Gain, bandwidth, and tunability of a high power millimeter-wave free-electron laser

Initial spectral measurements on a high-power super-radiant millimeter-wave free-electron laser experiment have produced emission spectra that agree well with theoretical predictions. Moderate linewidths in the range of 6% to 15% have been measured, demonstrating the broad gain bandwidth of the interaction. Additionally, this experiment has observed for the first time the predicted simple broadband tuning of the FEL interaction, with tuning demonstrated over a 50% range of frequencies through variation of the axial electron velocity by means of changing the strength of the wiggler field. Direct gain measurements have demonstrated a single-frequency gain of ∼2 dB/cm, through variation in the interaction length for the super-radiant emission. Additionally, a frequency shift, accompanied in some cases by a large increase in total power and efficiency has been observed through shortening the region of uniform axial field and tapering the strength of the axial magnetic field at the end of the uniform wiggler region. The highest power produced in this way is ≳ 75 MW at 6% efficiency.