, which are composed of one or more metals and a reactive gas, most often oxygen or nitrogen. When reactive gas is added, non-trivial consequences arise for the system because the target may become “poisoned,” i.e., a compound layer forms on the target surface affecting the sputtering yield and the yield of secondary electron emission and thereby all other parameters. It is emphasized that the target state depends not only on the reactive gas' partial pressure (balanced via gas flow and pumping) but also on the ion flux to the target, which can be controlled by pulse parameters. This is a critical technological opportunity for reactive HiPIMS (R-HiPIMS). The scope of this tutorial is focused on plasma processes and mechanisms of operation and only briefly touches upon film properties. It introduces R-HiPIMS in a systematic, step-by-step approach by covering sputtering, magnetron sputtering, reactive magnetron sputtering, pulsed reactive magnetron sputtering, HiPIMS, and finally R-HiPIMS. The tutorial is concluded by considering variations of R-HiPIMS known as modulated pulsed power magnetron sputtering and deep-oscillation magnetron sputtering and combinations of R-HiPIMS with superimposed dc magnetron sputtering.

Modeling of reduced effective secondary electron emission yield from a velvet surface

Complex structures on a material surface can significantly reduce total secondary electron emission from that surface. A velvet is a surface that consists of an array of vertically standing whiskers. The reduction occurs due to the capture of low-energy, true secondary electrons emitted at the bottom of the structure and on the sides of the velvet whiskers. We performed numerical simulations and developed an approximate analytical model that calculates the net secondary electron emission yield from a velvet surface as a function of the velvet whisker length and packing density, and the angle of incidence of primary electrons. We found that to suppress secondary electrons, the following condition on dimensionless parameters must be met: (π/2)DA tan θ≫1, where θ is the angle of incidence of the primary electron from the normal, D is the fraction of surface area taken up by the velvet whisker bases, and A is the aspect ratio, A ≡ h/r, the ratio of height to radius of the velvet whiskers. We find that velvets available today can reduce the secondary electron yield by 90% from the value of a flat surface. The values of optimal velvet whisker packing density that maximally suppresses the secondary electron emission yield are determined as a function of velvet aspect ratio and the electron angle of incidence.

Validation and benchmarking of two particle-in-cell codes for a glow discharge

The two particle-in-cell codes EDIPIC and LSP are benchmarked and validated for a parallel-plate glow discharge in helium, in which the axial electric field had been carefully measured, primarily to investigate and improve the fidelity of their collision models. The scattering anisotropy of electron-impact ionization, as well as the value of the secondary-electron emission yield, are not well known in this case. The experimental uncertainty for the emission yield corresponds to a factor of two variation in the cathode current. If the emission yield is tuned to make the cathode current computed by each code match the experiment, the computed electric fields are in excellent agreement with each other, and within about 10% of the experimental value. The non-monotonic variation of the width of the cathode fall with the applied voltage seen in the experiment is reproduced by both codes. The electron temperature in the negative glow is within experimental error bars for both codes, but the density of slow trapped electrons is underestimated. A more detailed code comparison done for several synthetic cases of electron-beam injection into helium gas shows that the codes are in excellent agreement for ionization rate, as well as for elastic and excitation collisions with isotropic scattering pattern. The remaining significant discrepancies between the two codes are due to differences in their electron binary-collision models, and for anisotropic scattering due to elastic and excitation collisions.

The role of Ohmic heating in dc magnetron sputtering

Sustaining a plasma in a magnetron discharge requires energization of the plasma electrons. In this work, Ohmic heating of electrons outside the cathode sheath is demonstrated to be typically of the same order as sheath energization, and a simple physical explanation is given. We propose a generalized Thornton equation that includes both sheath energization and Ohmic heating of electrons. The secondary electron emission yield is identified as the key parameter determining the relative importance of the two processes. For a conventional 5 cm diameter planar dc magnetron, Ohmic heating is found to be more important than sheath energization for secondary electron emission yields below around 0.1.

Secondary electron emission of graphene-coated copper

The secondary electron emission of graphene-coated copper has very interesting properties. In this work, we prepared graphene-coated copper foil and measured both the secondary electron emission yield (SEY) and the secondary electron energy spectra. Compared with the copper base, there is a significantly reduction for the SEY. For the copper base without coating, the max value of SEY is beyond 2.1 for the primary electron energy of 300eV. However, the corresponding value for the graphene-coated copper is about 1.5, reduced about 25%. For the secondary electron energy distribution, the elastically backscattered peak becomes higher and peak of the true secondary electrons is shifted from about 1.5eV to 4eV after coating the copper with graphene. The full width at half maximum of the true secondary electrons peak is also increased. Since the position of the peak is related to the surface potential barrier of the material, we believe that the graphene coating could increase the surface potential barrier of the material. This could also be the reason of SEY reduction because a higher potential barrier can reduce the electron emission from the material. A preliminary theoretical model of the surface potential distribution for copper surface with graphene coating was built and used to analyze the secondary electron emission of metal surface with graphene coating. The graphene coating is found to be an effective method to suppress the secondary electron emission.

The role of the metastable O2(b) and energy-dependent secondary electron emission yields in capacitively coupled oxygen discharges

The effects of including the singlet metastable molecule O2(b) in the discharge model of a capacitively coupled rf driven oxygen discharge are explored. We furthermore examine the addition of energy-dependent secondary electron emission yields from the electrodes to the discharge model. The one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 is used for this purpose, with the oxygen discharge model considering the species , , , O(3P), O(1D), , O+, O−, and electrons. The effects on particle density profiles, the electron heating rate profile, the electron energy probability function and the sheath width are explored including and excluding the metastable oxygen molecules and secondary electron emission. Earlier we have demonstrated that adding the metastable O2(a) to the discharge model changes the electron heating from having contributions from both bulk and sheath heating to being dominated by sheath heating for pressures above 50 mTorr. We find that including the metastable O2(b) further decreases the ohmic heating and the effective electron temperature in the bulk region. The effective electron temperature in the electronegative core is found to be less than 1 eV in the pressure range 50–200 mTorr which agrees with recent experimental findings. Furthermore, we find that including an energy-dependent secondary electron emission yield for -ions has a significant influence on the discharge properties, including decreased sheath width.