Third-Harmonic 1 THz Large-Orbit Gyrotron With Kilowatt Power Level: Longer-Pulse Experiments and Observation of Multimode Regimes

Recently, a pulsed third-cyclotron-harmonic large-orbit gyrotron (LOG) operating with a radiation power of 1.3 kW at a frequency of 1 THz was implemented on the basis of an 80 kV/0.7 A/ $10~\mu $ s version. A modified cavity with a reduced diffraction Q-factor and improved mode selection was used to ensure the kilowatt level of the output power, as well as to expand the range of parameters where the selective excitation of the operating third-cyclotron-harmonic wave was provided. In this article, we describe a new series of experiments, where the same power level was achieved for a longer ( $30~\mu $ s) electron beam pulse. Stable single-mode generation of various axial modes excited at different stages of the electron beam pulse was observed. Such regimes were reproduced in numerical simulations.