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Electron clouds hinder the operation of particle accelerators. In the Large Hadron Collider (LHC), the copper beam screens are located within close proximity to the beam path, resulting in beam-induced electron multipacting, which is the main source of electron cloud formation. Conditions for multipacting are encountered when such surfaces have a secondary electron yield (SEY) greater than unity. Roughening the surface through laser processing offers an effective solution for reducing secondary electrons. Laser ablation leaves behind a complex rough, multi-scale geometrical surface with an altered chemical composition. Current models often over-simplify the geometry, do not have sufficient experimental data to derive input parameters, and exclude SEY-reducing mechanisms such as the surface chemistry. Leading to electron-matter interactions which do not resemble that of a real surface. Here, this complex surface is studied on copper used in the LHC, and the influence of microgeometry, inhomogeneous nanostructure and complex surface chemistry on the SEY is investigated. A novel, improved model is proposed that characterises these sophisticated structures, enabling the efficient design of surfaces to reduce SEY. To validate the model, samples were made using a variety of laser parameters. Modelling insights revealed that secondary electron suppression is not only caused by the microgeometry but also the nanostructure and chemical modification play a role. Contrary to the conventional theory, high aspect ratio structures are not necessarily required for effective SEY reduction. Currently, the model is applicable to a variety of surface morphologies and could be employed for other materials. [ABSTRACT FROM AUTHOR]

Particle trajectory and collision simulation is a critical step of the design and construction of novel particle accelerator components. However it requires a huge computational effort which can slow down the design process. We started from a sequential simulation program which is used to study an event called Multipacting. Our work explains the physical problem that is simulated and the implications it can have on the behavior of the components. Then we analyze the original program's operation to find the best options for parallelization. We first developed a parallel version of the Multipacting simulation and were able to accelerate the execution up to ∼ 35 × with 48 or 56 cores. In the best cases, parallelization efficiency was maintained up to 16 cores ( ∼ 95 %) and the speed-up plateaus at around 40–48 cores. When this first parallelization effort was tried for multi-power simulations, we found that parallelism was severely limited with a maximum of 20 × speed-up. For this reason, we introduced a new method to improve the parallelization efficiency for this second use case. This method uses a shared processor pool for all simulations of electrons (OnePool). OnePool improved scalability by pushing the speed-up to over 32 × . [ABSTRACT FROM AUTHOR]

This article presents a theoretical and experimental comparative study of the different multipactor threshold values obtained in the rectangular waveguide (RW) and the groove gap waveguide (GGW). To this end, the multipactor effect has been first analyzed in an RW with a recently developed theoretical model. Then, the multipactor breakdown power levels in the equivalent GGW have been predicted with an accurate electron tracking code, showing a significant increment compared with the RW case. In order to validate these results, two $E$ -plane WR-90 RW transformers have been designed with a full-wave electromagnetic simulation tool. The central sections of these transformers have been implemented in RW and GGW, respectively, and their multipactor breakdown power levels have also been predicted. The two designed transformers have been fabricated in aluminum and then measured in terms of electrical response (scattering parameters) and RF multipactor effect (power threshold values). All the experimental results agree well with the set of simulated data, thus fully validating the performed study. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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

Multipactor effect is a well-known phenomenon of RF breakdown in satellite payloads which degrades components, generates undesirable harmonics, contributes to power dissipation and increases noise in communications. Traditionally, multipactor has been investigated with the aim of obtaining the so-called multipactor threshold voltage, or to present different multipaction detection methods. However, very little attention has been focused on analysing this phenomenon using a multimodal approach. The main goal of this work is to analyse the interaction between a multipactor current and a realistic microwave cavity by means of a rigorous and accurate formulation. For the first time to the authors’ knowledge, a full-wave method based on the 3-D Boundary Integral – Resonant Mode Expansion (BI-RME) technique is proposed to model the aforementioned undesired effect. In this work, the classical formulation used in the 3-D BI-RME method is exploited to achieve a full-wave representation of the multipactor effect in terms of an equivalent multimode network expressed in the form of a generalized admittance matrix. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

The aim of this paper is the study of the RF multipactor breakdown in coaxial transmission lines excited by a single carrier with a digitally modulated signal. Employing an in-house developed code, numerical simulations are performed to determine the RF multipactor voltage threshold for several digitally modulated signals under different modulations schemes: quadrature phase-shift keying, 16-quadrature amplitude modulation, 16-amplitude and phase-shift keying, and 32-amplitude and phase-shift keying. Moreover, a coarse method based on the envelope integration to determine the RF multipactor voltage threshold when involving arbitrary digital modulations is also presented. These results are also compared with the “20-gap-crossing” rule used in the space standard document ECSS-E20-1A. In order to validate the theoretical results, a test campaign was performed for realistic modulated signals, finding good agreement between theoretical predictions and experimental data. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The aim of this paper is to study the multipactor radio-frequency breakdown voltage in several ridge and multiridge waveguide configurations. First, multipactor susceptibility charts for several types of ridged waveguides have been computed using the commercial software FEST3D. Next, these charts have been used to predict multipactor threshold values for a bandpass filter and a quasi-low-pass filter both containing ridge waveguide sections. Furthermore, multipactor simulations using FEST3D are carried out to calculate the multipactor threshold of the aforementioned structures. A good agreement between predictions and simulations has been found for both filter examples. [ABSTRACT FROM AUTHOR]

This paper is aimed at studying the electromagnetic radiation pattern of a multipactor discharge occurring in a parallel-plate waveguide. The proposed method is based on the Fourier expansion of the multipactor current in terms of time-harmonic currents radiating in the parallel-plate region. Classical radiation theory combined with the frequency domain Green's function of the problem allows the calculation of both the electric and the magnetic radiated fields. A novel analytical formula for the total radiated power of each multipactor harmonic has been derived. This formula is suitable for predicting multipactor with the third-harmonic technique. The proposed formulation has been successfully tested with a particle-in-cell code. [ABSTRACT FROM AUTHOR]

• Ordered monolayer silver nano-bowl coatings prepared by an electrodeposition method. • Coating structure modulated by altering the deposition time and template diameter. Multipactor leads to the unrepairable damage to high-power microwave devices in the satellite. Restraining the secondary electron yield (SEY) is an effective method to suppress the multipactor of microwave devices. In this work, silver nano-bowl coating was fabricated via electrodeposition in AgNO 3 electrolyte at room temperature on the surface of silver-plated aluminum alloy using the monolayer colloidal crystal of polystyrene (PS) spheres as templates. The scanning electron microscope images show that the surface structure of the silver-plated aluminum alloy sample could be modulated by altering the deposition time and adjusting the size of the PS spheres. And the pore diameter and the aspect ratio of the nano-bowl have a great influence on the SEY of the silver nano-bowl coating. When the size of the PS spheres is 2000 nm and the electrochemical deposition time is 90 min, the maximum value of SEY value of silver nano-bowl coating drastically decreases by 39.3% from 2.44 to 1.48, compared with that of the initial silver aluminum alloy samples. The silver nano-bowl coating shows great multipactor suppression for potential application in a satellite. [ABSTRACT FROM AUTHOR]