CERN Accelerating science

Thesis
Report number	CERN-THESIS-2022-189
Title	Numerical calibration of the bead-pull measurement setup for beam coupling impedance evaluation
Author(s)	El Dali, Dalia (Universita e INFN, Perugia (IT))
Thesis note	Master : Universita degli Studi di Perugia : 2022
Thesis supervisor(s)	Passeri, Daniele ; Morozzi, Arianna ; Zannini, Carlo
Note	Presented 2022
Subject category	Accelerators and Storage Rings
Abstract	To identify the accelerator structures that are contributing to performance limitations and execute mitigation techniques, a precise estimate of the beam coupling impedance is required. The beam coupling impedance of a device should ideally be assessed by activating it with the beam itself. However, in most circumstances, this solution is not practicable, and additional approaches must be used to analyze the influence of the beam. A well-known approach is to replicate the beam by feeding a current pulse via a wire stretched along the beam axis. The stretched wire method is a popular and well-liked method for determining beam coupling impedance. Yet, because the introduction of the stretched wire perturbs the EM boundary conditions, the information obtained from wire measurements may not exactly represent the answer to our initial dilemma. The existence of another conductive medium in the center of the device under investigation has the most obvious effect of allowing TEM propagation across the device with zero cut-off frequency. The existence of a TEM mode among the EM issue solutions will have the unintended consequence of causing extra losses. The modeling of the beam coupling impedance of complicated or rounded-shaped accelerator parts is difficult, and other methods are needed. The Bead-Pull method is a popular electromagnetic field measuring technique for tuning an RF cavity to meet design criteria. A tiny dielectric or metallic bead is drawn through a cavity while electric field measurements are obtained. Inserting a metallic or dielectric bead into a resonant cavity alters its frequency. This frequency shift is proportional to the sum of the squared amplitudes of the electrical and magnetic fields at the bead’s position. The magnitude of the perturbation for a particular electromagnetic field is solely determined by the geometry of the perturbing item. As a result, calibration of the bead may be performed in a variety of resonant topologies without sacrificing generality. A method for doing precise bead calibration using electromagnetic simulations is proposed in this study. In comparison to the conventional technique of measuring a reference cavity, the simulation method’s flexibility in studying alternative bead shapes and sizes may be useful in optimizing the measurement setup. A calibrated bead-pull configuration allows you to measure the electric field and hence the shunt impedance of the cavity’s resonant modes. The beam coupling impedance obtained with the calibrated bead-pull arrangement is compared with well-established electromagnetic models.

Email contact: alessia.valenza@cern.ch