Bunch length is one of the key longitudinal parameters in beam diagnostics, a bunch length measurement system is urgently needed to monitor the variation of bunch length in HUST-FEL. Based on a thorough investigation, we propose a measurement system using streak camera based on the optical transition radiation (OTR). We discuss the classical theory of the transition radiation (TR) in HUST-FEL in detail, simulate the angular distribution of TR at different beam energies, and select the TR target material from the perspective of economy and practicability. Considering about the practical application, we also discuss the effects of finite size target and pre-wave zone of TR based on the Weizsäcker-Williams method of virtual photon.

Bunch length,FEL,Transition radiation,Angular distribution