Demonstration of a Selective Oversized Cavity in a Terahertz Second-Harmonic Gyrotron

A method of increasing the selectivity of highly-oversized open cavities is experimentally demonstrated in the electron cyclotron maser (gyrotron) operating at the second cyclotron harmonic in a high transverse mode (presumably, TE58,13) of a circular cylindrical cavity at a frequency near 1.2 THz in the magnetic field of 24 T. This method is purely electrodynamical and based on a simple deformation of the cavity wall. The proposed approach paves the way for increase of the gyrotron frequencies and for enhancing the selectivity of open cavities in other applications.