A mm wave amplifier experiment on a free electron laser was performed
using an intense, mildly relativistic electron beam (0.7-0.9 MeV) with
a combination of a focusing planar wiggler and a weak additional axia
l magnetic field in the so-called ''group 1'' region. A beam transport
ratio of over 80%, through the whole wiggler length, was realized. In
an amplification experiment, a spatial growth rate of 56 dB/m was obt
ained at a frequency of 45 GHz, and a power saturation was observed at
a level of 6 MW, where the total gain was estimated to be 52 dB. It w
as shown that the off-axis component of the beam contributes considera
bly to the rf amplification in the focusing wiggler.