• N.S. Ginzburg, V.I. Belousov, G.G. Denisov, R.M. Rozental, A. Sergeev, I.V. Zotova
  IAP/RAS, Nizhny Novgorod
• A.G. Reutova, K.A. Sharypov, V.G. Shpak, S.A. Shunailov, M.R. Ulmaskulov, M.I. Yalandin
  RAS/IEP, Ekaterinburg

Funding: This work was supported by the RFBR grant no.05-02-17553

Recently significant progress was archived in the generation of multimegawatt subnanosecond pulses in millimeter wave band utilizing the cyclotron and Cherenkov mechanisms of superradiance (SR) [1,2]. We study the novel mechanism of SR when the powerful pumping wave undergoes the stimulated back scattering on the intense electron bunch. Due to the Doppler up shift the radiation frequency can significantly exceed the frequency of the pumping wave. With the relativistic microwave generator as a pumping wave source such a mechanism can be used for generation of the powerful pulse radiation in the short millimeter and submillimeter wave bands. Experiments on the observation of the stimulated scattering in the superradiance regime were carried out at Institute of Electrophysics RAS with two synchronized accelerators. The 4 ns electron beam from the first accelerator is used for generation of the 38 GHz 100 MW pumping wave which subsequently scattered on the subnanosecond 250 keV 1 kA electron bunch produced by the second accelerator. The SR pulses with duration 200 ps and peak power about 1 MW were generated. The spectrum of scattered signal included the frequencies up to 150 GHz.

[1] Ginzburg N., et al. Phys.Rev.Lett.,1997, 78 (1997) 2365. [2] Ginzburg N., et al. Phys.Rev. E, 1999 60 3297.

• N.S. Ginzburg, A.M. Malkin, N.Yu. Peskov, R.M. Rozental, A. Sergeev, V.Yu. Zaslavsky
  IAP/RAS, Nizhny Novgorod

A novel Bragg FEL scheme is discussed in which an electron beam synchronously interacts with a propagating wave, and the latter is coupled to a quasi cut-off mode. This coupling is realized by either helical or asimuthally symmetric corrugation of the waveguide walls. The quasi cut-off mode provides feedback in the system leading to self-excitation of the whole system while the efficiency in steady-state regime of generation is almost completely determined by the propagating mode, synchronous to the beam. Analysis based on averaged time domain approach as well as on direct PIC code simulation shows that the efficiency of such a device in the single mode single frequency regime can be rather high. The main advantage of the novel Bragg resonator is provision of higher selectivity over transverse index than traditional scheme of Bragg FEL. The cold microwave testing of the Bragg structure based on coupling of propagating and trapped waves in the Ka band demonstrated a good agreement with theoretical considerations. The novel feedback scheme should be used on a JINR- IAP FEL as a method of increasing the operating frequency for fixed transverse size of interaction space.