High quality operation of high frequency gyrotron

High frequency, medium power gyrotrons (Gyrotron FU series) have been developed at Fukui University as radiation sources covering a broad frequency band from millimeter to submillimeter wave region. Conversion from the gyrotron output of circular waveguide mode into a Gaussian-like beam is useful for applications of the gyrotron to a wide field. A high mode purity output is necessary for an effective conversion. In order to improve mode purity, a nonlinear cavity which is designed using a scattering matrix code and fabricated, is installed in the Gyrotron FU VA. The measurement of radiation patterns confirmed a high purity mode operation.