Third harmonic CW gyrotron with operating frequency 1.2 THz for a DNP /NMA spectroscopy

The project of a third harmonic CW gyrotron with an output frequency of 1.185 THz (wavelength about $\pmb{250\ \mu}\mathbf{m}$ ) and an output power of 100–200 W, intended for DNP/NMR spectroscopy applications is described. The project based on the cryomagnet with a maximum magnetic field intensity of 15 T. The analysis of the modes spectrum, coupling factors and starting currents proposed TE -l5,6 or TE+21.4 modes as most promising candidates. To improve mode selection different schemes, including multi-beam electron optics are analyzed. The specific feature of magnetron injection gun is that it allows form two beams or one beam with the same electrode profile by eliminating of the additional emitter ring on the cathode surface. The important role of velocity spread for parasitic mode suppression is shown. In the conditions of an extremely dense spectrum of modes, this makes it possible to suppress the most dangerous parasitic traveling modes at the first and second cyclotron harmonics, which are very sensitive to the velocity spread.