Development of Third-Harmonic 1.2-THz Gyrotron With Intentionally Increased Velocity Spread of Electrons

We develop the concept of a continuous wave (CW) 1.185 THz (wavelength about 250- $\mu \text{m}$ ) gyrotron with an output power of several watts, intended for dynamic nuclear polarization (DNP)/nuclear magnetic resonance (NMR) spectroscopy applications. For a 15-T magnetic field provided by a commercially available cryomagnet, the required frequency can be achieved by operating at the third cyclotron harmonic. Under conditions of extreme density of the mode spectrum, we propose using interaction with an electron beam intentionally formed with a high (up to 40%) velocity spread as a selection mechanism. This ensures suppression of the most dangerous parasitic traveling modes at the first and the second cyclotron harmonics, which are very sensitive to the spread. The main parameters of the gyrotron are determined both using the start currents analysis and within the framework of a nonstationary self-consistent model. Experimental feasibility of the single-mode third-harmonic generation under conditions of mode competition is verified based on 3-D particle-in-cell (PIC) simulations.