# Novel Dual Beam Cascaded Schemes for 346 GHz Harmonic-Enhanced TWTs
## 1 Introduction  
Terahertz (THz) devices are widely used in high data-rate communication systems, plasma diagnostic, hazardous material detection, medical imaging, etc. However, the development of THz technology faces some challenges, such as lack of THz sources with high power, miniaturization, and low cost. Semiconductor THz sources and vacuum electronic THz sources are two common ones. Although semiconductor THz sources can produce high-power output up to milliwatt, they are usually troubled by over-high upfront cost. As a compromise, vacuum electronic devices (VEDs) may deliver higher output power with lower cost [1, 2, 3, 4, 5, 6, 7]. In 2004, a kind of compact THz free electron laser device was introduced by [PERSON], with 1 kW pulsed power from 0.3 THz to 3 THz [8]. In 2010, [PERSON] developed a high-gain multi-beam traveling wave tube (TWT) whose operating frequency range varies from 200 GHz to 250 GHz [9]. In 2011, Istok proposed a series of Backward-wave oscillators (BWOs) in which the grating line is utilized as slow wave structure (SWS). These devices can deliver several milliwatts output power at 1.4 THz [10]. In 2012, [PERSON] et al. presented a cascade backward-wave amplifier operating at 1 THz [11]. From 2012 to 2016, [PERSON] et al. discussed a series of vacuum electronic amplifiers including one 100 mW 670 GHz prototype device driven by a novel solid-statesource [12], a compact, microfabricated vacuum electronic amplifier with 39.4 mW of maximum output power from 0.835 THz to 0.875 THz [13], and a 29 mW 1.03 THz vacuum electronic with 20 dB of saturated gain and 5 GHz of instantaneous bandwidth [14]. In 2018, a folded waveguide (FWG) TWT is fabricated by [PERSON] et al., with over 300 mW power in 231.5-235 GHz [15]. In 2020, [PERSON] et al. proposed a G-band continuous wave TWT. Saturated power of 20 W is generated from 217 GHz to 219.4 GHz [16]. For the development of nuclear fusion energy, the understanding of a critical plasma phenomenon as transport of the plasma is necessary. The collective Thompson scattering at THz frequency has been proven to be an adequate technology to map anomalous density fluctuation of electrons in the plasma, without perturbing its plasma behavior. The optically pumped far-infrared (FIR) laser is a practical radiation source for this technology and applied in the National Spherical Torus Experiment (NSTX) [17]. However, the features of the high price, large volume, and relatively low power restrict its mapping region. To extend the dimension of the plasma diagnostic, the BWOs operating at 346 GHz can be promising devices due to low cost, large output power, easy assembly, and compact volume. [PERSON] et al. designed a 0.4 W double corrugated waveguide (DCW) BWO and a 1 W double-staggered grating (DSG) BWO [18]. [PERSON] et al. designed a Grooved Single Grating (GSG) structure for 346 GHz BWO. The GSG circuit was fabricated by UV-lithographie galvanoformung abformung (LIGA) microelectromechanical technologies [19]. It should be noted that the BWO is strict about power source because BWO requires very suitable power source to maintain frequency stable. Phase noise of THz signal generation, which caused by power supply voltage ripple, should be reduced to ensure low bit-error rate in THz communication [20]. To reduce requirement of high-performance power source and obtain pure frequency spectrum of output power, we developed a kind of THz source, named high-harmonic TWT (HHTWT) [6; 21]. Based on HHTWT, one new HHTWT and two novel types of THz sources operating at 346 GHz are proposed in this paper to improve the output power. The HHTWT can generate the THz band electromagnetic (EM) wave by amplifying the E-band signal. Compared with conventional THz signal sources, the application of high-power E-band signal source in HHTWT can input considerable signal into SWS. It can avoid input signal being interfered with and even down out noise, which is caused by electron gun and discordance of SWS fabrication.