Superconducting Cavity - Photoinjector Device Physics And Experimental Research,

Superconducting accelerator and photocathode injector is the first choice for high average power free electron laser study. In this paper, the experiment of a one-cell superconducting cavity accelerator with a normal conducting photocathode RF Gun was introduced. Some physics issues about this experiment system was analyzed.The use of superconducting niobium cavity allows a reduction in RF surface resistance by about 105 compared with room temperature copper, so the design choices for superconducting cavity shape is different from normal conducting cavity. For superconducting cavity design, the large bore 7r-mode elliptical cavity is used. A analytical treatment was made for 7i-mode pillbox cavities, the results of resonant frequency and axis electrical field has been compared with the computations of elliptical superconducting cavity done with the SUPERFISH code, and then the modified analytical result of axis electrical field distribution for superconducting cavity is given. The MAFIA code is used to calculate high order mode for one-cell superconducting cavity. A brief analyze of long range wakefields effect in superconducting cavity is given.To simplify the problem of matching the RF generator and the cavity with beam load, the beam load impedance is ascertained. By analyzing the stored energy of superconducting cavity, the relations between axis electrical field and beam load is obtained.In superconducting cavity coupler design, the coupler is divided into two parts: the waveguide impedance transform and the waveguide coaxial line conversion. By means of analytical treatment, numerical computations and model coupler experiment, the coupling parameter of superconducting cavity beam tube coupler is obtained.The study of electron beam dynamics in RF Gun is based on K. J. Kim's theory. The longitudinal motion of electron beam is analyzed with the variable length of the half cell of photocathode RF Gun. The relations between the half cell length and electron beam initial injection RF phase is given.To make the physics image clear in analyzing the transverse emittance growth, we introduced the conception of emittance growth factor for linear effect induced transverse emittance growth. The transverse motion of electron beam is also analyzed with the variable length of the half cell of photocathode RF Gun. The analytical result shows that by increasing the length of the half cell and decreasing initial injection phase, the longitudinal compressibility is increased, the emittance growth is reduced.The system of photocathode RF gun and superconducting accelerator is more complex than the normal conducting accelerator. This experiment system include Nd:YAG Mode-locked laser, Cs2Te cathode, 2+1/2 RF gun, L band 3.5MW Microwave source, 1.3GHz superconducting cavity, 500W continue microwave source, coaxial beam tube coupler, 4.2K cryostat, helium liquefied system, control system, beam diagnostic, and vacuum system. In June 2001, the experiment of this system have been done in CAEP. The electron energy is 3.4MeV, the micro-pulse current is 0.1 A, the electron beam energy gained in superconducting cavity is 0.58MeV, the macro-pulse width is 1 s.