Axially symmetric magnetic fields are provided about the longitudinal axis
of each beam of a multi-beam electron beam device. The magnetic field
symmetry is independent of beam voltage, beam current and applied magnetic
field strength. A flux equalizer assembly is disposed between the cathodes
and the anodes and near the cathodes of a multi-beam electron beam device.
The assembly includes a ferromagnetic flux plate completely contained
within the magnetic focusing circuit of the device. The flux plate
includes apertures for each beam of the multi-beam device. A flux
equalization gap or gaps are disposed in the flux plate to provide a
perturbation in the magnetic field in the flux plate which counters the
asymmetry induced by the off-axis position of the beam. The gaps may be
implemented in a number of ways all of which have the effect of producing
a locally continuously varying reluctance that locally counters the
magnetic field asymmetry. The flux equalizer assembly prevents or
substantially reduces beam twist and maintains all of the electron beams
of the device as linear beams.
