China Academy of Engineering Physics terahertz free electron laser(CAEP THz FEL, CTFEL) is the first THz FEL oscillator in China, which is jointly built by CAEP, Peking University and Tsinghua University. It is designed as a high-repetition-rate and high-duty-cycle linac-based FEL facility.This THz FEL mainly consists of a gallium arsenide(GaAs) photocathode high-voltage direct current(DC) gun, a superconducting radio frequency(RF) linac, a planar undulator, and a quasi-concentric optical resonator. The DC ...