Dottorato in FISICA DEGLI ACCELERATORI

Titolo della tesi: Beam Breakup and High Order Modes Instabilities Studies for Energy Recovery Linear Accelerators

Energy Recovery has the potential to become a significant accelerator advancement technique. This involves using the kinetic energy from a previously used beam to accelerate a newly injected beam, resulting in reduced power consumption. This concept, which was invented half a century ago, also takes advantage of the high brightness of injectors and disposes of excess energy at the injection level. This innovative accelerator concept is comparable to the revolutionary ideas of Wideroe, Lawrence, Veksler, Kerst, Van der Meer, and other accelerator technology pioneers from the last century. In principle, electron beams in an Energy recovery linear accelerator (ERL) are generated and accelerated through the linac, then transported to a magnetic arc lattice to their end use (e.g. to drive radiation sources). Later, the electrons are transported back to the entrance of the linac with 180° out of phase for deceleration and energy recovery to be dumped at an energy close to their injection energy. The study and simulation done in this Thesis, are based on the ERL model of BriSXinO, which is a test bunch demonstrator of BriXS (Bright and Compact X-Ray Source) and is currently under investigation at LASA INFN laboratory in Milan. The first stage of the study reported in this thesis will be mainly in a linac based on coupled Superconducting (SC) cavities considering only the forward propagation of the beam. The second stage will be then dedicated to the ERL working mode where more than 90% of the beam can be recovered due to the scheme proposed by BriXSinO. This leads to significantly reduced power consumption from the walls. The machine’s high current operation (5 mA) presents serious difficulties for the beam dynamics (BD). The primary aim of designing an accelerator is to deliver the required power while minimizing any losses that could reduce the efficiency of the machine and potentially cause harm to various crucial pieces of equipment. Beam loss in the accelerator can be attributed to various factors, and one of the significant sources is beam-excited high-order modes (HOMs), where HOM couplers are typically employed to mitigate their effects on the beam. HOMs investigations in SC cavities were obtained within a collaboration between INFN LASA and INFN-Napoli group. The impact of HOMs on the BD which leads to Beam Breakup (BBU) and the ways to suppress them are presented in this work. This has been combined with theoretical and numerical BD studies based on two models called HOMEN and Compact HOMEN respectively. These models are performed by using different numerical integration and accelerator figures of merit, like the accelerating voltage and the quality factor parameters. In addition, an ERL application that was born in the frame of BriXSinO research activity will be presented in this work, which consists of a dual source able to generate THz and X-Ray radiation simultaneously.