Cavity Beam Position Monitors State of the Art and Design Studies in the Context of the ILC Project

Abstract

The aim of this master thesis project is the study and evaluation of the design of cavity beam position monitors (BPM) for the International Linear Collider (ILC) project. The state-of-the-art of BPMs is investigated to determine the most appropriate existing BPM design for the requirements of the ILC. The theoretical background on resonant cavities, including the beam coupling to the BPM, is examined. This knowledge enables the understanding of how certain parameters can affect the performance of the detector. To this end, a design investigation is carried out by analyzing the frequency spectrum of the beam for choosing the operational frequency of the resonator. Different common-mode-rejection methods are evaluated as well. Theoretical predictions are confirmed by 3D electromagnetic simulations, with the commercial software CST, based on a basic design of cavity BPM. The simulations are also performed in order to understand the impact of the cavity geometry on the BPM. The identication of the dipole mode signal for an offset beam corroborates the working principle of the cavity BPM. The particle-in-cell (PIC) solver from CST is revealed to be the most adequate for the simulation of the cavity BPM, allowing an efficient extraction of the dipole field induced by the beam. The state-of-the-art and the numerical results support the design of a cylindrical cavity with operational frequency of 1.495 GHz, with rectangular waveguides output ports used as filters to reject the monopole mode signal.

Popular Abstract

Large particle accelerators are usually used in the research of fundamental particle physics. The International Linear Collider (ILC) project, proposed to be built in Japan, consists of two large linear colliders where electromagnetic fields are used to accelerate and guide protons and electrons to nearly the speed of light. These particles are aimed at each other to create a collision, and then scientists observe what particles are created. Since these machines require incredible precision, components are required with extremely tight tolerances.
This thesis focuses on one of the key components allowing a proper analysis of the particle beam: the beam position monitors (BPMs). These detectors allow one to determine the position of the beam, so it can be properly guided along the accelerator path until the collision. In general, one wants to keep the beam as centered as possible inside the pipe. Because the size of the beam is so small (only 1/100th of the width of a human hair!) and the particles must stay grouped closely together through their entire path, the BPM needs to detect the position of the beam with extremely high resolution. This resolution is determined by the smallest distance between the center of the pipe and the beam that the BPM can detect.
Many kinds of BPMs have been developed with different goals. When precision and resolution are essential, cavity BPMs are the best option. These BPMs consist of a cavity (usually cylindrical) placed along the beam pipe. When charged particles pass through the BPM, their electromagnetic fields remain trapped in the shape of the cavity. The beam will create different electric and magnetic fields inside the cavity that vibrate at different frequencies depending on its exact position. These frequencies can be measured to identify the beam’s exact position. As easy as it seems, the detection of these signals is not always straightforward. The electromagnetic field that the BPM is trying to identify is called dipole mode, and its signal is usually contaminated with noise. Once this noise is removed the high-resolution reading of the position can be determined.
This paper focuses in particular on how these detectors work. Part of the study focused on how the geometry of the cavity influences the vibration frequency of the electromagnetic fields. We found that the geometry of the detector can also be modified by adding components to reduce noise. A software was used to simulate fields that would be observed by the BPM allowing us to confirm our theoretical predictions, and different calculations in the software were used to obtain the dipole mode that is used to read the beam position.
Some areas could use more research, like how the material of the cavity influences the performance of the detector, or better methods to reduce noise in the detected signal. All of these studies may contribute to our overall goal of increasing the resolution of our BPM while keeping production costs as low as possible.

• author: Pedraza Motavita, Laura Karina
• supervisor: Daniel Esperante; Nuria Fuster; Benito Gimeno; Oxana Smirnova
• organization: Particle and nuclear physics; Department of Physics; Faculty of Engineering, LTH
• course: PHYM01 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• language: English
• id: 9124545
• date added to LUP: 2023-06-14 11:08:07
• date last changed: 2023-06-14 11:08:07