Generation of high mode purity output of a gyrotron and its conversion to a Gaussian beam

Summary form only given, as follows. Unlike a molecular vapor laser, a gyrotron generates spreading radiation with TE/sub 00/ mode structure. It is therefore necessary to convert the output radiation into a Gaussian beam (TEM/sub 00/ mode), which is suitable for an effective transmission and a well-collimated probe beam. High purity mode operation is convenient for effectively converting the output radiation into the Gaussian beam (TEM/sub 00/ mode). We have constructed a new gyrotron, Gyrotron FU VA, which has a carefully designed resonant cavity.