Study of electron bunching in vacuum devices: Electron gun, Linear accelerator

Abstract

Electron bunching is an interesting issue with practical and scientific applications. In this thesis, I will present two topics of electron bunching in vacuum devices: electron gun, linear accelerator. For the electron gun, I will present a theoretical approach of bunching condition in dc-biased ac-driven vacuum devices. There are two different approaches. The one is electron bunching from a DC-biased single surface multipactor. Generation of electron bunch from a dc biased, single surface multipactor was studied theoretically and by PIC simulations. The condition for a spatially narrow bunch was obtained and verified by PIC simulations. This kind of multipactor is proposed to be used as a compact electron gun for various applications, such as linear accelerators. The other one is a theoretical correlation between the periodicity of an electron micro-bunch train and the transit phase of each electrons passing through a vacuum gap in a dc-biased ac-driven diode was derived. The upper frequency limitation by the transit time effect could be explained by abnormal exclusion of a certain range of transit phase analyzed for the first time by our theory. In a particle-in-cell simulation guided by our theory to evade the deficiency, a micro-bunch train with 1.41 picosecond periodicity (0.707 THz) could be obtained from a gap of 50 μm regarded as excessively large owing to severe suffering from the transit time effect. For the linear accelerator, A 9.3 GHz 6 MeV linear accelerator (LINAC) was analyzed by using three-dimensional (3D) particle-in-cell (PIC) simulations with 3D time-domain electromagnetic simulations. In the 3D PIC simulations, field data of a p /2 standing wave mode extracted from the 3D time-domain field calculations were injected into a side-coupled LINAC composed of 25 acceleration and 24 coupling cavities. Acceleration of an electron beam of 20 kV, 300 mA in the 3D full LINAC structure was analyzed without spatial or time segmentations, which resulted in maximum energy of 6 MeV and average current of 68.5 mA when 9.3 GHz, 1.6 MW radio frequency (RF) power was assumed. By virtue of in-depth data from the 3D electromagnetic and PIC simulations aided by a one-dimensional (1D) particle code developed by ourselves, the analysis for a full LINAC structure being reported first in this paper, may be informative and useful because experimental reports about an X-band (9.3 GHz) 6 MeV side-coupled LINAC have been rare until now.