ISSN 0021-3640, JETP Letters, 2010, Vol. 91, No. 11, pp. 553–557. © Pleiades Publishing, Inc., 2010. Original Russian Text © M.I. Yalandin, A.G. Reutova, M.R. Ul’maskulov, K.A. Sharypov, S.A. Shunailov, N.S. Ginzburg, I.V. Zotova, E.R. Kocharovskaya, A.S. Sergeev, 2010, published in Pis’ma v Zhurnal Éksperimental’noі i Teoreticheskoі Fiziki, 2010, Vol. 91, No. 11, pp. 620–625. 553 1. The processes of the coherent resonant interac- tion of short (as compared to the relaxation times) laser pulses with active and passive media have long been studied both theoretically and experimentally in quantum electronics and plasma physics. Among such processes are self-induced transparency, photon echo, etc. [1, 2], as well as the amplification of laser pulses in a plasma in the process of inverse Raman scattering [3, 4]. Similar effects can undoubtedly accompany the interaction of electromagnetic pulses with the electron beams [5–8]. In this case, fundamental differences from the traditional mechanisms of the amplification of stationary monochromatic signals appear if the duration of an input electromagnetic pulse is much shorter than the so-called cooperative length. By anal- ogy with the corresponding processes in quantum electronics [9], this length can be defined as the shift length (slippage [10]) of the pulse with respect to the particles in the characteristic time of the development of instability, which is proportional to the inverse increment. The difference of the group velocity of radiation, V gr = β gr c, from the electron velocity V 0 = β 0 c is a necessary condition for the appearance of the indicated effects. In this case, the propagating input electromagnetic pulse sequentially interacts with dif- ferent electron fractions of the beam, which have no modulation in the velocity and density of the particles, and takes their energy. Correspondingly, an increase in the peak power in such a process is not limited by mechanisms of the capture of particles by a wave that are typical for traditional electronic amplifiers [11] and the peak power of the output signal under optimal conditions can be higher than the power of the elec- tron beam 1 [6, 7]. Moreover, the increase in the power in the amplification process is accompanied by a strong decrease in the effective duration of the input pulse. The above effects of the amplification and nonlin- ear compression of ultrashort pulses have been theo- retically investigated for the undulator (V gr > V 0 ) and Cherenkov (V gr < V 0 ) mechanisms of stimulated emis- sion in the optical and microwave ranges [5–7]. Nev- ertheless, experimental investigations of the indicated effects have not yet been performed. In particular, the absence of such investigations in the microwave range is explained by the absence of sources of sufficiently short electromagnetic pulses with a length comparable with the cooperative length. However, the develop- ment of a new class of short-pulse generators [12–15] based on the superradiance of long electron bunches made it possible to begin such investigations. In this work, we describe an experiment demonstrating the nonlinear compression of superradiance pulses of the millimeter range in the process of the Cherenkov ampli- 1 Note that the generation of microwave superradiance pulses with a power higher than the power of the electron beam was experimentally demonstrated in [13, 14] in the centimeter and millimeter wavelength ranges. The energy of the electromag- netic pulses in such a process naturally remains lower than the total kinetic energy of the electronic ensemble. Effect of the Nonlinear Compression of Ultrashort Microwave Pulses in the Process of the Amplification by Quasistationary Electron Beams M. I. Yalandin a , A. G. Reutova a , M. R. Ul’maskulov a , K. A. Sharypov a , S. A. Shunailov a , N. S. Ginzburg b , I. V. Zotova b , E. R. Kocharovskaya b , and A. S. Sergeev b a Institute of Electrophysics, Ural Division, Russian Academy of Sciences, ul. Amundsena 106, Yekaterinburg, 620016 Russia e-mail: yalandin@iep.uran.ru b Institute of Applied Physics, Russian Academy of Sciences, ul. Ul’yanova 46, Nizhni Novgorod, 603950 Russia Received March 11, 2010; in final form, April 27, 2010 An effect of the nonlinear compression of ultrashort microwave pulses has been observed in the process of the amplification of quasistationary electron beams. The Cherenkov mechanism of the interaction of a rectangular electron beam with a decelerated wave in a waveguide partially filled with an insulator is used. The experiment has been conducted on a setup consisting of two synchronized RADAN high-current accelerators. The first accelerator supplied a generator of 37-GHz superradiance pulses with a duration of about 300 ps. The second accelerator with a beam current of up to 1.2 kA and an electron energy of about 300 keV was used in an ampli- fying section. The theoretical analysis shows that the amplification of the electromagnetic pulses (at least by a factor of 4 in the power) is accompanied by a strong decrease in their duration (down to 100 ps). DOI: 10.1134/S0021364010110032