Observation of the operating mode in Raman free-electron lasers

The method of microwave-induced gas breakdown to diagnose the operating mode structure in a nanosecond-pulse high-power Raman free-electron laser is presented. Modes of low azimuthal and radial index, such as TE 11 or TM 01 , are observed in the Raman free-electron laser experiments. Occasionally, some mixed-mode photographs of the output wave are also obtained, such as the sum of modes TE 11 and TE 01 . In the region of gyroresonance and higher β1, modes with the high index of the electron cyclotron maser are obtained; combined low- and high-index modes are also observed within the same experimental parameters. The results of experimental operating mode observation are discussed and a prediction of mode analysis is made.