Subject: first lasing of a THz FEL at CAEP

The first lasing near a wavelength of 0.12mm was  
achieved in March 24,2005 on a THz free-electron laser  
(FEL) oscillator at China Academy of Engineering  
Physics (CAEP) [1].  
The simulated radiation was about 40 times of the  
spontaneous radiation, and was achieved at a special  
length of the optical cavity, or in other words, a  
point of the detuning curve with a length of about  
10mm. The relative line width (FWHM) of the simulated  
radiation is about 1.7%, and is many smaller than that  
of the spontaneous radiation of about 6%.  
 
   The FEL is driven by an L-band RF linear  
accelerator and has a linear planar wiggler. The  
envelope of the electron beam in the y-direction is  
governed by focusing effect of the wiggler, but the  
electrons drift free in the x-direction. To keep the  
transverse beam extension small the THz laser beam is  
guided in the y-direction by a plane waveguide with a  
gap of 14mm. The semi-closed optical cavity with a  
length of 2.54m composed of two symmetric spherical  
metal mirrors with a curvature of 1.77m and a hole  
with a diameter of 1mm on axis of the down-stream  
mirror is used to extract the radiation from the  
cavity [2]. The electron beam has an energy of 6.5MeV,  
a peak current of 4A, a micro bunch length of 25ps and  
a repetition rate of 1.3GHz, and a macro bunch length  
of 4ms and a repetition rate of 0.75Hz. The energy  
spread and emittance of the electron beam is about 1%  
in FWHM and 1.5 p mm mrad in rms. The wiggler has a  
length of 1.5m, a period of 3.2cm, and peak field of  
3.2kG.  
 
1.        Xiaojian Shu and Yuanzhang Wang, Simulation  
and design of the CAEP far-infrared FEL, Nucl. Instru.  
& Meth., A483, 2002:205-208.  
 
2.        Xiaojian Shu, Yuanzhang Wang, Yunqing Jiang,  
Zhichou Zhang and Wu Ding, Numerical simulations of a  
waveguide FEL oscillator using hole coupling, Nucl.  
Instru. & Meth., A407, 1998:76-80  
 
Your sincerely  
 
Xiaojian Shu  
Institute of Applied Physics & Computational  
Mathematics (IAPCM)  
Ming Li, Xiao Jin
Institute of Applied Electronics, CAEP