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 /spl mu/s 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 /spl mu/s 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.