Generation of coherent undulator radiation based on ultra-short electron bunch at Tohoku University

A test accelerator as a coherent terahertz source (t-ACTS) has been under development at Tohoku University, in which a generation of intense coherent terahertz (THz) radiation via an extremely short electron bunch. Velocity bunching scheme in a traveling accelerating structure is employed to generate the extremely short electron bunch and a subpicosecond electron bunch generation was demonstrated. A long-period undulator, which has 25 periods with a period length of 10 cm and a peak magnetic field of 0.41 T, has been also developed and installed to provide intense coherent THz radiation. Matching condition of beam injection to the undulator was derived from the undulator parameters. Numerical studies were performed about a coherent THz radiation from the undulator. Electric field and the spectrum of undulator radiation for single electron and Gaussian shape of electron bunch in longitudinal direction were calculated. By optimizing the bunch length, it was found that it is possible to generate a perfectly coherent THz light. The angular distributions of undulator radiation for the fundamental mode also derived from the undulator parameters. The results of this study for the undulator radiation is described in this paper. (author)