Radiation growth and saturation in a millimeter-wave superradiant free-electron laser

Frequency-resolved studies of radiation growth have been performed on a high-power millimeter-wave free-electron laser operating in the collective regime. These measurements have shown large radiation growth rates (approximately 2 dB/cm) over a broad instantaneous bandwidth (66-90 GHz), in good agreement with predictions of theory. Growth narrowing and saturation effects have also been observed. In addition, a large increase in experimental power and efficiency has been observed to result from tapering the strength of the axial magnetic field in the sense that compensates for kinetic energy extraction from the electron beam. Direct calorimetric measurements indicate the production of approximately equal to or greater than 75 MW centered at 75 GHz with 6 percent experimental efficiency.