  The electron beam with a virtual cathode (VC) in the drift tube is
investigated with the help of a 1.5-dimensional relativistic electromagnetic
code. The existence of complex modes, including chaotic modes,is demonstrated.
The dynamic nature of chaos in the considered system is revealed. Physical
processes in the flow are investigated, and it is found that the initiation of
chaotic dynamics of the electron beam with VC isrelated to the nonlinear
interaction of structures formed in the system.
