Design of Fel by the Eehg Scheme at Tsinghua University*

We adopt the Echo-Enabled Harmonic Generation (EEHG) scheme proposed by D. Xiang to design a device to generate intense narrow-band THz radiation based on the Tsinghua Thomson scattering X-ray (TTX) electron beam [1]. In this scheme the 50 MeV electron beam is firstly energy-modulated by two lasers with wavelengths of 800nm and 1560nm respectively. Then the modulated electron beam is sent into a chicane whose R56 is 20 mm to convent the energy modulation to density modulation. At last the density-modulated electron beam is sent to a radiator which is tuned to 31.2 μm and generates the narrow-band THz radiation with the peak power of 8 MW. We simulate above laser-beam interaction using GENESIS 1.3. INTRODUCTION There is a growing interest in generating high power and narrow-band THz radiation which has wide applications in nondestructive imaging and spectroscopic studies of materials and molecules. The typically category aims to provide such powerful narrow-band THz radiation using bunched electron beam with a density modulation repeated at THz frequency. Conventional methods are to use a train of laser pulses to generate a train of electron beams repeated at THz frequency or use a shaped laser with a quasisinusoidal envelope at THz frequency to modulate the relativistic electron beam [2]. D. Xiang proposed a scheme to generate intense narrow-band THz radiation [2]. The scheme is similar to Echo-Enabled Harmonic Generation (EEHG) except that there is only one dispersion section and the aim is to down-convert the frequencies of the lasers. In this scheme the electron beam is firstly energy-modulated by two lasers with the wave number of k1 and k2, respectively. The beam acquires energy modulation at wave number k = n k1+m k2, where n and m are positive or negative integers. After passing through a dispersion section the energy modulation is converted to density modulation. By properly choosing the parameters of the lasers and dispersion section, the electron beam can be densitymodulated at THz frequency and generates powerful narrow-band THz radiation. We can easily tune the central frequency of THz radiation by varying the wavelength of lasers and the strength of dispersion section to cover the whole THz range. We generate high quality 50MeV electron beam with the slice energy spread of 0.01% using the photo cathode and traveling-wave linac in ACC lab at Tsinghua University [1]. Employing the high quality electron beam we choose reasonable parameters to generate THz radiation using D. Xiang’s scheme. We use the upgraded code GENESIS 1.3 to simulate the whole process. We encounter some puzzles when simulating because GENESIS 1.3 only allows the up-conversion to a higher harmonics. We have written a code to read the output of particle distributions generated by GENESIS 1.3 and rewrite it for a next run. EEHG SCHEME The scheme proposed by D. Xiang for generating THz radiation is shown in Fig.1. It consists of two modulators and one dispersion section. Following the assumptions and derivations in Ref. [2], we can write the particle distribution function at the end of the dispersion section as