Permanent magnet beam focus structure for linear beam tubes

Claim 1

In a linear beam tube: electron gun means for forming and projecting a beam of electrons over an elongated beam path; collector means at the terminal end of the beam path for collecting and dissipating the energy of the collected electrons; electrical circuit means disposed along the beam path intermediate said cathode means and said collector means in energy exchanging relation with the beam to produce an output signal; magnet means for magnetically focusing the beam over said beam path between said electron gun means and said collector means, said magnet means including, electron gun and collector pole piece structures coaxially disposed of the beam path in axially spaced relation, said pole piece structures each being centrally apertured for passage of the beam therethrough and each being made of a magnetically permeable material, magnetically permeable magnetic yoke means coaxially disposed of said beam path and including axially spaced portions surrounding said electron gun and collector pole piece structures respectively in radially spaced relation therefrom, permanent magnet means for energizing said pole piece structures, said permanent magnet means comprising essentially only first and second axially spaced centrally apertured radially polarized permanent magnet means of opposite radial polarity interposed between said respective pole piece structures and said respective surrounding yoke portions for producing an axially directed beam focus magnetic field within the beam path and extending axially between said pair of axially spaced electron gun and collector pole piece structures, the magnetic field within said permanent magnet means being composed of flux lines which are substantially radially directed and centered substantially at the beam axis.