Secondary Emission Yield Research Articles

Special Features of Parasitic Current Formation in a Sealed-Off Cold-Cathode Thyratron with Trigger Unit Based On an Auxiliary Glow Discharge

Results of investigation of the glow discharge with hollow cathode and ring anode in the trigger unit of a coldcathode thyratron TPI1-10k/50 are presented. A specific feature of the discharge sustainment is that a highly emissive tablet containing cesium carbonate is placed in the cathode cavity. The current-voltage characteristics of the discharge in the trigger unit are obtained for different tablet compositions together with the measured parasitic current to the main cathode cavity. Stepwise transitions to the regime with decreased discharge burning voltage accompanied by an increase in the parasitic current are observed. A model of current sustainment in a hollow-cathode glow discharge is used to interpret the characteristics obtained. Instead of the conventional secondary emission yield, the model uses a generalized emission yield that takes into account not only the ion bombardment of the cathode, but also the emission current from an external source. Based on estimations of the discharge parameters in the trigger unit, a reason for a parasitic current increase is revealed.

Dynamic secondary electron emission in rough composite materials

The interaction of ionizing radiation with matter is of critical importance in numerous areas of science and technology like space and vacuum technology and even medicine and biotechnology. Secondary electron emission is a consequence of electron irradiation on materials. We achieve extremely low secondary electron emission yield values smaller than 0.2, even up to incident electron energies ~1 keV, due to an undocumented synergy between neighbouring metal and dielectric domains in composite samples. To investigate this experimental discovery, we propose a simple 3D model where the dielectric and metallic domains are arranged in parallel and interleaved. The proposed surface profile has a triangular shape to model the surface roughness. We obtain a continuous equation to describe the electric field that arises between grounded conductors and charged dielectrics domains. The calculated trajectories of secondary electrons in this 3D geometry are used to predict dynamic secondary emission yield, which strongly depends on the charge accumulated in the dielectric domains. This research paves the way to design new materials of low secondary emission yield, addressing the technological problem not yet resolved to inhibit the electron avalanche in RF equipment that limit their maximum working power.

Helical resonator filters with improved multipactor performance exploiting rigorous modelling and the large gap approach

The power handling capability of helical resonator filters is studied by means of the parallel plate model (employed in the European Cooperation for Space Standardisation) and more rigorous modelling techniques (like the one available in the commercial software tool SPARK3D™) as well as through an experimental test campaign. The results indicate that the parallel plate model provides conservative power handling capabilities for this class of filters, while rigorous modelling can better capture the impact of the geometrical features on multipactor evolution. Although the accuracy of the estimated power handling depends on the knowledge of the practical secondary emission yield values, the use of such rigorous modelling tools enables the design of helical resonator filters with improved power handling capability by exploiting the large gap approach, therefore opening opportunities to avoid additional dielectric fillings.

On the connection between secondary electron emission yield and the potential of an electron-beam-irradiated target

An unambiguous correspondence between the potential of an insulated metal target, irradiated by a continuous electron beam, and secondary electron emission yield from the target, in the medium vacuum pressure range (at fore-vacuum pressures), is demonstrated. This connection allows one to estimate the secondary emission properties of the target material and to predict the beam-plasma parameters. Moreover, this can help to estimate the intensity and energy of the ion bombardment of the target when it is irradiated by the electron beam. The metallic target potential value was found to be dependent on its material (aluminum, stainless steel, titanium and copper), which is related to the secondary emission properties of the surface. Since an electrically insulated metal target can be regarded as an experimental model for exploring the effect of energetic electron beam bombardment of dielectric surfaces, this work also provides a firm basis for the study of electron-beam modification of dielectric materials. A method for estimation of the potential of a dielectric having a secondary emission yield less than 1 and irradiated by a dc electron beam in fore-vacuum pressures is proposed. This method is based on measurements of the ionic sheath length and plasma parameters near the surface of the target irradiated by the electron beam.

Calculations of Multipactor Growth in Rectangular Waveguides

Multipactor growth in rectangular waveguides is probed based on a kinetic approach. Unlike most studies relying on the Vaughan model, a probabilitic approach for random multiple secondary particle emissions is used. Spread in electron emission velocities, the angular dependence of secondary emission yields, and an external radio frequency (RF) driving field due to a TE10 mode, were all built in. The calculations predict the secondary emission yield for copper, probe the population growth dynamics, and obtain the susceptibility diagram. Despite a maximum field at the waveguide center from the RF excitation, maximum electron densities are predicted at locations symmetrically displaced from the center. The secondary electron yield (SEY) characteristics, its local maxima, and the role of oblique incident angles, collectively lead to multipactor finding its place at off-center locations.

Comparison of the Techniques for Multipactor Discharge Damping in Coupling Cells of CDS Accelerating Structure

In the CDS structure proposed to substitute the first cavity of the main part of the linac at the Institute for Nuclear Research of the Russian Academy of Sciences, the appearance of a multipactor discharge (MP) in coupling cells at the operating level of the accelerating field was detected. Earlier, an option for MP damping by alternating neighbor accelerating cell frequency detuning was considered. This option complicates the cavity section tuning. An option for MP removal by changes in the coupling cell geometry inducing the destruction of phase conditions of the electron cloud resonance motion was studied. The analytical estimation and results of numerical simulations are presented. In conditions of the first cavity of the INR linac, the options considered were compared with respect to the efficiency of MP damping and the enhanced resistivity to the increase of the secondary emission yield.

Multipacting in an RF Window: Simulations and Measurements

Electron guns are used in the accelerators of the European XFEL and FLASH. They are operated at 1.3 GHz. The RF peak power is 5 MW at 650 μs pulse width and 10 Hz repetition rate. In order to understand the multipacting that occurs during conditioning, it was simulated in the RF window type that is used for the electron gun in the XFEL. The reduction in secondary emission yield associated with conditioning was taken into account. Since the RF windows are tested with high power on a test stand before their use, without the electron gun, measurement results are available which are compared with the simulation results. The main advantage of the simulation compared to the measurement is that the locations of multipacting can be determined in the RF window. This could be helpful for the development of high-power RF components in the future, in order to detect pronounced multipacting resonances even before production and to avoid them by design changes.

A new fit to secondary emission yield in the low impact voltage regime: An improvement of Vaughan’s expression

Reducing the emission of secondary electrons from materials is critical to improved efficiency and increased performance in high power vacuum electronics. A new mathematical expression for the secondary emission yield (SEY) as a function of the impact voltage up to a maximum of 5 kilovolts is proposed which is an extension of a formula first suggested by Vaughan. The new analytical fit and Vaughan’s fit are compared with SEY experimental data reported by others and measured by our group. The new analytical expression gives good fits to SEY experimental data in all cases, even when the SEY maximum is either slightly larger or below unity, two situations for which Vaughan’s fit is either inadequate or inapplicable.

The effects of secondary emission on the sheath structure in an electrostatic dusty plasma containing energetic electrons and charged nanoparticles

Multi-fluid numerical simulations are utilized to explore the effects of secondary emission by nanosize dust particles on the structure of a dusty plasma sheath in the presence of a beam of fast, mono-energetic electrons. It was found that the sheath dynamics depends strongly on the magnitude of the secondary emission yield δm. For δm smaller than unity, the secondary emission is weak, and the sheath width always increases with increasing beam flux, such that it experiences a sharp transition from the regime of thin sheath to the regime of thick sheath, at a given beam flux. For δm larger than unity, the secondary emission dominates the dust dynamics, and the sheath width always decreases with increasing beam flux. The sheath thickness decreases very quickly with the secondary emission yield, but increases with Em, the characteristic energy corresponding to the maximum secondary emission. As δm is increased, the absolute dust charge and hence the accelerating ion drag force are reduced. Then, the dust is decelerated and as a result the dust number density is enhanced. Increasing the dust radius and/or the dust number density leads to an enhanced secondary emission effect and thus to a narrower sheath width.

25aZB-9 炭素クラスターイオン照射による2次イオン放出(放射線物理(クラスター・2次粒子放出),領域1,原子・分子,量子エレクトロニクス,放射線物理)

25aZB-9 炭素クラスターイオン照射による2次イオン放出(放射線物理(クラスター・2次粒子放出),領域1,原子・分子,量子エレクトロニクス,放射線物理)

Stationary statistical theory of two-surface multipactor regarding all impacts for efficient threshold analysis

Statistical multipactor theories are critical prediction approaches for multipactor breakdown determination. However, these approaches still require a negotiation between the calculation efficiency and accuracy. This paper presents an improved stationary statistical theory for efficient threshold analysis of two-surface multipactor. A general integral equation over the distribution function of the electron emission phase with both the single-sided and double-sided impacts considered is formulated. The modeling results indicate that the improved stationary statistical theory can not only obtain equally good accuracy of multipactor threshold calculation as the nonstationary statistical theory, but also achieve high calculation efficiency concurrently. By using this improved stationary statistical theory, the total time consumption in calculating full multipactor susceptibility zones of parallel plates can be decreased by as much as a factor of four relative to the nonstationary statistical theory. It also shows that the effect of single-sided impacts is indispensable for accurate multipactor prediction of coaxial lines and also more significant for the high order multipactor. Finally, the influence of secondary emission yield (SEY) properties on the multipactor threshold is further investigated. It is observed that the first cross energy and the energy range between the first cross and the SEY maximum both play a significant role in determining the multipactor threshold, which agrees with the numerical simulation results in the literature.

Photoelectron backscattering in the microchannel plate photomultiplier tube

The late pulse occurring in photomultiplier tubes (PMTs) is attributed to photoelectron backscattering. The small effective open area of microchannel plates (MCPs) increases the probability of photoelectrons bouncing on the front surface of MCPs and aggravates late pulses. In this work, a three-dimensional MCP-PMT model is developed in CST particle studio, a powerful electromagnetic field simulation program, to evaluate the effects of secondary emission yield (SEY) property of the MCP input facet and electric field on the photoelectron backscattering. Timing properties and the number of collected electrons are extracted without smearing due to electronic noise and finite pulse width. Results predict that the collection efficiency (CE) and the time performance can be improved by coating the MCP Nickel–Chromium input electrode with a high SEY material and operating in a proper photocathode-MCP bias voltage.

Progress on RE2O3-Mo/W matrix secondary emitter materials

Two types of secondary emitter materials, the rare earth oxides (RE2O3) doped Mo cermet cathodes and the Y2O3-W matrix pressed cathode, are introduced in this paper. According to the calculation results, Y2O3 exhibits the best secondary emission property among Y2O3, La2O3, CeO2 and Lu2O3. The rare earth oxides co-doped Mo cathodes in which Y2O3 is the main active substance exhibit better secondary emission property than single RE2O3 doped Mo cathode. The results obtained by the Monte-Carlo calculation method show that the secondary electron emission property is strongly related to the grain size of the cathode. The decreasing of the grain size reduces the positive charge effect of the rare earth oxide due to the electrons supplement from the metal to the rare earth oxide, whereby the secondary electrons are easier to escape into the vacuum. Y2O3 is introduced into Ba-W cathode to fabricate a pressed Y2O3-W matrix dispenser cathode. The result indicates that the secondary emission yield of the Ba-W cathode increases from 2.13 to 3.51 by adding Y2O3, and the thermionic emission current density (J 0) could reach 4.18 A/cm2 at 1050 °Cb.

Effect of space charge on the negative oxygen flux during reactive sputtering

Negative ions often play a distinctive role in the phase formation during reactive sputter deposition. The path of these high energetic ions is often assumed to be straight. In this paper, it is shown that in the context of reactive magnetron sputtering space charge effects are decisive for the energetic negative ion trajectories. To investigate the effect of space charge spreading, reactive magnetron sputter experiments were performed in compound mode with target materials that are expected to have a high secondary ion emission yield (MgO and CeO2). By the combination of energy flux measurements, and simulations, a quantitative value for the negative oxygen ion yield can be derived.

Study of multipactor effect with applications to superconductive radiofrequency cavities

In this paper a one-dimensional Particle-in-Cell/Monte Carlo collision code has been used in order to study characteristics of multipactors. For multipactor to occur each electron striking the surface must generate more than one secondary on average. The ratio of primary to secondary electrons is given by the secondary emission yield. For this study, calculations were carried out by using Sternglass model that includes energy dependence of the secondary emission yield. The obtained simulation results for the pressure dependence of the breakdown time follow the scaling law. Number of electrons increases in time, while their mean energy decreases. Since secondary electron emission at the cavity surface plays an important role, simulation results, presented here, can help cavity designers predict multipacting issues before fabrication.

Ignition and monitoring technique for plasma processing of multicell superconducting radio-frequency cavities

An in-situ plasma processing technique has been developed at the Spallation Neutron Source (SNS) to improve the performance of the superconducting radio-frequency (SRF) cavities in operation. The technique uses a low-density reactive neon-oxygen plasma at room-temperature to improve the surface work function, to help remove adsorbed gases on the RF surface, and to reduce its secondary emission yield. SNS SRF cavities have six accelerating cells and the plasma typically ignites in the cell where the electric field is the highest. This article details the technique to ignite and monitor the plasma in each cell of the SNS cavities.

Secondary Electron Emission of Pt: Experimental Study and Comparison With Models in the Multipactor Energy Range

Experimental data of secondary emission yield (SEY) and electron emission spectra of Pt under electron irradiation for normal incidence and primary energies lower than 1 keV are presented. Several relevant magnitudes, as total SEY, elastic backscattering probability, secondary emission spectrum, and backscattering coefficient, are given for different primary energies. These magnitudes are compared with theoretical or semiempirical formulas commonly used in the related literature.

Auxiliary glow discharge in the trigger unit of a hollow-cathode thyratron

Results from studies of a low-current glow discharge with a hollow cathode are presented. A specific feature of the discharge conditions was that a highly emissive tablet containing cesium carbonate was placed in the cathode cavity. In the absence of a tablet, the discharge ignition voltage was typically ≥3.5 kV, while the burning voltage was in the range of 500–600 V. The use of the tablet made it possible to decrease the ignition voltage to 280 V and maintain the discharge burning voltage at a level of about 130 V. A model of the current sustainment in a hollow-cathode discharge is proposed. Instead of the conventional secondary emission yield, the model uses a generalized emission yield that takes into account not only ion bombardment of the cathode, but also the emission current from an external source. The model is used to interpret the observed current−voltage characteristics. The results of calculations agree well with the experimental data. It is shown that, in some discharge modes, the external emission current from the cathode can reach 25% of the total discharge current.

Secondary Emission From Clusters Composed of Spherical Grains

Dust grains or their clusters can be frequently found in many space environments—interstellar clouds, atmospheres of planets, tails of comets, or planetary rings are only typical examples. Space dust grains are formed by various processes and their shapes are complex. These grains are exposed to electrons with different energies, and thus, they can acquire positive or negative charge during this interaction. We present a systematic study of well-defined systems—clusters consisting of different numbers of small spherical (1 $\mu \text{m}$ ) grains and such objects can be considered as examples of real irregularly shaped space grains. The charges acquired by investigated objects as well as their secondary emission yields are calculated using the secondary emission model. We have found that: 1) the charge and surface potential of clusters exposed to the electron beam are influenced by the number of grains and by their geometry within a particular cluster; 2) the model results are in an excellent agreement with the experiment; and 3) there is a large difference between charging of a cluster levitating in the free space and that attached to a planar surface. The calculation provides a reduction of the secondary electron emission yield of the surface covered by dust clusters by a factor of 1.5 with respect to the yield of a smooth surface.

Hydrogen isotopic fractionation in secondary ion mass spectrometry using polyatomic ions

Isotopic measurements at the micron scale using secondary ion mass spectrometry (SIMS) require an accurate control on the instrumental mass fractionation (IMF). The IMF correction is challenging since substantial mass fractionation can occur during both emission and detection processes. In this work, we report hydrogen isotopic measurements on a series of organic standards using polyatomic ions (CH−, CD−, C2H−, C2D−) with the NanoSIMS-50 instrument. We show that the D/H ratios resulting from the measurement of CD−/CH− and C2D−/C2H− ratios are close to the true D/H ratios of the samples over more than 3 decades and allow accurate hydrogen isotopic measurements to be performed in solid samples. Large secondary emission yields are usually associated with high electron affinities and correlate presently with low IMF effects. Moreover, we show that even when large variations in secondary ionic current are observed, an accurate isotopic ratio can still be inferred.