The report is devoted to the results of the numerical study of the virtualcathode formation conditions in the relativistic electron beam under theinfluence of the self-magnetic and external axial magnetic fields. Theazimuthal instability of the relativistic electron beam leading to theformation of the vortex electron structure in the system was found out. Thisinstability is determined by the influence of the self-magnetic fields of therelativistic electron beam and it leads to the decrease of the critical valueof the electron beam current (current when the non-stationary virtual cathodeis formed in the drift space). The typical dependencies of the critical currenton the external uniform magnetic field value were discovered. The effect of thebeam thickness on the virtual cathode formation conditions was also analyzed.
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