Development Of C-band Deflecting Cavity For The Measurement Of Ultra-shortelectronbunch Length

Deflectingcavityisastructuretodeflectthe electronsusing RFelectromagneticfield.Electrons change the direction of movement kicked by horizontal Lorentz force along the central axis of the cavity.The sizes of the deflection of the electrons varying in the direction of the electron beamare different due to the different magnetic field.Based on this principle,deflecting cavity can be used for beam diagnostics,etc.The C-band deflecting cavity researched in this paper is used for the project of MeV UED facility at Shanghai Jiao Tong University.It will be used for the measurement of 20 femtosecond beam length.The deflecting voltage is up to 1MV with 1MW power input.The working mode of deflecting cavity is TM110 mode,which is a dipole mode,existing degenerate mode.For a multi-cell structure,we need to ensure that the polarization direction of the microwave coupling between cells doesn't rotate.Two method to solve this problem is discussed in this paper including making holes on the disk between cells and making slots on the edge of cells.And we choose the latter to make the cavity.The 3-Cell standing-wave deflecting cavity is designedby analytical solution and electromagnetic simulation program like CST.The dimensions of the cavity are determined by the simulation results of electromagnetic field and particle dynamics.Meanwhile,the coupler of the cavity is designed.And water-cooling system is designed by thermal simulation.After machining,we need to ensure the working frequency and magnetic field flatness meet the requirement.To simplify the tuning process,the relation between field flatness change and working frequency change while adjusting radius of each single cell is researched by numerical simulation.And it presents great linear relationship,based on which,the process of tuning will be much simplified and it will be easier to ensure the accuracy.Besides,a bead-pull method using two wires to measure the electromagnetic field is presented.This method can void the inaccuracies of field measurement caused by the rotation of the perturbation bead.After tuning,the parameters of the cavity match the physical design of the cavity.It is expected to be able to work properly in the next high-power testing.