Ultra-short bunch length measurement via observation of OTR using a streak camera (2)

A test accelerator as a coherent terahertz source (t-ACTS) has been under development at Tohoku University, in which an intense coherent terahertz (THz) radiation is generated from an extremely short electron bunch. An injector of t-ACTS is consisted of a thermionic RF gun, an alpha-magnet as an energy filter, and a 3m-long travelling wave accelerating structure. A velocity bunching scheme is employed for the production of short electron bunches. According to a numerical simulation, ∼100 fs bunch can be produced by using the t-ACTS accelerator configuration. An experiment of short bunch production has been carried out and the bunch length was measured with a streak camera by measuring optical transition radiation from the electron bunch. We have succeeded in producing a sub-picosecond electron bunch, however a time resolution of the measurement was approximately 400 fs. To improve the time resolution, we performed a modification of the optical transport line of OTR for the measurement of streak camera with the fast range and the optical system was evaluated by ray tracing. The time resolution of the measurement system and measurement results are described in this paper. (author)