## 1 Introduction  
Terahertz (THz) radiation contributes to many scientific frontiers by serving as a nonionizing probe in nonlinear optics, pump experiments, spectroscopic, and time-resolved applications [1; 2; 3]. In the last decades, there has been significant interests in the generation of terahertz (THz) radiation with superior properties, for instance, ultra-high pulse energy, tunable wavelength at full THz frequency range, and full temporal coherence. Many fantastic scientific applications such as THz-assisted high-order harmonic generation, THz-triggered chemistry, and single-shot THz bioimaging require THz radiation with pulse energy up to millijoule (mJ) level [4; 5; 6], which remains a challenge for most THz generation methods. Furthermore, generating THz radiation with mJ level pulse energy at full THz frequency range can be even more difficult for the existing THz techniques. Currently, the prominent methods to generate high-power THz pulses are generally based on the ultrafast laser techniques [7; 8; 9; 10; 11], laser-produced plasmas techniques [12; 13; 14; 15; 16; 17], or electron accelerator-based methods. The ultrafast laser technique, called optical rectification and optical parametric amplifier, has been employed to generate THz pulses with a maximum pulse energy of about 900 \(\upmu\)J from organic crystals [9], while it has limitations on further increases in the peak power and repetition rate due to the low inherent optical damage threshold of crystals [11]. The laser-produced plasma technique is generally immune to the damage threshold since a damage-free medium is utilized to achieve an intense THz pulse. Laser-driven gas-density plasma can generate THz radiation with a pulse energy of a few mJ level, then saturation happens with the increasing of the energy of the pump laser [13; 14]. By contrast, laser-solid interaction can achieve THz radiation with a pulse energy of several hundreds of mJ but the spectral coverage is quite limited [15; 16]. Recently, coherent THz radiation with exceeding multi-millijoules is demonstrated by utilizing an ultra-intense, picosecond (ps) laser pulse to irradiate a metal foil, while it can only produce THz radiation at relative low frequency (lower than 3 THz) [17]. The electron accelerators have reliable abilities to produce THz radiation via several different ways: coherent synchrotron radiation (CSR) [18; 19; 20; 21], optical transition radiation (OTR), coherent optical transition radiation (COTR) [22; 23; 24; 25], or undulator radiation [26; 27; 28]. The CSR radiation at the THz range can be simply generated from a magnetic dipole in the storage ring [18; 19]. Generation of tunable coherent THz radiation with a pre-bunched electron beam has also been demonstrated in the storage ring, but with a relatively low power and long pulse length [20]. The CSR radiation can also be achieved with the linac accelerator where one single pass radiation will limit the radiation at low gain level and the output bandwidth is quite broad [21]. The OTR emits when the relativistic electron beam crosses the boundary of two different medias, which can generate the radiation from X-ray to microwaves, while the radiation generally has a broadband spectrum and a maximum pulse energy of hundreds \(\mu\)J (350 \(\mu\)J [24] and 140 \(\mu\)J [25]). In addition, a special Smith-Purcell radiation from cylindrical grating is applied to obtain coherent THz radiation below 1 THz with a pulse energy of 0.8 \(\mu\)J from high harmonics [29]. Free electron laser (FEL) [30; 31; 32] is recognized as a new generation light source, which has the capability of providing ultra-short pulses with gigawatt (GW) level peak power and tunable wavelength from THz to hard X-ray.