Study of an Eighth-Harmonic Large-Orbit Gyrotron in the Terahertz Band

A 0.42-THz eighth-harmonic large-orbit gyrotron (LOG) operating with TE_8,1,1 mode has been designed and studied. The LOG design is characterized by the requirement of low external magnetic field and short cavity length to limit the ohmic loss. The cold cavity analysis is presented and followed by the hot cavity simulation with the 3-D particle-in-cell code. It is shown in the initial hot test that by proper choice of the electron beam source, the eighth harmonic of the electron cyclotron resonance can be selectively excited at 0.42 THz with an output power of 3.8 kW, which requires an external magnetic field as low as 2.8 T. Based on the model, the starting oscillation process of TE_8,1,1 mode at the eighth harmonic and the dynamic of beam-wave interaction are investigated. Upon verification of the design and modeling, a number of parameter variation studies are conducted to draw broader conclusions about the beam-wave interaction behavior in a high-harmonic LOG. The design procedure and the conclusion of this paper can be generalized and applied to other LOGs.