ISSN 1063-7850, Technical Physics Letters, 2011, Vol. 37, No. 2, pp. 144–147. © Pleiades Publishing, Ltd., 2011. Original Russian Text © S.A. Kurkin, A.E. Hramov, A.A. Koronovskii, 2011, published in Pis’ma v Zhurnal Tekhnicheskoі Fiziki, 2011, Vol. 37, No. 3, pp. 102–109. 144 Investigation of electron beam–plasma systems with virtual cathodes (VCs) is among important tasks of the microwave electronics and plasma physics. This is related, on one hand, to the basic significance of this research, since the beam–plasma systems with VCs represent typical active distributed electron–wave sys- tems capable of featuring various nonlinear effects such as the formation and interaction of electron structures, chaotic generation, electron turbulence, etc. [1–5]. On the other hand, oscillators that employ electron beams with VCs (vircators) are promising sources of high-power broadband microwave radiation and can be used in various practical applications [1–3, 6–8]. Vircators can generate high-power microwave radiation and are advantageous in having relatively simple design, being capable of operating without magnetic field, and admitting easy control of the fre- quency and regime of generation. The principle of vircator operation is based on the phenomenon, whereby a VC that reflects back a cer- tain fraction of electrons toward the plane of injection is formed when the electron beam current exceeds a certain limiting (critical) level [1–3]. The dynamics of a VC in the electron flow is characterized by complex spatiotemporal oscillations, which frequently lead to the generation of broadband microwave radiation in vircator systems. In recent years, special interest is attracted to low-voltage vircators, in which VCs are formed in a retarding field. In these systems, the com- plex dynamics of a VC in the static retarding field is used to generate a broadband chaotic signal of medium power [6–8]. It was previously established [9–12] that the mag- nitude and configuration of an external magnetic field significantly influence the character of processes in intense electron beams with VCs and the dynamics of vircators. However, an important question concerning the effect of shielding of the electron source from a homogeneous external magnetic field on the nonlin- ear dynamics of the electron beam with a VC has not been studied until now. The importance of this issue is related to the fact that the electron guns typically used in vircators are not specially shielded and occur in stray fields of the magnetic system. Apparently, the presence of these inhomogeneous fields in the region of electron bean formation can strongly influence the dynamics of a VC, which is highly sensitive to external conditions [7, 9, 11]. This Letter presents the results of numerical simu- lation of the effect of shielding of the electron source from a homogeneous external magnetic field of the drift chamber in a model vircator on the nonlinear dynamics of the electron beam with a VC. We have studied the VC formation and dynamics in an electron beam using a classical vircator model [13]. According to this, the electron beam propagates in a drift space representing a cylindrical waveguide sec- tion of length L and radius R, which is bounded by electrodes at both ends. An axisymmetric, monoener- getic annular beam of electrons with the initial velocity v 0 , total current I, radius R b = 0.5R, and thickness a = 0.1R is injected into the interaction space through the input grid and extracted via the out- put grid. Accordingly, electrons can leave the interac- tion space via both output grid (and strike a collector) and input grid (upon reflection from the VC) and can also be lost on side walls of the drift chamber. An inhomogeneous, external focusing magnetic field with induction B is generated by a solenoid and applied along the drift tube axis in the region of VC Chaotic Signal Generation in Low-Voltage Vircator with Electron Source Shielded from External Magnetic Field S. A. Kurkin*, A. E. Hramov, and A. A. Koronovskii Saratov State University, Saratov, 410026 Russia *e-mail: KurkinSA@gmail.com Received September 15, 2010 Abstract—The effect of shielding an electron source from a homogeneous external magnetic field of the drift chamber on the nonlinear dynamics of the electron beam with a virtual cathode (VC) and on the character- istics of output microwave radiation in a low-voltage vircator have been numerically simulated within the framework of a two-dimensional model. It is established that the increased degree of shielding of the electron source from the external magnetic field leads to the complication of the VC dynamics in the system and the corresponding chaotization of the output microwave radiation. Physical processes that account for the observed effect of shielding are analyzed. DOI: 10.1134/S106378501102009X