Abstract
Intense relativistic electron beam having very short bunch length of a hundred femtoseconds or less is the key for many advanced applications such as free electron laser, Compton scattered x-ray, and coherent terahertz radiation. A radio frequency (rf) gun that adopts the thermionic cathode as the electron source is with the capability to produce multi-bunched electron beam stably. Nevertheless, the study of thermionic rf gun linac system has not been well invstigated so far. In this study, we intend to explore the potential ability of a thermionic cathode rf gun linac system for the production of sub-hundred femtosecond electron bunches. The electron beam dynamics in a thermionic rf gun linac system which consists of an α-magnet and a traveling wave accelerating structure are thoroughly examined and analyzed. It is found that the space charge effects overrule the beam dynamics seriously in the low energy side of such injector system. We thererfore consider a scheme called velocity bunching in the traveling wave accelerating structure for generation of sub-hundred femtosecond beam. We have demonstrated that thermionic rf gun possesses a promising advantage for velocity bunching. The compressed beam with bunch length of ~ 50 fs can be attained efficiently. In order to optimize the beam longitudinal phase space for velocity bunching, a novel thermionic rf gun equipped two independent cavity cells has been developed. Since the coupling between the two cells is small, the electric field and the phase difference between two cells can be adjusted independently. This allows the flexible manipulation of electron distribution in the longitudinal phase space. The proof-of-principle experiment which shows this feasible manipulation has been demonstrated successfully in this study. The aggressive bunch compression in the thermionic rf gun linac system is made possible with this advanced electron rf gun.