Progress in investigations on microwave FEL driven by microsecond sheet beam

For production of mm-radiation with 10 GW power one needs a few tens of kAs current of 1 MeV electrons passing through a generator. Ribbon or sheet E-beams allow one to reach this value of the current. In the result of our investigations at U-2 accelerator in 1994 this type of the beams with 30 kA current at 10 μs duration was generated and transported through the slit vacuum channel. In 1996 at this accelerator in modelling experiments with the beam current 3-5 kA in a passive magnetic undulator of 4 cm period we also obtained 200 J energy of 4 mm radiation in a few μs pulse. The next step of investigations was aimed to reach the maximal efficiency of the microwave generation at a single mode operation of the generator. On this goal in 1997-98 we made reconstruction of the experimental setup intended to investigate FEM-oscillator. In order to increase productivity of the investigations this setup was replaced from the U-2 accelerator to the U-3 one that was specially reconstructed for generation of sheet beams