Determination of electron beam radius at the cavity in gyrotron FU CW I

A 300 GHz fully continuous wave gyrotron FU CW I has been developed for application to material processing in the submillimeter wave range. The output power from this gyrotron has not reached the designed one. As a reason of power shortage, it is considered that the electron beam radius shifts from the optimum radius due to installation of additional gun coils to suppress a large anode current. For improvement of the output power, the present beam radius is determined from measurement. With the determined radius, the cause of power shortage is discussed.