Comparative study of free electron laser experiments in longitudinal and transverse wiggler geometries

The results of an experimental investigation of an Intense Relativistic Electron Beam (IREB) operating in the axially periodic and transverse wiggler geometries are presented. The particle energy of the IREB was 0.6 MV, and the peak intensity and duration of the beam pulses were 10 kZ, and 10 ns, respectively. A detailed description of the experimental apparatus is given, and some preliminary results of IREB experiments in the operational regimes of a collective Raman FEL and an electron-cyclotron maser are discussed. On the basis of the experimental results, it is shown that: (1) operation in a smooth cylindrical cavity produced intense ECM emission in the range of the guide field; and (2) little ECM was observed in the transverse wiggler geometry with a rectangular cavity. In the cylindrical cavity, hollow beam, and longitudinal wiggler geometries, the wiggler was found to enhance the sharp spectral peak at 10 GHz. The frequency spectra of waves generated in the cylindrical and rectangular cavities with and without the longitudinal and transverse wigglers are described in a series of graphs.