Design and Experimental Verification Study of Non-invasive Short Electron Bunch Length Monitor for AWAKE Run 2

Abstract

The Advanced Wakefield Acceleration (AWAKE) experiment at CERN is a proof of-principle experiment in which a 400 GeV proton beam from the Super Proton Synchrotron (SPS) is used as a drive beam to excite plasma for the first time in a beam-based plasma wakefield accelerator (PWFA). The motivation of the AWAKE program is to achieve high accelerating field gradients to accelerate the externally injected electron bunch while preserving beam quality, and ultimately produce highbrightness and high-energy particle beams for particle physics research. AWAKE Run 2 consists of four phases, Run 2a, 2b, 2c, and 2d, each with different tasks to complete for the ultimate goal of Run 2. The aim of AWAKE Run 2c is to inject electrons with a 200 fs bunch length, equal to the distance between two proton microbunches, to achieve and utilize the maximum accelerating gradient above 0.5 GV/m. Therefore, a precise, non-invasive, and real-time bunch length measurement technique must be provided to ensure and maintain beam quality during operation. In this thesis, a new concept of a non-invasive longitudinal electron bunch length monitor designed and experimentally commissioned for AWAKE Run 2c, based on the detection of the coherent Cherenkov Diffraction Radiation (ChDR) phenomenon. Coherent ChDR is emitted due to dynamic polarization of a dielectric medium induced by a charged particle passing in the near vicinity of it. The coherence of the radiation is attained once the electrons within the bunch radiate in phase, i.e., the radiation wavelength is comparable to or larger than the electron bunch length. The coherent ChDR bunch length monitor consists of three identical radiators placed on one side of the traveling bunch to minimize the impact of beam position and angular jitter on the measurements. The novelty of the ChDR setup is that it possesses two independent bunch length measurement techniques. One method aims to provide shot-by-shot RMS bunch length measurements, and the other is a detailed bunch profile measurement based on interferometric measurements. Both ChDR measurement methods can operate spontaneously to provide cross-checks between measurements. A comprehensive theoretical study is carried out and demonstrated to design the radiator shape and optimize the ChDR detection hardware. The design of the ChDR vacuum chamber, including modifications to the radiator and additional components used in the beam line, is described in detail. The results of the experimental tests performed at the CLEAR facility to calibrate and commission the system are shown and analyzed. It is demonstrated that the ChDR bunch length and bunch profile results are consistent with the invasive RF deflector technique used as the gold standard during the tests. Further tests to increase the accuracy of the calculation technique and potential improvements to install the ChDR setup as an operational instrument for Run 2c are discussed. Any accelerator that uses short bunches, such as Free Electron Lasers (FELs), can adapt the ChDR bunch length monitor as a real-time, precise, and non-invasive diagnostic tool.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Darren Graham (Supervisor) & Guoxing Xia (Supervisor)