  The presence of a pulse train structure of an oscillator-type free-electron
laser (FEL) results in the immediate damage of a solid target upon focusing. We
demonstrate that the laser-induced damage threshold can be significantly
improved by gating the mid-infrared (MIR) FEL pulses with a plasma mirror.
Although the switching pulses we employ have a nanosecond duration which does
not guarantee the clean wavefront of the gated FEL pulses, the high focusablity
is experimentally confirmed through the observation of spectral broadening by a
factor of 2.1 when we tightly focus the gated FEL pulses onto the Ge plate.
