We have observed the oscillation of a waveguide mode free-electron laser using the coherent synchrotron radiation emitted from electron bunches. The oscillation was observed at 2.73 mm with a linewidth of 0.4%. This spectral selection was attributed to the dispersion of the radiation in the waveguide. We found that the oscillating radiation had to have the same group velocity as the velocity of the electrons passing through the wiggler field.
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