A moderately relativistic high-harmonic Large Orbit Gyrotron (LOG) has been considered as a source of high power radiation in the millimeter/submillimeter wavelength range. The experiments on a LOG designed to operate at 5th cyclotron harmonic at the wavelength of 2 mm with output power of 50-100 kW are in progress. The LOG electron-optical system with a thermionic cathode is constructed and checked experimentally. It produces a thin rectilinear electron beam with an energy of 250 keV, current of 18 A, pulse duration of 10 /spl mu/s and diameter of 0.6 mm in an axial magnetic field of 16 kG. A pumping system transforms the straight beam into a beam of electrons gyrating around the axis with a velocity ratio of 1.5 and sufficiently small velocity spread. A possibility of selective excitation of the 5th cyclotron harmonic at the wavelength of 4 mm has been proved in a short-pulse experiment on LOG driven by 300 keV/30 A/20 ns electron beam.
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