We report on a new regime of free-electron-laser operation with reversed axial guide magnetic field, in which the rotational motion of the electrons in the helical wiggler field is opposed by the presence of the uniform guide field. The 33.3-GHz free-electron-laser amplifier is driven by a mildly relativistic electron beam (750 kV, 300 A, 30 ns) and generates 61 MW of radiation with a 27% conversion efficiency. Measurements with the conventional orientation of the axial field show a considerable loss of power and efficiency.

• Received 22 July 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.3082