ANALYSIS OF MULTIPACTION EFFECT IN S-BAND COMPONENTS FOR PARTICLE ACCELERATORS

This work has been developed within a cooperation between the Microwave and Optical Engineering MOE group – Department of Electrical and Information Engineering of Politecnico di Bari and the enterprise LinearBeam (Ruvo). It is an in-depth analysis of the multipaction effect, in the context of waveguide-based components for particle accelerators. This effect consists in an electronic avalanche inside a RF component which normally operates under vacuum conditions. If present, multipaction can not only physically damage the device by giving birth to arcing and corona discharges, but can also be responsible of other phenomena which should be avoided in RF systems. After a brief overview of particle accelerator systems, a specific focus on the structure and components used in their RF power distribution system is provided. Then the theoretical framework extensively employed in literature for the understanding of the multipaction effect is briefly illustrated. In addition, techniques used for its evaluation by means of Particle-In-Cell simulations have been employed in this work for the detection of multipaction thresholds. In particular, Particle-In-Cell simulations have been performed to investigate a waveguide-based directional coupler, namely a Riblet short slot coupler, at our disposal for the measurements. The results obtained with PIC simulations have shown the limits of operation of this device, which should not be exceeded in order to avoid the aforementioned effects if one wants to keep the device under its safety operational limits.