Stability of sheet electron beam with cyclotron rotation in the axial symmetric magnetic field was studied. It was shown, that at the expense of increase in area of beam cross-section in the divergent magnetic field smaller deformation of a transverse cross-section of the beam may be reached
$^1$ Department of photonics and microwave physics, Faculty of Physics, Lomonosov Moscow State University. Moscow 119991, Russia.
$^2$ FGUP “NPP THORIY”. Moscow 119191, Russia.