Measurements of millimeter and submillimeter wavelength emission (240 GHz<ω/2π<470 GHz) from a free-electron laser are reported. The laser operates as a superradiant amplifier and without an axial guide magnetic field; focusing and transport of the electron beam through the wiggler interaction region are achieved by means of the bifilar helical wiggler field itself. Approximately 18 MW of rf power has been observed at a frequency of 470 GHz, corresponding to an electronic efficiency of 0.8%. Frequency spectra are measured with a grating spectrometer and show linewidths Δω/ω∼2%–4%. The experimental results are in very good agreement with nonlinear numerical simulations.
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