US3896329A - Permanent magnet beam focus structure for linear beam tubes - Google Patents

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7/// PERMANENT MAGNET BEAM FOCUS STRUCTURE FOR LINEAR BEAM TUBES DESCRIPTION OF THE PRIOR ART Heretofore, X-band medium power CW klystrons have had their beams magnetically focused by means of a pair of centrally apertured pole pieces disposed at opposite ends of the beampath and energized by means of transversely polarized bar magnets of Alnico IX material energized and loaded such as to have a Bm/Hm ratio of approximately 9.0. Two such bar magnets were provided at opposite lateral sides of each pole piece and a pair of magnetically permeable yoke plates interconnected the two energized pole piece assemblies to provide a return flux path therebetween. In a typical X-band CW klystron tube, the aforedescribed magnet structure, which is capable of providing an average focusing magnetic field of 2,200 gauss over a beam path length of 1.8 inches, weighs approximately 14 pounds.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2, there is shown an X-

band klystron tube

1 employing a prior art permanent magnet

beam focus structure

2. The

tube

1 includes an

electron gun assembly

3 for forming and projecting a beam of

electrons

4 over an elongated beam path to a

beam collector structure

5 disposed at the terminal end of the beam path for collecting and dissipating the energy of the collected electrons. A

microwave circuit

6 is disposed along the.

beam path

4 intermediate the

electron gun

3 and

beam collector

5 in energy exchanging relation with the beam to produce an output microwave signal which is coupled from the

circuit

6 to a suitable load, such as an antenna, not shown. The microwave circuit.6 comprises an electrically conductive

block body portion

7, as of copper, having a plurality of

reentrant cavity resonators

8 formed therein. The

cavities

8 include a deformable wall portion defining a tuning diaphragm, not shown, which is moved in and out (relative to the plane of the paper) for changing the volume of the cavity and thus tuning the resonant frequency of the cavity. The diaphragm is deformed by means of a

tuning bridge structure

9 projecting out from the body of the tube along one side thereof. A pair of waveguides, not shown, project out from the first and last cavity on the opposite side of the

body

7 from the

tuners

9.

As best shown in FIG. 2 the permanent magnet

beam focus structure

2 includes a centrally apertured electron gun pole piece 11 disposed at the upstream end of the

beam

4 and a similar

collector pole piece

12 disposed adjacent the

collector

5. In a typical example,

pole pieces

11 and 12 are madeof soft iron. The. pole pieces are energized by transversely directed pairs of permanent bar-

shaped magnets

13 and 14, said pairs of

permanent magnets

13 and 14 being transversely polarized as indicated by the arrows to make one of the magnetic poles ll of one polarity such as north, and the other

magnetic pole piece

12 of opposite polarity, such as south, to produce a beam focusing magnetic field within the

beam

4 which is longitudinally directed of the beam and of generally uniform magnitude as of 2,200 gauss. A pair of magnetically permeable

rectangular plates

15, as of soft iron, are disposed on opposite sides of the body of the

tube

7 interconnecting thepairs of

permanent magnets

13 and 14 to provide a magnetic flux return path.

The

magnets

13 and 14 of the prior art were made of Alnico IX and energized and loaded such as to have an operating point with a ratio of Bm/Hm of approxiing structure for focusing an X-band klystron of the type shown in FIG. 1 having a gap length between poles ll and'l2 of 1.8 inches and providing a magnetic field of an averageintensity of 2,200 gauss within'the beam path weighs approximately 14 pounds. Several problems, aside from weight, are encountered in the structure of FIG. 2. One of these problems is that if the

magnets

13 and 14 are not properly balanced, i.e., of equal prior art magnet structure of FIG. 2 by substitution of more powerful permanent magnets, such as one of the rare earth cobalt magnets, for example samarium cobalt, .it is found that excessive leakage of magnetic flux around the permanent magnets is experienced. More particularly, as shown in FIG. 3, the Bm v. Hm curve for samarium cobalt is shown by

line

23. In order to obtain a higher energy product (Bm X Hm) for a rare earth cobalt magnet material, the load line 24 (Bm/Hm) ratio is chosen within the range of 0.8 to 1.3. This yields a higher energy product magnet than that offered by Alnico IX. However, it requires that the magnet have a larger cross-sectional area and this results in producing more self leakage of flux around the =

individual magnets

13 and 14 in the geometry of FIG.

It has been found that the self leakage associated with the end regions of the magnets at 26, in FIG. 2, can be substantially eliminated by shaping the

magnets

13 and 14 to figures of revolution, i.e., annular members, such that there are no

end portions

26.

Referring now to FIG. 4, there is shown a magnetic

beam focusing structure

31 incorporating features of the present invention. The

beam focus structure

31 includes an annular

gun pole piece

32 disposed at the gun end of the tube and-having an axially directed

tubular projecting portion

33 surrounding the

thermionic cathode emitter

34 of the

electron gun

3. An annular collector pole piece 35 is disposed at the collector end of the

interaction circuit region

6. A pair of annular radially polarized

permanent magnets

36 and 37 are dis-- posed encircling the respective gun and the collector

pole piece structures

32 and 35. A cylindrical

magnetic return yoke

38, as of soft iron surrounds both the gun and collector

permanent magnets

36 and 37 and the intervening region therebetween. The

yoke

38 is apertured for access to the

tuners

9 and for'passage of the wave-guides through the yoke.

The

permanent magnets

36 and 37 are preferably rare earth cobalt magnets operated in a region having a ratio of (Bm/Hm)within the range of 0.8 to 1.3, as indicated by the cross-hatched region of the plot of FIG. 3.

Magnets

36 and'37 are oppositely radially polarized as shown by the arrows of FIG. 4 to produce an axial magnetic focusing field B in the gap between the

pole pieces

33 and 35. The mutually opposed

inner faces

41 and 42 of the gun pole piece and collector pole piece, respectively, are planar and parallel to facilitate emitter of the gun such that the magnetic flux lines pass through the emitting surface perpendicular thereto to obtain a confined flow type of magnetic beam focusing.

The magnetic

beam focusing structure

31 of the present invention, as used for focusing the beam of the tube of FIG. 1, results in reducing the weight of the beam focus magnetic structure by a factor of 3.5. That is, the weight is cut from 14 pounds to 4 pounds. In addition, a better beam focusing field is obtained because the

cylindrical yoke

38 in combination with the annular

permanent magnets

36 and 37 yields cylindrical symmetry which assures that the opposite sides of the yoke are operating at the same magnetic potential as the yoke constitutes a magnetic equipotential member.

What is claimed is: I 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 I 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

7 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 I manent magnet means has-a (Bm/Hm) ratio falling within the range of 0.8 to 1.3.

5. The apparatus of

claim

4 wherein said permanent I magnet material is a rare-earth-cobalt.

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.

2. The apparatus of claim 1 wherein said first and second permanent magnet means are each figures of revolution with an axis of revolution along the axis of the beam path.

3. The apparatus of claim 1 wherein said yoke is a figure of revolution.

4. The apparatus of claim 1 wherein each of said permanent magnet means has a (Bm/Hm) ratio falling within the range of 0.8 to 1.3.

5. The apparatus of claim 4 wherein said permanent magnet material is a rare-earth-cobalt.

6. The apparatus of claim 1 wherein said electron gun and collector pole piece structures have opposed generally planar transverse inner faces.

7. The apparatus of claim 1 wherein said electron gun includes a thermionic cathode emitter having a generally spherically concave emitting surface, and wherein said electron gun pole piece includes an axially directed tubular projection surrounding said electron gun for shaping the magnetic field in the region of said gun to cause the beam focusing magnetic field lines to pass through said concave emitting surface generally perpendicular to said surface, whereby a magnetically confined convergent flow electron gun is obtained.