  In this Letter we research the space charge limiting current value at which
the oscillating virtual cathode is formed in the relativistic electron beam as
a function of the external magnetic field guiding the beam electrons. It is
shown that the space charge limiting (critical) current decreases with growth
of the external magnetic field, and that there is an optimal induction value of
the magnetic field at which the critical current for the onset of virtual
cathode oscillations in the electron beam is minimum. For the strong external
magnetic field the space charge limiting current corresponds to the analytical
relation derived under the assumption that the motion of the electron beam is
one-dimensional [High Power Microwave Sources. Artech House Microwave Library,
1987. Chapter~13]. Such behavior is explained by the characteristic features of
the dynamics of electron space charge in the longitudinal and radial directions
in the drift space at the different external magnetic fields.
