Research On Electron Beam Dynamics Of MHz Repetition Rate And Its Wakefield Effect

It is necessary for advanced X-ray flash radiography to develop linear induction accelerator (LIA) with multi-pulse high-current electron beam because of its multi-flash imaging technology. For needs of good quality of MHz repetition rate (pulse interval less than 1μs) high-current electron beam and multi-flash X-ray imaging, it is important to initiate beam dynamics research. A miniature double-pulse LIA (Mini-LIA) was constructed by CAEP and Tsinghua University in order that multi-pulse high-current beam dynamics is studied and parts of some new key techniques are preliminarily tested on it.Three main dynamics factors——space charge effect, corkscrew and wake field effect influencing high-current beam transportation are compared and contrasted from their physical origins to their deteriorations on the beam. It is pointed out that the emphases of multi-pulse high-current beam dynamics are wake field effect produced among multi-bunch and cumulative beam breakup instability in beam transportation. Decay of wake field generated in LIA accelerating gap and its influence on multi-pulse high-current transportation are analyzed in this paper. A code named Minienvelope is written to simulate beam envelope changes of single-pulse beam moved on Mini-LIA and the variation of beam centroid is simulated by using PARMELA code.By using CST microwave studio code, two structures of LIA cavity——empty cavity and metglas cavity are simulated respectively, and the resonant frequencies of several TM electromagnetic modes in two kinds of cavity as well as their corresponding longitudinal characteristic impedances R/Q are obtained. A bench measurement system is constructed for investigating characteristics of higher order modes (HOM) in Mini-LIA cavity. Discerning and confirmation of TM electromagnetic modes in two cavities are performed, and several TM modes in two cavities as well as their corresponding quality factors are achieved. The transverse impedances of two kinds of cavity are calculated by using a formula deriving from longitudinal characteristic impedances of dipole mode (TM1n0 mode). This paper describes the principle, method, and techniques of transverse impedance measurement through coaxial twin-lead (ηfactor method). A bench measurement system based on vector network analyzer is constructed for transverse impedance measuring of Mini-LIA cavity. Theηfactors and transverse impedances of TM₁₁₀ mode in two kinds of cavity are measured by experiment, and the result is consistent with the calculated one. A practical technique of transverse impedance measuring of LIA cavity with low Q is available. At last, analysis to several kinds of measuring error in transverse impedance measurement method withηfactor is performed and methods to correct the errors are discussed in this paper according to simulation result.