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In this article, two ferrite circulators with wedge-shaped cross section geometry have been designed, manufactured, and tested. Inclination angle of wedge-shaped geometry for two circulators is different. First, the electrical performance of the ferrite circulators is optimized using high-frequency electromagnetic simulation software (HFSS), and then measurements of the electrical performance are completed. Also, the two ferrite circulators have equivalent electrical characteristic. The isolation is greater than 20 dB, and the bandwidth is 15%. Finally, the multipactor thresholds of the circulators involved in this article are simulated using particle solver in CST simulation software and the actual measurement experiment is also performed. The simulation and measurement results show that the circulator with a larger wedge angle has a larger multipactor threshold. So, this structure can provide a reference for the design of multipactor-free ferrite circulator.

This work presents a novel low-pass filter based on using dielectric materials, in the critical areas of the structure, to prevent high-power space condition adverse effects, like multipactor breakdown. It is shown that the use of dielectric pieces in the inverters of the filter leads to increased gaps between metallic parts in the irises. In addition, by completely filling up the air space in critical gaps, the multipactor breakdown thresholds can be increased. Moreover, they also lead to a volume reduction, increasing compactness of the structure. To demonstrate the proposed concept, a prototype has been manufactured with an aluminum housing and a Teflon dielectric material, and tested for multipactor effect at ESA-VSC Laboratory. Tests confirmed that the manufactured filter has good electrical performance with cut-off frequency 13.6 GHz, insertion losses 0.32 dB, return losses better than 15 dB, and additionally does not show multipactor breakdown up to the maximum power tested in the experimental facility (5.5 kW). This research work has been financially supported by the Spanish Ministerio de Ciencia e Innovación in the frame of the project “Green an Efficient Technologies for Advanced Telecommunication Systems (GRETAS)” with Ref. TPID2019-103982RB-C42.

The aim of this article is to establish a general and practical analytical model of the one-sided multipactor on a dielectric of a metal surface and analyze the resonant multipactor effect for the recent growing interest in spacecrafts. A theoretical multipactor model involving a normal static electric field, a radio frequency (RF) electric field with both tangential and normal components, and the influence of the desorption gas is proposed. The kinetic properties and the resonant condition of the electron are studied by analytical calculations. The results show that the transit time of the electron decreases with the increase in the normal static electric field and the pressure of the desorption gas. The emission phase of the electron is the paramount factor that has a direct influence on the occurrence of the resonant multipactor. If the resonant multipactor effect could appear, the emission phase of the electron should be larger than a critical phase, and the ratio between the amplitude of the normal component of the RF electric field and the normal static electric field should be larger than a specific value as well.

Suppressing the harmful multipactor effect by, for instance, reducing secondary electron yield (SEY) is crucial in the design of RF space technologies. Therefore, improving fundamental understanding of how structural and electronic features of materials affect the SEY is necessary. In this benchmark work, we use density functional theory calculated properties to simulate, via Monte Carlo method, the SEY of simple metals copper and silver. As in these simulations the dielectric property i.e. the energy and momentum dependent energy loss function plays a major role, we focus our study on the computational procedures necessary to obtain reliable first principles data.

We have studied nanometric high aspect ratio Ag nanopillar coatings exhibiting reduced secondary electron emission for the mitigation of multipactor effect in radio-frequency space devices of high frequency and high power. The Ag nanopillars have been grown by glancing angle deposition with DC magnetron sputtering. Some samples have been covered by a gold capping layer to reduce oxidation and aging effects. The secondary emission yield of the surfaces of these samples has been measured and compared to those of flat Ag and Au reference samples. The results show that high aspect ratio surface roughness at the nanometer scale significantly reduce the secondary emission yield of the surface. This reduction is more important for low electron energies, which is the most influencing energy range of electrons for multipactor. The multipactor region for the nanopillar coating presenting the best secondary emission yield properties has been simulated, finding practical suppression of multipactor effect. The high-frequency surface resistance of these samples has also been estimated from published computations for similar surface roughness patterns. It was found that such nanopillar coatings are compatible with the best accomplishments of present space industry., This investigation has been funded by the MINECO of Spain through the AYA2012-39832-C02-02, FIS2015-67367-C2-1-P, MAT2015-69035-REDC and MAT2014-59772-C2-1-P projects and by the Comunidad de Madrid through the NANOMAGCOST-CM Ref: P2018/NMT4321 project. One of the authors (C.M.) thanks MECD for FPU14/02020 grant. The authors also want to thank SEGAINVEX and SIdI from UAM for technical support, and to acknowledge the service from the MiNa Laboratory at IMN funded by Comunidad de Madrid (S2018/NMT-4291 TEC2SPACE), MINECO (CSIC13-4E-1794) and the European Union (FEDER, FSE).

In this work, a comparative study of the susceptibility chart in a partially dielectric-loaded rectangular wave-guide and in its equivalent partially dielectric-loaded parallel-plate wave-guide with the same height is performed. This study shows how the inhomogeneity of the electric field inside the rectangular wave-guide modifies the edges of the multipactor susceptibility chart with respect to that predicted by the parallel-plate wave-guide case. Moreover, comparisons of the evolution of the population and the trajectory of the effective electron and the DC field appearing in the wave-guide in a point well inside the multipactor region obtained with both models are performed, showing some quantitative differences in the multipactor evolution, but a similar qualitative behaviour is observed.

RF window which serves as a barrier between the vacuum and air sides is a very important device for RF cavity of cyclotron. But the performances of a basic RF window are not good in impedance matching characteristics and service life. In this paper, two kinds of RF window are developed to improve performances from three candidate designs as impedance matching optimization, thermal consideration during RF operation, and multipactor effect analysis. The multiple comparisons indicate that one of the designed RF windows is far better than the others, and a low-power test has verified its impedance matching characteristic. This optimal RF window may be widely used in other low energy cyclotron and similar devices.

[EN] The objective of this paper is to advance the state of the art in the characterization of the multipactor effect in dielectric materials. The materials studied are the most commonly used dielectrics in space applications, namely, Alumina, Rexolite, Rogers RT5870, Rohacell, Teflon, and Ultem 1000. In this paper, a new family of coaxial waveguide components, covering the L- and S-bands, with a wideband, low-pass response has been designed, and six different prototypes have been specifically optimized and manufactured. The six prototypes have then been used to simulate and measure the multipactor breakdown susceptibility charts for the six dielectric materials investigated. Finally, the simulation results are compared with the results of the measurement campaign indicating good agreement., This work was supported in part by the European Space Agency through Research Project "Novel Investigation in Multipactor Effect in Ferrite and Other Dielectrics Used in High Power RF Space Hardware" under Grant AO 1-7551/13/NL/GLC and in part by MINECO (Spanish Government) through Research and Development Project under Grant TEC2016-75934-C4-1-R. This paper is an expanded version from the IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes IMWS-AMP 2017, Pavia, Italy, September 20-22, 2017.

In view of the multipactor effect caused by the resonant multiplication of secondary electrons in space high-power microwave components, this paper proposes an intra-expansion trap surface to strongly suppress the emission of secondary electrons, and analyzes the suppression characteristics on multipactor via numerical simulation. We track each electrons' trajectory inside the trap to obtain secondary electron emission, and compare with a traditional straight hole trap to present its suppression effects. In addition, the influence of the structure parameters on the secondary electron emission characteristics and the power threshold of multipactor was studied. Results show that comparing with the traditional straight hole trap surface, the proposed intra-expansion trap surface performs 13% more remarkable suppression effect on secondary electron emission, and the power threshold of multipactor can be improved from 8969W to 15437W. A deeper intra-expansion trap surface can enhance the multipactor power threshold further.

International audience; Characterized by a very fast growth of the electron population in vacuum of Radio-Frequency (RF) devices, the multipactor effect has been widely studied during the past decades. As it limits the transmitted RF power and may degrade RF devices, its understanding is primordial. Multipactor simulation tools give accurate results in the simplest cases, but are less accurate for advanced configurations: complex geometries, dielectric materials, presence of magnetic fields, etc. In such cases, an accurate modelling of the electron emission phenomena becomes essential. We extended a one-dimension electron emission (EE) model to three dimensions. The obtained model is compared to measured electron emission yields. The impact of this new model on the simulated multipactor threshold of parallel plane wave-guide is also investigated.

High-power multipactor effect always leads to high risk of passive intermodulation and electromagnetic compatibility problems. In this paper, multipactor in ferrite circulators has been simulated, analyzed, and measured. Using the Electromagnetic Particle-In-Cell method, the discharge breakdown thresholds of multipactor in circulators with electron-blocking dielectric and without electron-blocking dielectric have been predicted and verified by simulations. To improve the simulation accuracy of electron emission and collision on the ferrite dielectric, Secondary Electron Emission properties have been measured and fitted into the numerical models. Through power scanning method, transient evolution of electron numbers has been obtained and threshold predicted. Simulation results show that the threshold power of these circulators with and without electron-blocking were 1000W and 9700W, respectively. It demonstrates that multipactor breakdown power in ferrite circulators have been improved more than 900% utilizing the electron-blocking dielectric, which is promising for high-power applications, especially in space industry.

Multipactor is an electron discharge that may appear in particle accelerators and microwave devices such as filters, multiplexers, and RF satellite payloads in satellite on-board equipment under vacuum conditions. When some resonance conditions are satisfied, secondary electrons get synchronized with the RF fields, and the electron population inside the device grows exponentially leading to a multipactor discharge. This multipactor discharge has some negative effects that degrade the device performance: increase of signal noise and reflected power, heating of the device walls, outgassing, detuning of resonant cavities, and even the partial or total destruction of the component. The main aim of this investigation is to show a new model for the analysis of multipactor effect in partially dielectric-loaded rectangular waveguides. For this purpose, a CAD software has been developed which allows the simulation and design of these structures out of the operation points in which this undesired multipactor effect occurs, including the main factors that drive it.

In this paper, the design of a 1.3GHz high power coupler cold window is presented for the Shanghai light source hard X-ray free electron laser system. The cold window consists of 3 parts: outer conductor, inner conductor and ceramic window. The multipactor effect is a resonant vacuum electron discharge that appears in components operating with high power electromagnetic field. When certain RF resonance condition is met, the secondary electrons grow exponentially, which leads to remarkable power losses and heating of the ceramic window, so that it becomes impossible to increase the fields by raising the incident power.

[EN] This paper reports on the experimental measurement of power threshold levels for the multipactor effect between samples of ferrite material typically used in the practical implementation of L-and S-band circulators and isolators. For this purposes, a new family of wideband, nonreciprocal rectangular waveguide structures loaded with ferrites has been designed with a full-wave electromagnetic simulation tool. The design also includes the required magnetostatic field biasing circuits. The multipactor breakdown power levels have also been predicted with an accurate electron tracking code using measured values for the secondary electron yield (SEY) coefficient. The measured results agree well with simulations, thereby fully validating the experimental campaign., This work was supported in part by the European Space Agency (ESA) through the Research Project Novel Investigation in Multipactor Effect in Ferrite and Other Dielectrics Used in High Power RF Space Hardware under Contract AO 1-7551/13/NL/GLC, in part by the Ministerio de Economia y Competitividad (Spanish Government) through Research and Development under Project TEC2016-75934-C4-1-R and Project TEC2016-75934-C4-2-R, and in part by the European Regional Development Fund under Project TEC2014-55463-C3-3-P.

Secondary electron yield (SEY) is relevant for widely used characterization methods (e.g., secondary electron spectroscopy and electron microscopy) and materials applications (e.g., multipactor effect). Key quantities necessary for understanding the physics of electron transport in materials and simulation of SEY are electron mean free paths (MFPs). This paper explores the impact of alloying on MFPs and SEY for Cu-Ni, Cu-Zn, and Mo-Li alloys relative to their component metals Cu, Ni, Zn, Mo, and Li. Density functional theory calculations yield density of states, Fermi energy, work function, and frequency- and momentum-dependent energy loss function. These material properties were used to calculate MFPs and Monte Carlo simulations were performed to obtain energy dependent SEY for the alloys as well for the component metals. The results show that MFPs and SEYs of the studied alloys lie between those of component pure elements but are not a simple composition weighted average. Detailed analysis of the secondary electron generation and emission process shows that the changes in the SEY of alloys relative to the SEY of their component metals depend on the changes in both electronic structure and dielectric properties of the material.

Surface flashover at the solid dielectric/vacuum interface is a fundamental physical phenomenon, which restricts the development of pulsed power system and high voltage electrical equipment. The initiation and developing processes of surface flashover at the solid dielectric are associated with electrode-dielectric interface, dielectric surface and other external factors. Especially, the surface charges transport properties that related with surface flashover developing process are determined by the physicochemical of the dielectric surface. Therefore, the surface insulating performance can be enhanced by deposited with appropriate functional films. TiN film has a low secondary electron yield (SEY), which can help to suppress the secondary electron multipactor effect and suppress the surface flashover. In this paper, a preparation method of film contained TiCxOyNz using atmospheric pressure plasma jet (APPJ) is proposed. The experimental devices include AC power supply, plasma jet chamber and gas/precursor system. The AC power supply has a power of 1000 VA and frequency of 20 kHz. The precursor tetraethyl titanate (TTEO) is pumped into plasma jet chamber by argon gas and the working gas is nitrogen. The experimental results show that the maximum SEY of sample with TiCxOyNz film is approximately decreased to 1.5. The flashover voltage of sample with TiCxOyNz film is increased obviously. The morphology of the dielectric surface indicates that the rough surface is introduced after modification. Also, the physical processes of film deposition are investigated by the emission spectroscopic analysis.

The secondary electron emission (SEE) property of materials is closely related to the multipactor, which is one of the potential risk factors affecting the reliability of high power microwave components for space application. In the presented work, the influence of atmospheric exposure on the secondary electron yield (SEY) of the inert metal is investigated. It is found that the SEY increases gradually with exposing time, and the increase in SEY results from the surface adsorption layer consisting of carbon and oxygen elements. A two-layers SEY model is established to explain this evolution and obtain the corresponding parameters of the adsorption layer. Simulation on a C band impedance transformer shows that 45 days of exposure will result in a 7.5 dB decrease of multipactor threshold.

The reduction of the total secondary electron emission during electron-matter interactions is required to improve the maximum power limit of the vacuum RF equipments prone to multipactor discharge. In this work, a chemical deposition technique was developed to create the appropriate micron or submicron surface roughness to significantly improve the typical characteristics of the flat silver coatings used for high-power microwave devices for space applications. Rough silver coated RF filters with a high surface aspect ratio, >3, was achieved. It is found that the secondary electron emission yield (SEY) remains lower than 1 throughout the primary electron energy range and therefore multipactor electron avalanche is avoided. The influence of aging on the multipactor discharge is also drastically reduced in rough silver coatings. An experimental linear correlation was found between the multipactor power level and the minimum interaction energy for which the number of incident and emitted electrons is equal.

Multipactor and vacuum breakdown are serious problems for RF systems that utilize vacuum electronic devices. These devices contain metallic surfaces that serve as the source of unwanted particles released into vacuum. The primary culprits are desorbed gas molecules and secondary electrons. Outgassing results from stimulated or thermal desorption of molecular species in metals and can lead to undesired plasma formation in the vacuum gap. Secondary electrons (SEs) can lead to the multipactor effect when an avalanche of SEs obtain resonance with the internal RF field. Here we discuss surface treatments for mitigating these problems. Laser surface melting (LSM) entails irradiating a metallic sample with the output of a high energy, continuous laser beam, thereby causing melting, flow and re-solidification of the material. LSM processing reduces H outgassing by decreasing the number of grain boundaries through which H can diffuse. We have demonstrated a 50x reduction over an untreated stainless steel surface1. For SE reduction, both surface roughening and patterning have been investigated. Surface roughening was accomplished by using specific grades of metallographic polishing pads imparting controlled levels of roughness and surface features. Electron beam bombardment experiments showed that the secondary electron yield (SEY) increased with surface roughening2. Surface patterning with micro-pores has also been shown to reduce SEY. Modeling has shown that the amount of reduction depends on the aspect ratio of the pore (the ratio of the pore height to the pore diameter), with the greatest reductions occurring for aspect ratios less than two. In order to validate these models, a variety of micro-porous gold surfaces were designed and fabricated using photolithography and electroplating processes, and the models were validated with experimental results3, These results will be discussed.

The multipactor effect is a discharge phenomenon which may occur when secondary electron emission (SEE) is in resonance with an electric field under certain vacuum conditions [1]. Liberated electrons will oscillate between two surfaces driven by a radio frequency (RF) field; this discharge cultivates and sustains itself by SEE through reoccurring impacts with the surface. This can lead to deterioration of vacuum RF electronics' performance [2]. To suppress the multipactor effect, the secondary electron yield (SEY) must be reduced.

An experimental setup was designed and implemented for the study of the multipactor effect in a rectangular waveguide under high vacuum. The experiment and test piece were constructed using WR 284 waveguide sections operating at S- Band frequencies. Multipactor effects are to be detected via an Electron Multiplier Tube (EMT). The test setup includes sections of standard WR 284 waveguide which will be in line with a custom test piece. Design of the test setup incorporates a standard piece of waveguide with a custom steel collar which can be used to insert different tapers, and even periodic structures into the hollowed out broadside wall. A solid state RF transmitter with 2.8 kW peak power and a center frequency of 2.85 GHz is used to inject an RF signal into the test section.

This paper 1 presents a study of the multipactor 2 effect in a partially dielectric-loaded rectangular waveguide. 3 To obtain the simulations presented in this paper, a detailed 4 analysis of the dynamics of the electron inside this waveguide 5 has been performed, taking into account the radio frequency 6 electromagnetic fields propagating in the waveguide and the dc 7 electric field that appears because of the charging of the dielectric 8 layer. This electrostatic field is obtained by computing the electric 9 potential produced by an arbitrary charge distribution on the 10 dielectric layer in a dielectric-loaded waveguide. The electron 11 trajectory is then found by numerically solving the equations of 12 motion. The results obtained show that multipactor discharges 13 do turn off by themselves under certain circumstances when they 14 occur in such dielectric-loaded waveguide. 10.13039/5011000110033-Agencia Estatal de Investigación; Unión Europea through the Fondo Europeo de Desarrollo Regional

[EN] Secondary electron emission has an important role on the triggering of the multipactor effect; therefore, its study and characterization are essential in radio-frequency waveguide applications. In this paper, we propose a theoretical model, based on equivalent circuit models, to properly understand charging and discharging processes that occur in dielectric samples under electron irradiation for secondary electron emission characterization. Experimental results obtained for Pt, Si, GaS, and Teflon samples are presented to verify the accuracy of the proposed model. Good agreement between theory and experiments has been found., The authors would like to thank the European High Power Space Materials Laboratory for its contribution-a laboratory funded by the European Regional Development Fund-a way of making Europe. Many thanks to the University of Valencia (Spain) for supporting this research activity with the internal program "Assistance for temporary stays of invited researchers within the framework of the Subprogramme Attraction of Talent 2015".

La fusion nucléaire contrôlée par confinement magnétique avec les réacteurs de type Tokamaks et les applications spatiales ont en commun d’utiliser des composants Haute-Fréquence (HF) sous vide à forte puissance. Ces composants peuvent être sujets à l’effet multipactor qui augmente la densité électronique dans le vide au sein des systèmes, ce qui est susceptible d’induire une dégradation des performances des équipements et de détériorer les composants du système. Ces recherches consistent à améliorer la compréhension et la prédiction de ces phénomènes. Dans un premier temps nous avons réalisé une étude de sensibilité de l’effet multipactor au rendement d’émission électronique totale (noté TEEY). Cette étude a permis de montrer que l’effet multipactor est sensible à des variations d’énergies autour de la première énergie critique et dans la gamme d’énergies entre la première énergie critique et l’énergie du maximum. De plus, les composants HF utilisés dans les réacteurs Tokamak et dans le domaine du spatial peuvent être soumis à un champ magnétique continu. Nous avons donc développé un nouveau dispositif expérimental afin d’étudier ce phénomène. Le fonctionnement du dispositif et la méthode de mesure ont été analysées et optimisées à l’aide de modélisations numériques avec le logiciel PIC SPIS. Une fois que l’utilisation du dispositif a été optimisée et que le protocole de mesures a été validé, nous avons étudié l’influence d’un champ magnétique uniforme et continu sur le TEEY du cuivre. Nous avons démontré que le rendement d’émission électronique totale du cuivre est influencé par la présence d’un champ magnétique et par conséquent également l’effet multipactor. Space communication payload as well as magnetic confinement fusion devices, among other applications, are affected by multipactor effect. This undesirable phenomenon can appear inside high frequency (HF) components under vacuum and lead to increase the electron density in the vacuum within the system. Multipactor effect can thus disturb the wave signal and trigger local temperature increases or breakdowns. This PhD research aims to improve our understanding and the prediction of the multipactor effect. The multipactor phenomenon is a resonant process which can appear above a certain RF power threshold. To determine this power threshold, experimental tests or/and simulations are commonly used. We have made a study to evaluate the multipactor power threshold sensitivity to the TEEY. Two particular critical parameters have been found: first cross-over energy and the energies between the first cross-over and the maximum energies. In some situations, the HF components are submitted to DC magnetic fields which might affect the electron emission properties and hence the multipactor power threshold. Current multipactor simulation codes don’t take into account the effect of the magnetic field on the TEEY. A new experimental setup specially designed to investigate this effect was developed during this work. Our new experimental setup and the associated TEEY measurement technique were analysed and optimized thanks to measurements and SPIS simulations. We used the setup to study the influence of magnetic field perpendicular to the sample surface on the TEEY of copper. We have demonstrated that the magnetic field affects the copper TEEY, and hence multipactor power threshold.

Space communication payload as well as magnetic confinement fusion devices, among other applications, are affected by multipactor effect. This undesirable phenomenon can appear inside high frequency (HF) components under vacuum and lead to increase the electron density in the vacuum within the system. Multipactor effect can thus disturb the wave signal and trigger local temperature increases or breakdowns, which can cause components deteriorations or even their destruction. This PhD research aims to improve our understanding and the prediction of the multipactor effect. The multipactor phenomenon is a resonant process which can appear above a certain RF power threshold. To determine this power threshold, experimental tests or/and simulations are commonly used. The power threshold is closely related to the electron emission properties of the RF component materials, in particular, to the Total Electron Emission Yield (TEEY). Therefore, we have made a study to evaluate the multipactor power threshold sensitivity to the TEEY. Two particular critical parameters have been found: first cross-over energy and the energies between the first cross-over and the maximum energies. Six TEEY models frequently used in multipactor simulations have been benchmarked with respect to their relevance in modelling these two previous regions of interest of the TEEY. It was found that only one TEEY model is suited to predict accurate multipactor power threshold. In some situations, the HF components are submitted to DC magnetic fields which might affect the electron emission properties and hence the multipactor power threshold. Magnetic fields are generated by the coils and the plasma in a tokamak while for space application we use isolators and circulators which are built with permanent magnets. Current multipactor simulation codes don’t take into account the effect of the magnetic field on the TEEY. A new experimental setup specially designed to investigate this effect was developed during this work. We aimed to have experimental conditions closed to RF components actual use. Our new experimental setup and the associated TEEY measurement technique were analysed and optimized thanks to measurements and SPIS simulations. SPIS, for Spacecraft Plasma Interaction System, is a PIC code which allows us to simulate plasma-matter interactions and in particular to analyse electron trajectories under uniform magnetic field. We used the setup to study the influence of magnetic field perpendicular to the sample surface on the TEEY of copper. We have demonstrated that the magnetic field affects the copper TEEY, and hence multipactor power threshold. It is clearly established that the magnetic field has to be taken into account on both electron emission properties and multipactor simulation., La fusion nucléaire contrôlée par confinement magnétique avec les réacteurs de type Tokamaks et les applications spatiales ont en commun d’utiliser des composants Haute-Fréquence (HF) sous vide à forte puissance. Ces composants peuvent être sujets à l’effet multipactor qui augmente la densité électronique dans le vide au sein des systèmes, ce qui est susceptible d’induire une dégradation des performances des équipements, de détériorer les composants du système et de les rendre partiellement voire totalement inutilisables. Ces recherches consistent à améliorer la compréhension et la prédiction de ces phénomènes. L’effet multipactor dépend des propriétés d’émission électronique des matériaux des composants HF. Dans un premier temps nous avons donc réalisé une étude de sensibilité de l’effet multipactor au rendement d’émission électronique totale (noté TEEY). Cette étude a permis de montrer que l’effet multipactor est sensible à des variations d’énergies autour de la première énergie critique et dans la gamme d’énergies entre la première énergie critique et l’énergie du maximum. Nous avons aussi comparé six modèles d’émission électronique totale usuels, un seul permet d’obtenir des seuils multipactor proches des données expérimentales. De plus, les composants HF utilisés dans les réacteurs Tokamak et dans le domaine du spatial peuvent être soumis à un champ magnétique continu qui est généré respectivement par les bobines du Tokamak ainsi que le plasma au sein de la chambre du réacteur et par des aimants permanents utilisés dans les circulateurs et isolateurs HF. Les études multipactor précédentes ne prennent pas en compte l’influence du champ magnétique sur l’émission électronique des matériaux. Nous avons donc développé un nouveau dispositif expérimental afin d’étudier ce phénomène. Le dispositif a été conçu pour réaliser des mesures les plus représentatives de la situation réelle d’utilisation des composants HF. Le fonctionnement du dispositif et la méthode de mesure ont été analysées et optimisées à l’aide de modélisations numériques avec le logiciel PIC SPIS (pour Spacecraft Plasma Interaction System). Développé par un consortium ONERA – ESA – Artenum, SPIS permet de simuler les interactions plasma – matériaux et en particulier de déterminer les trajectoires électroniques dans un milieu représentatif de l’environnement expérimental. Une fois que l’utilisation du dispositif a été optimisée et que le protocole de mesure a été validé, nous avons étudié l’influence d’un champ magnétique uniforme et continu sur le TEEY du cuivre. Nous avons démontré que le rendement d’émission électronique totale du cuivre est influencé par la présence d’un champ magnétique et par conséquent également l’effet multipactor. Ainsi la présence d’un champ magnétique doit être prise en compte vis-à-vis de l’émission électronique et lors des calculs prédictifs de puissance seuil multipactor.

Multipactor effect in a coaxial transmission line was studied theoretically and numerically. The results show that significant radio frequency breakdown likely originates from gas desorption in the material and further gas ionization induced by electrons. The multipactor effect using different surface gas pressures was analyzed with particle-in-cell simulations. The simulation results indicate that plasma originating from locally desorbed gas causes significant power reflection and absorption. At the same time, plasma electrons play a more significant role than plasma ions in the multipactor process. A numerical method such as the one used here can thus be used to predict whether radio frequency breakdown can be caused by the multipactor effect.

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. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. O171037 and Grant no. III45006]

The main aim of this paper is the analysis of the feasibility of employing permanent magnets for the multipactor mitigation in a coaxial waveguide. First, the study of a coaxial line immersed in a uniform axial magnetic field shows that multipactor can be suppressed at any RF if the external magnetic field is strong enough. Both theoretical simulations and experimental tests validate this statement. Next, multipactor breakdown of a coaxial line immersed in a hollow cylindrical permanent magnet is analyzed. Numerical simulations show that multipactor can be suppressed in a certain RF range. The performed experimental test campaign demonstrates the capability of the magnet to avoid the multipactor electron multiplication process., This work was supported by the European Regional Development Fund-A Way of Making Europe. The review of this paper was arranged by Editor R. Carter.

The aim of this paper is the analysis of the multipactor effect in a parallel-plate waveguide when a ferrite slab, transversally magnetized by a static magnetic field parallel to the waveguide walls, is present. Employing an in-house developed code, numerical simulations are performed to predict the multipactor radio frequency voltage threshold in such a ferrite-loaded waveguide. Variations of the ferrite magnetization field strength and the ferrite slab height are analyzed. Effective electron trajectories are also shown for a better understanding of the breakdown phenomenon, finding different multipactor regimes.

In this paper, the multipactor RF breakdown in a parallel-plate waveguide partially filled with a ferrite slab magnetized normal to the metallic plates is studied. An external magnetic field is applied along the vertical direction between the plates in order to magnetize the ferrite. Numerical simulations using an in-house 3-D code are carried out to obtain the multipactor RF voltage threshold in this kind of structures. The presented results show that the multipactor RF voltage threshold at certain frequencies becomes considerably lower than for the corresponding classical metallic parallel-plate waveguide with the same vacuum gap, This work was supported in part by the European Space Agency under Novel Investigation in Multipactor Effect in Ferrite and other Dielectrics used in high power RF Space Hardware under Contract AO 1-7551/13/NL/GLC, in part by the Spanish Government under Coordinated R&D Project TEC2013- 47037-5-R and Project TEC2014-55463-C3-3-P, and in part by the European Commission (ERDF). The review of this paper was arranged by Editor R. Carter.

[EN] The objective of this paper is to study the time evolution of multipactor discharges in a parallel-plate dielectric-loaded waveguide. The electron discharge phenomena investigated in this paper include the effects of space charge, as well as the presence of time varying static fields. In order to perform the simulations presented in this paper, an approach considering simultaneously multiple effective electrons has been used. The results obtained demonstrate that multipactor discharges do turn off by themselves under certain circumstances when they occur in such dielectric-loaded structures., This work was supported by the Spanish Government's Ministerio de Educacion y Ciencia under Project TEC2007-67630-C03-01. The review of this paper was arranged by Editor W. Menninger.

In this work, an ultra-high vacuum reactive evaporation procedure has been tested and optimized for the growth of TiN thin films, throwing very good results both in composition of the TiN layers and secondary emission yield values and performance. Stoichiometry and composition are analyzed by means of X-ray photoemission spectroscopy which, together with the secondary electron emission measurements, allow us determine the appropriate growth parameters in order to obtain in a reproducible manner optimum results towards the multipactor effect suppression. After refining the growth on test layers, several waveguide pieces, specially designed and constructed for testing multipactor level (maximum RF power attainable without multipactor discharge), have been coated with TiN films following those optimized growth conditions.

The development of alumina RF windows for use at the output section of high-power RF sources is an important issue for accelerators. The breakdown of RF windows is caused by 1. the multipactor effect (electron multiplication on the surface) and/or 2. the discharge of accumulated charges (due to the multipactor effect). Measurements of secondary electron emission (SEE) and surface charging were carried out to determine the durability of various alumina ceramics in this application. Since excessive surface heating takes place during surface discharge, SEE yields at high temperature were also measured. The results indicate that SEE yields become lower at high temperature, and that higher purity alumina ceramics show higher SEE yields. High-power tests using a resonant ring were conducted in order to observe surface charging and electron accumulation. Higher purity alumina ceramics showed superior performance in the high-power tests, probably due to lower surface charging.

The main goal of this letter is the analysis of the multipactor effect within a coaxial waveguide structure when an external axial dc magnetic field is applied. We have designed and manufactured a coaxial waveguide sample that has been immersed within a long solenoid. Numerical and experimental results confirm a significant change in the RF breakdown behavior with regard to the case without the axial dc magnetic field, as well as the existence of single- and double-surface multipactor regimes. Good agreement between theory and experimental data has been found., Manuscript received January 23, 2012; accepted January 31, 2012. Date of publication March 16, 2012; date of current version April 20, 2012. This work was supported in part by the Spanish Government through R&D project TEC2010-21520-C04-01. The review of this letter was arranged by Editor E. A. Gutierrez-D.

The secondary electron emission (SEE) from materials used in high power RF devices in space is the main trigger and sustaining mechanism of the resonant avalanche electron discharge known as the multipactor effect. It limits the attainable power of those devices. During recent decades, some scientific research has been focused on material properties for obtaining anti-multipactor coatings of low secondary emission yield (SEY). The European Space Agency (ESA) is leading a technological research on a new approach based on surface roughness that might perform as a kind of blackbody or Faraday cage effect. A multilayer coating structure was adopted for fulfilling the stringent requirements of the space. The surface of a standard silver plating was modified by a two-step treatment. First, a wet chemically etching process created a roughness of high aspect ratio, in the scale of microns. Secondly, the surface was coated with a protective 2 μm overlayer of gold, using magnetron sputtering. This anti-multipactor coating has been tested on several types of Ku-band WR75 waveguide transformers and band-pass filters, with excellent results. The multipactor effect was suppressed for two waveguides, even when applying the maximum available power levels. As for the other final four, the increase of multipactor power level was in the range of 4-6 dB. These results were obtained after more than one year of air exposure. In spite of the strong roughness, the insertion losses were diminished by 25% with respect to the values attained in the tests of the standard anti-multipactor coating, Alodine., This work was supported by the European Space Agency (ESA)/European Space Research and Technology Centre (ESTEC), under the research program “Porous Inert Metal Coatings for Controlling Secondary Electron Emission”, ESA 17025/03/NL/EC (CCN-02) and by the Ministry of Economy and Competitiveness of Spain, the National Plan of R&D&I, coordinated Projects no. AYA2012-39832-C02-01 and -02.

The objective of this letter is to study the time evolution of a multipactor discharge in a simple dielectric waveguide structure. In particular, the investigation is performed on the case of a parallel-plate waveguide structure partially filled with a dielectric layer. The main consequence of the specific case studied in this letter is the fact that the dielectric layer charges negatively, allowing a negative static field to build up. This dc field eventually leads to a single-surface multipactor in the metallic surface before finally turning off the electron discharge.

This paper deals with the analysis of the Multipactor effect in RF components with ferrites performed through the preliminary measurements of ferrites secondary emission coefficient and the simulations of the effect of a continuous magnetic field on the electron bunch spread. The impact of this effect on the Multipactor threshold was quantified.

The objective of this letter is to study the effect of multipactor on the dielectric resonator filter response. First, a dual-mode cylindrical dielectric resonator waveguide filter is designed and Multipactor breakdown threshold of this filter has been obtained by using effective electrons approach in Monte Carlo method. Then multipactor current density in the radial space between the internal dielectric and the external metallic cavities is calculated. The magnetic field generated by the multipactor current is calculated and its equivalent volume impedance is applied to the solution boundaries and is used in the filter simulation. The simulated results are obtained using the Ansoft simulation software high-frequency structure simulator and clearly show the effect of multipactor on the frequency response of the filter.

The possible application of amorphous carbon nitride thin films to prevent the multipactor effect is discussed. The main difficulty lies in the increase of the secondary electron emission coefficient after air exposure. Adsorption processes on amorphous carbon nitride surfaces due to air exposure and subsequent changes due to thermal annealing have been studied by means of x-ray photoemission spectroscopy with the purpose of understanding the changes of secondary electron emission characteristics.

Based on an improved multipactor dynamic model, the multipactor phenomenon on a dielectric surface existing longitudinal RF electric field was simulated using Monte Carlo method. The temporal evolution images of the multipactor discharge were investigated. The power deposited on the dielectric surface by the multipactor was also obtained in terms of an S-band RF dielectric window. The results show that, the longitudinal RF electric field may intensify the single-surface multipactor effect, which is likely to result in dielectric crack and detrimental to RF transmission.

Breakdown phenomena on alumina RF windows for S-band high-power use were investigated using a resonant ring. Occasional flashovers were observed on TiN-coated surfaces, though the multipactor effect is suppressed by the coating. The flashovers show luminescence with a treelike pattern, resulting in a limitation of the transmittable power through the window. This flashover threshold depends on the dielectric materials; a sapphire with F-center defects shows a lower threshold. For further investigations of the flashover mechanism under a RF field, in situ measurements of the charges built up on the surface both before and after flashover and high-speed photography observations of the flashover avalanches were made. The results indicate an electron communication; its relaxation leaves positive charges on the surface. The relaxation time is found to be 10/sup 6/ m/s. >

Thales Components & Subsystems as a manufacturer of High Voltage components and vacuum tubes has developed ceramics coating in order to minimize multipactor effect. Two prototypes of TTF III couplers have been manufactured and tested, one with a Cr2O3 coating, the other one with a TiN coating

To evaluate the Dielectric-Loaded Accelerating (DLA) structure as a potential alternative high-gradient accelerator, a series of high power RF experiments have been carried-out by a joint Argonne National Laboratory/Naval Research Laboratory program. We have tested DLA structures loaded with two different ceramic materials, alumina and Mg x Ca 1-x TiO 3 . For the alumina tube, we concentrated on the study of the multipactor effect in the DLA structures under the high power rf field. In the Mg x Ca 1-x TiO 3 tests, we mainly investigate the local field enhancement that caused dielectric joint breakdown of the DLA structures. In both cases, physical models have been set up, and the corresponding engineering solutions are being implemented.

High gradient accelerating structures using dielectric‐lined circular waveguides have been developed for a number of years at Argonne National Laboratory. In this article, we first report the experimental results of high power rf testing on the quartz based Dielectric‐Loaded Accelerating (DLA) structure carried out on Feb. 2006 at the Naval Research Laboratory. The motivation for this experiment is to test the multipactor effect on different materials under high power and high vacuum condition. Up to 12 MW pulsed rf went through the tube without breakdown. Multipactor appeared during the experiment but with different features compared to other materials like alumina. Photomultiplier Tube (PMT) measurements were introduced into the experiment for the first time to observe the light emission time and intensity. In the second part of this paper, ways to achieve higher gradient for DLA structures are proposed: 1) smaller ID and longitudinal gap free DLA structures to reduce multipactor and obtain higher gradi...

Summary form only given. A joint program is under way at the Naval Research Laboratory (NRL) and Argonne National Laboratory (ANL) to develop and test dielectric-loaded structures for use in high-gradient RF linear accelerators. DLA structures offer the potential of a simple, inexpensive alternative to conventional copper disk-loaded structures for use in high-gradient linear accelerators. The study makes use of structures developed and fabricated by ANL, and carries out its testing program on the X-band magnicon facility at NRL at drive powers up to 10 MW. We have carried out tests of DLA structures employing cylindrical dielectric liners fabricated from both high purity alumina (epsi~9.4) and magnesium calcium titanate (epsi~20). In the case of alumina, we have tested structures either with or without a low-secondary-coefficient TiN coating. In all cases, a strong multipactor effect has been observed. However, the effect has been found to depend on the both the dielectric used and the experimental geometry. Further tests are planned, including tests using other dielectric materials. The latest results will be presented and discussed

A joint Argonne National Laboratory/Naval Research Laboratory program is under way to investigate X-band dielectric-loaded accelerating (DLA) structures, using high-power 11.424-GHz radiation from the NRL Magnicon Facility. DLA structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient RF linear accelerators. The purpose of the high-power tests is to find the RF breakdown limits of these structures and to test their ability to produce high accelerating gradients. We have recently tested two different dielectric tubes, one with alumina and another with MgCaTi liners. For alumina, we have shown no evidence of RF breakdown at up to 5 MW drive power (equivalent to 8 MV/m accelerating gradient). However, multipactor effect was found to absorb an increasing fraction of the incident microwave power, as the power level was increased, ultimately absorbing half of the incident power. Detailed experimental results and solutions for curing the multipactor will be discussed. We will also report on the most recent test results of MgCaTi dielectric tubes

Electrical breakdown is one of the most serious problems for developing compact and/or higher-voltage insulation in a vacuum. High secondary electron emission (SEE) yields result in the multipactor effect (electron multiplication on the dielectric surface). Multipactor induces not only discharging, but also excess surface heating, leading to localized surface melting. Thus, SEE at high temperature is important for understanding the actual breakdown process. The SEE yields of sapphire were measured at high temperature by a single-pulsed beam method with a scanning electron microscope (SEM) so as to avoid surface charging. In general, the effective SEE decreases by multipactor due to surface charging. Since the electrical conductivity becomes higher at a high temperature, effective SEE can be larger due to less surface charging. In order to estimate the surface charging, multi-pulse beams were injected to sapphire disks at room and high temperature.

Summary form only given. A joint Argonne National Laboratory/Naval Research Laboratory program is under way to investigate X-band dielectric-loaded accelerating (DLA) structures, using high-power 11.424-GHz radiation from the NRL Magnicon Facility. DLA structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient rf linear accelerators. The purpose of the high-power tests is to find the rf breakdown limits of these structures and to test their ability to produce high accelerating gradients. Recent tests using a copper accelerating tube with an alumina liner have shown no evidence of rf breakdown at up to 5 MW drive power (equivalent to 8 MV/m accelerating gradient). However, substantial absorption of the incident microwave radiation was observed, accompanied by visible light emission from the dielectric surface, indicating a strong multipactor effect in the tube. The multipactor was found to absorb an increasing fraction of the incident microwave power, as the power level was increased, ultimately absorbing half of the incident power. This behavior is unlike that observed in multipactor on rf windows, which typically saturates at /spl sim/1% power absorbed, independent of incident power. We present a simple model that explains these observations, and discuss means to eliminate the effect.

Our presentation describes computer modelling of a possible multipactor effect in the output system of an experimental S band magnetron. The output system is a cylindrical waveguide with two coaxial lines feeding a dipole. A metal rod supports the dipole in the middle. It has been observed that certain areas of the two coaxial coupling rods would melt during some operations. Computer simulation shows no indication of possible current overheating or 'ghost-mode' resonance in the melted region. This leads us to suspect that the multipactor. or the related gas discharge is responsible for the melting of the coupling rods.

A plasma waveguide switch based on multipactor effect for switching low/high power microwave is proposed. No external triggering and no actuator are needed for the switch. Through controlling the electron flow formed on the waveguide sidewall based on field-induced electron emission and secondary electron multipactor, the high power capacity, low loss, and rapid activation time are realized in the microwave switch simultaneously. The power loss is about −20 dB, turn-on time is 5 ns, and the power capacity of hundreds of megawatt can be achieved. The switch has been used for backward wave oscillator injection locking.