Broadly tunable THz free electron laser amplifier
陳家祥1*, 趙芙涵2, 邱鈺中1, 甘逸凱2, 黃冠諺1, 周炳榮3, 黃衍介1,2
1Institute of Photonics Technologies, National Tsing Hua University, Hsinchu, Taiwan
2Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
3Beam Dynamics Group, National Synchrotron Radiation Research Center, Hsinchu, Taiwan
* presenting author:Chia-Hsiang Chen,
Most THz FEL only needs a MeV electron beam, which can be comfortably generated from a photoinjector. However, a photoinjector, despite of its superior beam quality, often operates with a 10-100 Hz pulse rate. Such a slow pulse rate is not suitable to build an FEL oscillator. Although self-amplified spontaneous emission could be a choice for a THz FEL driven by a photoinjector, the space-charge debunching in a low-energy beam hinders the power buildup of such a single-pass THz SASE FEL. In this paper we present a broadly tunable MW THz FEL amplifier driven by a photoinjector with a sub-kW seed THz source tunable between 0.7-2.0 THz. Specifically an S-band photoinjector at 2.856 GHz generate a 3-5 MeV electron bunch with 500 pC charge in a 4.25 ps rms bunch length, which is injected into a 2-m long undulator with a period of 18 mm and an rms undulator parameter of 0.98. The driver laser of the photoinjector is a frequency quadrupled amplified, mode-locked Nd:YVO4 laser at 1064 nm. We recycle the unconverted infrared laser at 1064 nm to pump a THz parametric amplifier using a lithium niobate crystal as its gain crystal. This THz parametric amplifier generates a transform-limited THz pulse with sub-kW power between 0.7 and 2.0 THz, which is seeded into the undulator to produce broadly tunable, transform-limited, sub-MW THz radiation through FEL amplification with a gain of about 3000 at 2 THz. Since the pump laser of the THz OPA is derived from the driver laser of the photoinjector, the seed THz pulse is fully synchronized and overlapped with the electron bunch. Experimental progress of this work will be presented in the conference.

Keywords: free electron laser , THz , amplifier