  The paper considers the dynamics of the plasma generated by the
explosive-emission cells of the cathode spot of a vacuum arc in a magnetic
field. It is shown that the expansion of the (high-density) plasma in a
transverse magnetic field may cause asymmetry in the plasma density
distribution at the cathode spot boundary. The asymmetry, in turn, increases
the probability of the ignition of new explosive-emission cells in the region
of a stronger magnetic field in the 'anti-amperian' direction of BxI. The
disturbed plasma density distribution estimated in the MHD approximation is
presented. In addition, the velocity of the 'retrograde' spot motion (ignition
of new explosive-emission cells) in the stronger field region is estimated as a
function of the external magnetic field strength. The velocity estimates (a few
to tens of m/(s T)) are shown to agree with experimental data
