Secondary Electron Yield Curve Research Articles

Charging and Mobilization of Dust Particles on a Surface in Plasma.

We present first experimental results showing that single dust particles on a dielectric surface are mobilized and lofted due to exposure to an electron beam or ultraviolet radiation. It is shown that secondary electrons and/or photoelectrons emitted from a substrate surface are recollected on the surfaces within microcavities between a dust particle and the substrate surface, resulting in large negative charges and subsequently causing mobilization of the dust particle due to Coulomb repulsion. Dust mobility tested against the electron beam energy is shown to follow the secondary electron yield curve of the substrate surface in both the experimental and modeling results. The results verified the role of emitted electrons from the substrate surface in charging and mobilization of single dust particles.

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.

Investigation on Multipactor in Double-Sided Dielectric-Loaded Microwave Components

This article presents a study of the multipactor effect in a double-sided dielectric-loaded parallel-plate waveguide. To this end, a 1-D electrostatic particle-in-cell model is established, which considers the radio frequency electric fields, space charge fields, and surface charge fields generated by the accumulated charge on the dielectric surface. The dynamic evolution of multipactor breakdown for different operating voltages and dielectric materials with different secondary electron yield (SEY) properties is investigated by numerical calculation. The results obtained show that multipactor does occur self-extinguishing phenomenon due to the presence of the surface charge fields under certain circumstances in the double-sided dielectric-loaded parallel-plate waveguide. Moreover, a comparative study of multipactor between metal, single-sided, and double-sided dielectric-loaded parallel-plate waveguides is carried out. The simulation results show that the self-extinguishing time of the multipactor in the double-sided dielectric-loaded model is less than that in the single-sided dielectric-loaded model. Finally, a multipactor susceptibility diagram for the SEY curve of different bottom dielectric materials in the parallel-plate waveguide is constructed. The results show that the single-sided dielectric-loaded microwave components are more beneficial to suppress the multipactor effect. The results reported in this study can be used to guide the design of space high-power microwave components.

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

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

Non-monotonic dependence of heat loads induced by electron cloud on bunch population at the LHC

Electron cloud effects are among the main performance limitations for the operation of the Large Hadron Collider with 25 ns bunch spacing. Electrons impacting on the beam screens of the superconducting magnets induce a significant heat load reaching values close to the full cooling capacity available from the cryogenic system in some LHC sectors. To better understand this performance limitation, numerical simulations with the PyECLOUD code were performed to study the dependence of the heat load on different beam and machine parameters, in particular the bunch population, which is foreseen to be considerably increased with the impending HL-LHC upgrade. The simulations predict a complex, non-monotonic behavior of the heat load with bunch population which has important implications in defining the upgrade of the cryogenic system required for coping with HL-LHC beam intensities. An in-depth analysis of the simulation results shows that the non-monotonic dependence of the heat load on the bunch population is driven by an interplay between the spectrum of the impacting electrons and the shape of the Secondary Electron Yield curve. Experimental data were collected at the LHC during normal operation and dedicated experiments in order to validate the simulation model and confirm the expected non-monotonic behavior. The simulation results are found to reproduce very well the measurement data.

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.

Secondary electron emission yield measurements of dielectrics based on a novel collector-only method

The total electron-induced Secondary Electron Yield (SEY) of alkali-free glass has been determined by a novel technique which is solely based on the use of a collector electrode and of two interchangeable dielectric samples. The current from a charge-saturated sample provides a direct measure of the primary beam current. The SEY from the (uncharged) sample under investigation can then be determined simply as the ratio between the collector currents measured at the same electron energy from both samples. We further apply a low-current, short-pulse procedure to limit sample charging, needed to mitigate distortion of the SEY by charging effects. The potential build-up during the experiment could be reconstructed based on the primary electron currents and the measured SEY. This strategy permitted us to measure a 6-point SEY curve, without the need for intermediate discharging of the sample, as the total induced surface potential rise could be kept below +2.55 V.

Evolution of dielectric surface potential induced by electron beam radiation

<p indent=0mm>Spacecrafts operate in a complex space plasma environment and are constantly exposed to cosmic ray radiation. The charging and discharging phenomena that occur on the dielectric surfaces result in major reliability issues for the spacecraft’s normal operation. Secondary electron emission (SEE) caused due to electron radiation is the primary inducement of dielectric surface charging, and the SEE level severely affects the charging and discharging status. In this paper, to discover the dynamic evolution process of dielectric surface charging under the circumstances of electron beam irradiation, we have conducted a systematic study on the surface potential and SEE behaviors under various irradiation situations. First, we fitted the secondary electron yield (SEY) curves for three dielectric materials with high resistance, namely MgO, Al₂O₃, and ZnO, and extracted the two critical energies (<italic>E</italic>₁ and <italic>E</italic>₂, where the SEY value equals 1 for the first and second time, respectively), SEY peak values, and the corresponding primary electron energy of the SEY peak value from the three SEY curves. Then, for the situation of electron beam radiating dielectric materials, we developed a physical model and simulated the dynamic evolution processes of the dielectric surface potential. The radiation source is a pulsed electron beam, and some radiating conditions include the following: The intensity of the beam current is set as 1 × <sc>10⁻⁹ A,</sc> the diameter of the electron beam spot is 1 × <sc>10⁻⁸ m,</sc> the width of a single pulse is 0.9 × <sc>10⁻⁹ s,</sc> and the period duration of a single pulse is 1 × <sc>10⁻⁹ s.</sc> We calculated the charge accumulating amount after pulse radiation and the corresponding variation quantity of the surface potential based on these assumed radiation parameters and the ideal charging situation without charge leakage. We discovered the evolution regularities of the potential variation rate, potential level, and potential balanced duration for the three irradiated materials by calculating the transient surface potential. Furthermore, we have simulated the dependence of surface potential on the radiation energy (<italic>E</italic>_p) of the electron beam. Several conclusions are summarized by analyzing the simulation and calculation results. First, the SEE characteristics of dielectric materials, particularly the two critical energy values <italic>E</italic>₁ and <italic>E</italic>₂, are the two key factors that affect positive or negative charging, as well as the balanced duration and balanced surface potential level. Second, the negative balanced surface potential induced by low energy electrons (<italic>E</italic>_p><italic>E</italic>₂) is only a few tens of volts; the balanced time is a few hundred pulses, and the surface potential variation rate first increases and then decreases. Third, the negative balanced surface potential induced by the high energy electrons (<italic>E</italic>_p><italic>E</italic>₂) is relatively high and its magnitude can reach <sc>10000 V;</sc> moreover, the balanced duration is relatively long (about several tens of thousands pulse duration). Fourth, because the positive balanced surface potential induced by the middle energy electrons (<italic>E</italic>₁<<italic>E</italic>_p<<italic>E</italic>₂) is extremely low, and the balanced time is short (less than a few tens of seconds), the variation rate of the positive surface potential rapidly decreases. Fifth, the dielectric material with a higher <italic>E</italic>₂ value exhibits a lower negative balanced surface potential and a long balanced duration, and this property makes the dielectric material with a high <italic>E</italic>₂ value play a more significant role in the suppression of surface charging. In this paper, we have discussed in detail the dynamic evolution regularities of surface potential when the dielectric is irradiated by an electron beam, which is beneficial for further analysis about dielectric surface charging, discharging, and further research on the charging defense.

Suppression of single-surface multipactor discharges due to non-sinusoidal transverse electric field

It is of importance to suppress single-surface multipactor discharges in high power microwave devices. In this work, both particle-in-cell (PIC) and Monte-Carlo simulations demonstrate that multipactor discharges can be significantly suppressed by a temporal Gaussian-type transverse electric field waveform. Decreasing the half peak width of the Gaussian electric field can reduce the time-averaged positive charge density on the surface, corresponding to the strength of the multipactor, by an order of magnitude at fixed time-averaged input power. The underlying physical mechanism is revealed by examining the electron impact energy and angle distribution in detail, as well as the dynamic secondary electron yield (SEY) from PIC simulation. For the smaller half peak width and fixed average input power, more electrons striking the surface have energies either below the first crossover or higher than the second crossover of the SEY curve, giving rise to weaker secondary electrons emission and finally resulting in a weaker multipactor discharge. In addition, we give the analytical expressions of the frequency spectrum and phase shift needed to recover a Gaussian-type waveform, which is in excellent agreement with numerical calculations.

SEY and low-energy SEY of conductive surfaces

The study of Secondary Electron Yield (SEY) is widely performed to address important properties of materials to be used in a very wide spectrum of applications. It is, therefore, extremely important to understand the SEY dependence on material type, surface contaminants, structural quality and surface damage. We review here our recent studies of such items performed by looking at some representative conductive materials as noble metals and carbon based surfaces. Polycrystalline Ag, Au and Cu samples have been studied as introduced in the ultra-high vacuum chamber (therefore with an significant surface contamination) and after having been cleaned by ion sputtering. The comparison between the curves confirms that the SEY behavior is strongly influenced by the chemical state of the metal surfaces. We demonstrate the ability of SEY to determine work function values with high accuracy if the experimental system allows using very slow primary electrons. We also investigated, for the Cu sample, the effect on SEY of minimal amount of contaminants in the sub-monolayer regime showing that SEY is highly sensitive to the presence of adsorbates even at such very low coverages, specially for low energy primary electrons. In the case of C surfaces we summarize here the effect that the structural ordering of the C lattice has on the macroscopic SEY properties of ultrathin C layers. In particular we followed the SEY evolution during the thermal graphitization of thin amorphous carbon layers and during the amorphization of highly oriented pyrolytic graphite by means of Ar+ bombardment. In the first case the SEY decrease observed with the progressive conversion of sp3 hybrids into six-fold aromatic domains was related to the electronic structure of the C-films close to the Fermi level. We found that a moderate structural quality of the C layer, corresponding to aromatic clusters of limited size, is sufficient to obtain a SEY as low as ∼1. For the bombarded graphite, the strong lattice damage remains limited to the near surface layer, where the high density of defects reduces the transport of incoming and secondary electrons. Then, the SEY curves resulted differently modified in the low and high primary energy regions, but their maximal values remained favorably low. Our findings demonstrate that SEY, besides being an indispensable mean to qualify technical materials in many technological fields, can be also used as a flexible and advantageous diagnostics to probe surfaces and interfaces.

Study on the maximum stable output of a novel s-band micro-pulse electron gun

A novel S-Band Micro-Pulse electron Gun (MPG) which works at the left crossover point energy EcI on the Secondary Electron Yield (SEY) curve was proposed. The working principles of the MPG were presented. The maximum output beam current limited by space charge effects and beam loading effects was investigated by theoretical analysis. The result shows that the maximum beam current is decided by the parameters of the MPG such as resonant frequency, cavity length, shunt impedance and the secondary emission property of the cathode and grid, having nothing to do with the power flowed into the cavity. The low shunt impedance and the low slope of the SEY curve can help increasing the maximum beam current. According to the principles, a MPG with the frequency of 2.856 GHz has been designed and constructed. The steady working state was achieved by using oxygen free copper and molybdenum grid with different transmission coefficient. It was found a good agreement between the analysis and the experiments.

The secondary electron yield of noble metal surfaces

Secondary electron yield (SEY) curves in the 0-1000 eV range were measured on polycrystalline Ag, Au and Cu samples. The metals were examined as introduced in the ultra-high vacuum chamber and after having been cleaned by Ar+ ion sputtering. The comparison between the curves measured on the clean samples and in the presence of contaminants, due to the permanence in atmosphere, confirmed that the SEY behavior is strongly influenced by the chemical state of the metal surface. We show that when using very slow primary electrons the sample work function can be determined with high accuracy from the SEY curves. Moreover we prove that SEY is highly sensitive to the presence of adsorbates even at submonolayer coverage. Results showing the effect of small quantities of CO adsorbed on copper are presented. Our findings demonstrate that SEY, besides being an indispensable mean to qualify technical materials in many technological fields, can be also used as a flexible and advantageous diagnostics to probe surfaces and interfaces.

Secondary electron emission characteristics of graphene films with copper substrate**Supported by the National Natural Science Foundation of China (11475166) National Natural Science Foundation of China (11205155) and National Natural Science Foundation of China (11575214).

For modern and future circular accelerators, especially high-intensity proton synchrotrons or colliders, the electron cloud effect is a key issue. So, in order to reduce the electron cloud effect, exploring very low secondary electron yield (SEY) material or coating used in vacuum tubes becomes necessary. In this article, we studied the SEY characteristics of graphene films with different thicknesses which were deposited on copper substrates using chemical vapor deposition. The SEY tests were done at temperatures of 25 °C and vacuum pressure of (2 − 6) × 10−9 torr. The properties of the deposited graphene films were investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The SEY curves show that the number of graphene layers has a great effect on the SEY of graphene films. The maximum SEY of graphene films decreases with the increase of the number of layers. The maximum SEY of 6–8 layers of graphene film is 1.25. These results have a great significance for next-generation particle accelerators.

The effect of structural disorder on the secondary electron emission of graphite

The dependance of the secondary electron yield (SEY) on the degree of crystallinity of graphite has been investigated during the amorphization of a highly oriented pyrolytic graphite (HOPG) samples by means of Ar+ bombardment. Photoemission and Raman spectroscopies were used to follow the structural damage while the SEY curves were measured from very low energies up to 1000 eV. We found that the increase of lattice defects lowers the contribution of the π electrons in the valence band and loss spectra and smears out the intense modulations in the low energy secondary electron yield (LE-SEY) curve. Raman spectroscopy results showed that ion induced lattice amorphization is confined in a near-surface layer. The evolution of SEY curves was observed with the progressive Ar+ dosage after crystal damage as due to the modification of the electronic transport properties within the damaged near surface layer.

Detailed Investigation of the Low-Energy Secondary Electron Yield (LE-SEY) of Clean Polycrystalline Cu and of Its Technical Counterpart

The detailed study of the secondary electron yield (SEY) of technical surfaces for very low electron landing energies (from 0 to 20 eV) is a very important parameter in many fields of research. Some of the devices used in all those fields of research base some of their essential functionalities on the number of electrons produced by a surface when hit by other electrons, namely, its SEY and, in most cases, its very-low-energy behavior SEY (LE-SEY). Despite such interest, the very low electron landing energy part of an SEY curve has been rarely addressed due to the intrinsic experimental complexity to control and detect very low energy electrons. Furthermore, several results published in the past have been recently questioned to suffer from experimental systematic errors. In this paper, we critically review the experimental method used to study LE-SEY and define more precisely the energy region, in which the experimental data can be considered valid. By analyzing the significantly different behavior of LE-SEY in atomically clean polycrystalline Cu and in its as-received technical counterpart, we solve most, if not all, of the apparent controversy present in the literature, producing important inputs for better understanding the devices performances related to their LE-SEY.

Effect of the surface processing on the secondary electron yield of Al alloy samples

In this study we have investigated the relation between the secondary electron yield (SEY) and the surface chemical state for technical Al alloy samples cut from the inner walls of the Petra III storage ring. SEY curves measured after prolonged electron beam irradiation at 500 eV showed maximum values (${\ensuremath{\delta}}_{\mathrm{max}}$) between 1.8 and 1.5. By combining x-ray photoelectron spectroscopy with SEY measurements, we have been able to relate the surface chemical composition to the ${\ensuremath{\delta}}_{\mathrm{max}}$ values for the ``as-received'' surface (${\ensuremath{\delta}}_{\mathrm{max}}=2.7$), for the electron beam conditioned sample (${\ensuremath{\delta}}_{\mathrm{max}}=1.8\ensuremath{-}1.5$), and after substantially removing the surface contaminating layer by means of ${\mathrm{Ar}}^{+}$ ion sputtering (${\ensuremath{\delta}}_{\mathrm{max}}=1.3$). Our detailed chemical analysis shows that the SEY strongly increases in the presence of the thin surface oxide film which unavoidably forms on the clean Al alloy sample under electron beam irradiation even in ultrahigh vacuum conditions, and suggests that the high reactivity of pure Al and Al alloys to oxygen could be the cause of the difference among the SEY values measured in different ultrahigh vacuum environments.

Secondary Electron Yield Curve for Liquid Water

We have measured the secondary electron yield curve for liquid water using an Environmental SEM. The secondary electron emission coefficient, measured as a function of incident electron energy, is important for interpreting contrast in hydrated biological and inorganic specimens. This information is even more critical for water than other materials, as it is a factor of prime importance in understanding radiation damage in biological tissues.[1]These measurements were taken using a Philips XL-30 field emission ,ESEM, and repeated on an Electroscan E3 ESEM, equipped with a CeB6 filament. A specially designed Faraday cup was fashioned from brass and fitted with a removable graphite cup having an inset for a platinum aperture. This assembly was placed into an electrically floating Peltier cooling stage, and connected to a KE Instruments probe current meter.

Numerical simulation of secondary electron emission charging at insulator surfaces

The charging process of insulator surfaces in vacuum at HV due to secondary electron emission has been calculated using numerical simulation. The systems considered consist of two electrodes at a HV difference, separated by a glass plate or a hollow glass channel. The simulations give insight into the critical HV positions and into the influence of the geometry, the initial voltage distribution and the material properties on the charging profiles and the leakage current paths. A HV performance increase of these systems can be obtained by decreasing the impact of free electrons, adding a coating on all insulating surfaces which have a high value of E/sub 1/ (i.e. the first crossover point of the secondary electron yield curve) and/or are weakly conductive, and breaking up the total voltage difference in smaller parts by adding electrodes to all surfaces at intermediate voltages.

Power deposited on a dielectric by multipactor

We use a simple transmission line model to evaluate the RF power deposited on a dielectric window by a multipactor discharge. The calculation employs Monte Carlo simulation, using realistic secondary electron yield curves as input, and taking into account the distributions in the emission velocities and emission angles of the secondary electrons. Beam loading on the external RF, as well as the evolution of the DC electric field due to dielectric charging, are also accounted for. It is found that the buildup of the multipactor space charge, rather than beam loading, causes saturation. Over a wide range of operating conditions and materials, it is found quite generally that the multipactor delivers on the order of 1 percent, or less, of the RF power to the dielectric. A simple estimate is given in support of this ratio, using the susceptibility diagram that was constructed from kinematic considerations. Comparison with experimental results is given.

Background and applications of electron beam test techniques

The use of electron beams for contactless testing of electrical functions and electrical integrity of different active devices in VLSI - chips - has been demonstrated over the past years. This method of testing electronic networks is based on an electron probe which is moved from point to point in the network and a current of secondary electrons emitted in response to the impingement of the probe is converted to a signal indicating the presence of a voltage or varying potential at the different points. Voltage contrast, electron beam induced current, dual potential approach, stroboscopic techniques and other methods have been developed and are used to detect different functional failures in devices.Very little attention in most of the applications has been paid to the electron optical environment, mostly SEM's were upgraded or converted to do the job of a “voltage contrast” machine. This by no means will satisfy all requirements and new thoughts will have to be given to aspects such as: low voltage electron guns, two lens systems, different means of detection, signal processing and storage are a few of them.Besides the VLSI application, the contactless testing of three dimensional conductor networks of a 10 cm × 10 cm × .8 cm multilayer ceramic poses a different and new application. The mechanism of electron-solid interaction, the secondary electron generation, the energy distribution, the secondary electron yield, are important parameters to understand for any electron beam test technique. According to the secondary electron yield curve in Figure 1, floating samples can be charged positively or negatively with respect to ground, depending on the primary electron energy.The surface potential of a sample in turn determines the impact energy of the primary electrons and also energy of the secondaries that leave the surface. These two effects generate different voltage contrasts to be used in electron beam testing. The test method for packages uses multiple electron beams to both generate and detect voltage differences between the surface terminations of conductors in order to identify opens and short in the package networks.