Preliminary Physical Design Of A Microwave Gate - Controlled DC Electron Gun

In the IR-FEL, the gain and power are sensitively dependent on the energy spread of the electrons. These parameters are determined by design of the electron gun. The thesis introduces various electron guns in the world. Short bunch length at the exit of gun can reduce the increase of energy spread in the pre-bunching and bunching section. To meet the requirement of energy spread of IR-FEL, the paper designd a gridded thermionic electron gun with rf-gated.In the rf-gated gun, the electronic capture phase is modulated by rf field(The frequency of dual-frequency cavity are952MHz and2856MHz) and dc bias to get short bunch length (about50ps) at the exit of gun. The short bunch length can reduce the spread energy of low energy accelerator. In the paper a dual-frequency rf-gated gridded thermionic electron gun is design and simulated. For the rf-gated gun, a dual-frequency cavity is designed between cathode and grid wire, the modes of the microwave field of TM010and TM020are excited respectively by the feeding of power of the fundamental and3rd harmonic wave. The thermionic electron gun is designed on the base of DC gun of HLS â…¡. The static and rf field are simulated by POISSON and SUPERFISH. Then, beam dynamics simulation is executed by PARMELA to show the evolution of the bunch distribution in the structure. Lots of simulations are run to compare different beam parameters and gun parameters. By the optimization of gun parameters, a rf-gated gridded thermionic electron gun can meet the demanding of infrared free electron laser.In the paper we compared the beam tracking result of three different thermionic electronic gun(dc gun, rf-gated with3rd harmonic and rf-gated without3rd harmonic), it show that rf-gated with3rd harmonic can get shorter bunch length than without3rd harmonic when the charge is identical, so the rf-gated gun with3rd harmonic is more suitable for the gun of IR-FEL than without3rd harmonic. Meanwhile by the analysis of the influence of structure of grid wire to beam parameters, we got an optimal parameters of structure of grid wire. Finally, we also analyzed the influence of dc bias and rf field on beam parameters, and got optimal parameters of dc bias and rf field.The paper also designed a frequency of952MHZ pre-bunching cavity. The cavity can compress the bunch length to about9.2ps. The increasing of energy spread is about15.6keV in the per-bunching cavity. The increasing of energy spread is far less than other thermionic gun.