Abstract: A series of detailed numerical simulations are used to investigate the properties ofmultipactor breakdown in circularwaveguidespropagating the TM01 mode. AMonte Carlo model is constructed to track the motion of the electrons, study the multipactor scenarios, and predict the multipactor thresholds. The theoretical and numerical analyses indicate that the product of the frequency and the gap (f . D) affects both the intensity of the ponderomotive force and its spatial distribution, which results from the nonuniformity of the radio frequency (RF) field and significantly influences the electrons' trajectoriesandmultipactor trends. The decrease in f . D results in a remarkable enhancement in the magnitude of the ponderomotive force, while the maximal intensity gradually moves toward the half radius R/2 area. Low values of f . D correspond to high ponderomotive potential, which sustains the short-range electrons and triggers the single-sidedmultipactor. In contrast, high values of f . D correspond to low ponderomotive potential, contributing to long-range electrons and exciting the double-sided multipactor. Fitting to the susceptibility diagram produces the border line and a modified f . D threshold of (f . D) th approximate to 338.4 GHz mm, which separates the susceptibility diagram into single-sided, double-sided, andmixed-sided zones. The initial electron energy influences their trajectories at high f . D and low RF power. This effect tends to dominate the multipactor behavior in the mixed-sided region.

Abstract: RADES (Relic Axion Detector Exploratory Setup) is a project with the goal of directly searching for axion dark matter above the 30 μeV scale employing custom-made microwave filters in magnetic dipole fields. Currently RADES is taking data at the LHC dipole of the CAST experiment. In the long term, the RADES cavities are envisioned to take data in the BabyIAXO magnet. In this article we report on the modelling, building and characterisation of an optimised microwave-filter design with alternating irises that exploits maximal coupling to axions while being scalable in length without suffering from mode-mixing. We develop the mathematical formalism and theoretical study which justifies the performance of the chosen design. We also point towards the applicability of this formalism to optimise the MADMAX dielectric haloscopes.

Abstract: Dielectric Assist Accelerating (DAA) structures based on ultralow-loss ceramic are being studied as an alternative to conventional disk-loaded copper cavities. This accelerating structure consists of dielectric disks with irises arranged periodically in metallic structures working under the TM02-pi mode. In this paper, the numerical design of an S-band DAA structure for low beta particles, such as protons or carbon ions used for Hadrontherapy treatments, is shown. Four dielectric materials with different permittivity and loss tangent are studied as well as different particle velocities. Through optimization, a design that concentrates most of the RF power in the vacuum space near the beam axis is obtained, leading to a significant reduction of power loss on the metallic walls. This allows to fabricate cavities with an extremely high quality factor, over 100,000, and shunt impedance over 300 M omega/m at room temperature. During the numerical study, the design optimization has been improved by adjusting some of the cell parameters in order to both increase the shunt impedance and reduce the peak electric field in certain locations of the cavity, which can lead to instabilities in its normal functioning.

Abstract: The recent high demand of secondary electron emission yield (SEY) measurements in dielectric materials from space industry has driven SEY laboratories to improve their facilities and measurement techniques. SEY determination by the common capacitive method, also known as pulsed method, is well accepted and has given satisfactory results in most cases. Nevertheless, the samples under study must be prepared according to the experimental limitations of the technique, i.e. they should be manufactured separated from the devices representing faithfully the surface state of the own device and be as thin as possible. A method based on the Kelvin probe (KP) is proposed here to obtain the SEY characteristics of electrically floating Platinum, Kapton and Teflon placed over dielectric spacers with thicknesses ranging from 1.6 to 12.1 mm. The results are compared with those of the capacitive method and indicate that KP SEY curves are less sensitive to spacer thickness. An explanation based on the literature is also given. In all, we have established that KP is better suited for the analysis of dielectric samples thicker than 3 mm.

Abstract: 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.