First bandwidth measurements of a novel gyrotron amplifier are presented. T
he coupling between the second harmonic cyclotron mode of a gyrating electr
on beam and the radiation field occurred in the region of near infinite pha
se velocity over a broad bandwidth by using a cylindrical waveguide with a
helical corrugation on its internal surface. With a beam energy of 185 keV,
the amplifier achieved a a maximum output power of 1.1 MW, saturated gain
of 37 dB, linear gain of 37 dB, saturated bandwidth of 8.4 to 10.4 GHz (21%
relative bandwidth), and an efficiency of 29%, in good agreement with theo
ry.