  Removal of residual linear energy chirp and intrinsic nonlinear energy
curvature in the relativistic electron beam from radiofrequency linear
accelerator is of paramount importance for efficient lasing of a high-gain
free-electron laser. Recently, it was theoretically and experimentally
demonstrated that the longitudinal wakefield excited by the electrons itself in
the corrugated structure allows for precise control of the electron beam phase
space. In this Letter, we report the first utilization of a corrugated
structure as beam linearizer in the operation of a seeded free-electron laser
driven by a 140 MeV linear accelerator, where a gain of ~10,000 over
spontaneous emission was achieved at the second harmonic of the 1047 nm seed
laser, and a free-electron laser bandwidth narrowing by about 50% was observed,
in good agreement with the theoretical expectations.
