  The subject of the present theoretical and experimental investigations is the
effect of the external magnetic field induction on dark current and a
possibility of breakdown. The generalization of the Fowler-Nordheim equation
makes it possible to take into account the influence of a magnetic field
parallel to the cathode surface on the field emission current. The reduction in
the breakdown voltage due to the increment in electron-impact ionization was
theoretical predicted. Experimentally shown that the presence of a magnetic
field about a tenth as a large as the cutoff magnetic field [18] reduces the
breakdown voltage by 10% to 20% for practically all cathodes no matter what
their surface treatment.
