Simulation for THz coherent undulator radiation from combination of velocity bunchings

We study the effect of a combination of velocity bunchings and its application to THz coherent undulator radiation at LEBRA, Nihon U. by simulations. The velocity bunching is a technique that is commonly used to make the bunch length shorter at lower energies. However, since one velocity bunching has a correlation between bunch energy and length, we may not have so much room to change energies to obtain different coherent radiation wavelengths. Hence we propose a combination of velocity bunchings that relaxes the restrictive correlation. We have three 4 m traveling-wave accelerator tubes at LEBRA, Nihon U. The undulator is installed after the acceleration tubes and 2 x 45 degree bending magnets. Since the design of current undulator requires less than 25 MeV beam energy to obtain the radiation in THz region, the velocity bunching is reasonable for coherent radiation. We show the simulation results of a combination of velocity bunchings of the three tubes and the magnetic bunching at bending magnets, suitable for coherent undulator radiation.