US2787728A - Electron discharge device with toroidal permanent magnet - Google Patents

Description

April 2, 1957 P'. w. CRAPUCHETTES ELECTRON DISCHARGE DEVICE WITH TOROIDAL PERMANENT MAGNET Filed March 24, 1954 2 Sheets-Sheet 1 FIG. I.

MAGNET/C NEUTRAL PLANE INVENTOR W CE/JPUCHE 7' 7' E 5 ATTORNEY April 2, 1957 P. w. CRAPLICHETTES 2,787,728

ELECTRON DISCHARGE DEVICE WITH TOROIDAL. PERMANENT MAGNET Filed March 24, 195 I 2 Sheets-Sheet 2 FIG. 2.

6004 //V6 HOJE INVENTOR w. CAAPl/CHETTES ATTORNEY 'force and flux density.

ELECTRON DISCHARGE DEVICE WITH TOROIDAL PERMANENT MAGNET Paul W. Crapuchettes, Palo Alto, Calif., assignor, by mesue assignments, to Litton Industries, Inc.

Application March 24, 1954, Serial No. 418,310

6 Claims. (Cl. 31539.71)

This invention relates to the construction of an electron discharge device of the type requiring a transverse magnetic field with respect to the flow of electrons for the operation thereof, and more particularly, to the form of the permanent magnet utilized for producing this magnetic field.

In an electron discharge device such as the magnetron, wherein a permanent magnet is employed for the production of a magnetic field, the magnet is the largest single contributor to the overall weight of the device. Accordingly, it is a first object of this invention to provide a permanent magnet which operates at optimum efficiency, thus permitting a reduction in the size of the permanent magnet for a flux density and coercive force comparable to that attained by magnets conventionally used.

Permanent magnet materials are rated in terms of their energy density, which is the product of coercive If minimum magnet weight is to be achieved, all parts of the structure must be operating at the maximum energy density. In addition, magnet design must be such as to reduce the unavoidable leakage fields to a minimum relative to the working field of interest.

The most common types of magnets heretofore employed in conjunction with the magnetron are the C and E shaped magnets, in both of which it is necessary :to employ a relatively thick cross-section of material at the magnetic neutral plane in order to supply a magnetic field of sufficient strength at the working gap. As a result, however, these designs have two serious drawbacks. Firstly, the magnets are usually so thick that the flux path in the magnet, going from pole to pole, is twice as long at the outer periphery of the magnet as at the inner periphery, which means that the coercive force in the magnet varies in the ratio of approximately 2:1. Therefore, the material cannot at all places he working at maximum energy density. Secondly, the leakage fields of any given magnet are a function of the magnet shape, and for conventional non-symmetrical magnets there exists the following leakage field components:

1. Leakage flux between the top and bottom of the inside of the magnet;

2. Side leakage flux from the sides of the top to the sides of the bottom of the magnet;

3. Outside leakage flux from the top to the bottom of the outside ofthe magnet.

In accordance with the present invention, on the other hand, there is provided a magnet in which the second and third leakage flux components are eliminated and the first component is minimized, thereby to provide minimum leakage flux concomitant with maximum energy density. In addition, as stated hereinabove, the invention provides a magnet all parts of which provide maximum energy density and which is therefore approximately half the weight of known magnets heretofore employed for the same purpose.

I nited States Patent O r 2,787,728 T'Patented Apr. 2, 19:57

In accordance with the broadest aspect of the invention, there is provided in an electron discharge device, of the type requiring a transverse magnetic field with respect to the flow of electrons for the operation thereof, a permanent magnet substantially in the shape of a toroid surrounding the device and magnetically coupled thereto for the production of the magnetic field.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a cross-sectional view showing essentially the magnetic circuit of a magnetron in combination with a toroidal magnet, in accordance with the invention;

Fig. 2 is a diagrammatic view showing a magnetron in combination with a conventional C-shaped magnet of the prior art;

Fig. 3 is a side view of the complete magnetron using the toroidal magnet; and

Fig. 4 is a view taken along lines 4 -4 of Fig. 3.

Although the invention is applicable to electron discharge devices of the general type requiring a transverse magnetic field with respect to the flow of electrons, to facilitate an understanding of the invention its applicability to magnetrons in particular will be described in detail hereinbelow. The application of the invention and its basic concepts to other forms of electron discharge devices will then be readily. apparent.

With reference now to the drawings, Fig. l illustrates essentially the magnetic circuit of a magnetron in which unnecessary details of the structure have been eliminated for simplicity in illustration and discussion. There will be described briefly only that portion of the magnetron which isnecessary for an understanding of the invention. The magnetron comprises. an evacuated envelope or housing including a supporting body 2 and a pair 'of tubular members'18 extending from opposite ends thereof, only a portion of these members being shown in Fig. l. A plurality of vanes 4 are mounted inside the body 2 and are preferably brazed or welded to the body 2. The vanes form together with the body a high frequency generating structure which provides a plurality of cavity resonators (notseen in the figures but-existing between adjacent vanes). Magnetic pole pieces 5 and 6, respectively, are fixedly mounted inside the supporting body and positioned on opposite sides of theresonators. A movable magnetic pole piece 7 (axially slidable) is provided to vary. the flux path between itself and magnetic pole piece 5. A cathode 8 is also provided for supplying electrons which are emitted radially therefrom and travel in the plane of magnetron anode structure. The energy is supplied to the cathode over leads coupled to a source of power supply (not shown).

Referring now to Fig. 3, there is shown an extension 9 which accommodates a cathode support assembly and encloses one end of the evacuated tube envelope. The details of this assembly may be found in U. S. patent entitled Cathode Support, No. 2,474,263, filed November 1, 1945, and issued June 28, 1949. The power leads are connected to the power supply over the filament terminals 10, -11 and 12,- respectively.

An extension 13 is provided to enclose the opposite end of the evacuated housing or envelope 1 and encloses a frequency section'of the magnetron. For example, this section may include a sylphon bellows enclosing .a tuning plunger which may be attached to an L-C ring as described in my co-pending application entitled Tunable Magnetron," Serial No. 375,532 and filed August therefore; will notbe-described herein.

the well known mannerm fhe'.impedance transformer and the method of extracting energy from the magnetron is explainedin detail in-the above-mentioned patent and,

Referring now to Fig. 2, there is shown diagrammatically the cathode 8, vanes 4 and pole pieces 5 and 7 of a magnetro'n'using aconvent'ionalC type permanent magnet: As is well known", the operation of the magnetron as an oscillator is-dependentu'pon an axial magnetic field whichis usually supplied: by a permanent magnet. -An axial magnetic field -is construed to mean a field axial with-respect. to the axis ='of the cathode and is a field substantially transverse-to the'flow of electrons from the cathode'to the resonators." Several types of permanent magnets have been designed for-producing this field. One of the more conventional types is the C-shaped magnet. 'As may-be seen-in-Fig. 2 the' perimeter of the inside of the-magnet, e; gathe contourspassing through point A, is about half the outside perimeter, e. g. the contour passing through point- B. I Ihis' means that'the maximum energy density varies from point A to point B in the ratio of approximately 2-:l Moreover, since the C- shapedmagnet is unsymmetrical in design, there, of course, exists leakage fields as described above.

'The present invention is characterized by providing a permanent magnet of substantially toroidal design, whereby the inside and outside perimeters are substantially equal and in which all portions of the magnet operate essentially: at maximum efiiciency. Fig. 1 illustrates in cross-section a permanent magnet substantially in the shape of aitoroidal shell surrounding the magnetron-supporting body and having its poles contiguouswith opposite ends thereof. The magnetic neutral plane 16 is .symmetrically'Iocated-with' respect to the axial extremities-ofthe toroid and perpendicular to the axis of symmetry'thereof. Accordingly, the north pole of the magnetisat one end of thetoroidal axis as shown on' the drawing,-and the south pole is at the opposite end. The permanent magnet'l5=is magnetically coupled to :the pole pieces-"5, 6 and -7, respectively, preferably over 'steel shoulders, 17 and copper housing 18. The

steel shoulders 'andthe copper-housing are conventional,

and provide the required rigidity and heat conductivity necessary in such devices." It'Will -be' recognized, of course, that from a magnetic standpoint thesteel shoulders constitute-an extension or continuation of the toroidal magnet. 1 i

The leakage field off the side of the toroidal magnet is "zerobecause-the' magnet is symmetrical in form. Opening the gap between the top and bottom '(north and south poles) .of the toroid is thus possible and results in less internal leakage. As this is' done, the axial dimension of the toroid contributes to the development of the-coercive force; accordingly, the physical size (the circumference) may be reduced. 'Also since the magnet is a surface of revolution, it is possible to provide the increased area at the neutralplane'without changing the approximately equal inner andouter paths of the flux lines in the material. The toroidal shaped magnet provides: i

1.= Maximum gauss in'the working gap per pound of material.

2. Minimum intensity external fields.

3. Minimum attraction between the magnet and adjacent magnetic materials because of the absence of high local leakage fields, thus minimizing accidental damage to the magnet or to the electron discharge device operating in conjunction therewith w To obtain maximum efficiency, the toroidal magnet is designed preferably without air gaps; however, for practical expediency, it may be necessary that one or'r'nore" air gaps be provided to relieve stresses due to temperature expansion in the magnet material. One such gap is shown by way of example in Fig. 4.

As shown in Figs. 3 and 4, the magnet 1 is provided with an opening in which the output coupling wave guide 14 is positioned and coupled to the resonator. In addition, other openings of suitable shape in the magnet may be provided for input and output coolant leads for the magnetron." One such lead is shown in Figs. 3 and 4.

It is also 'to be understood that'the magnet may be made up of two or more sector parts.

While the foregoing description sets forth the principles of the invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope of the inventionas set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. In a magnetron having a supporting body, a plurality of vanes mounted inside said body integrally there with and forming together with said body a plurality of resonators, and'magnetic pole pieces mounted inside -said'supporting body and positioned on opposite sides of said resonators, the combination comprising a permanent'magnetsubstantially in the shape of a toroidal shell surrounding 'said'supporting body, the magnetic neutral plane of the magnet being symmetrically located with respect to the axial extremities of the toroid and perpendicular'to the axis thereof, whereby the north pole is at one end of the'toroidal axis and the south pole is at the opposite end,'and means magnetically coupling said north and south poles respectively, to said pole pieces mounted on opposite sides of said resonators.

2. The device according to claim 1, wherein said permanentmagnet is formed with an aperture, whereby an output wave guide may be coupled to one of. said resonator's'."

3. The device according to claim 1, wherein said permanent magnet is formed with an'air-gap extending from one pole towards the other, whereby stresses due to temp'ei'ature expansion may be relieved.

4. In a magnetron the combination comprising: an evacuated envelope having a central axis and first and second ends; means forming a plurality of cavity resonators'within 'said envelope, said cavity resonators being spaced from and disposed about said axis; and a toroidal permanent magnet'surrounding said envelope and having first "and second poles, the magnetic neutral plane of said ma'guet'being perpendicular to said axis and said first and'second poles being contiguous with said first and second ends of said envelopes, respectively, whereby a static magnetic field parallelto said axis is produced in said cavity resonators;

"5.)11'1 'an electron discharge device wherein the magnetron effect of a transverse magnetic field upon electrons is employed to generatemicrowave signals, the combina tion' comprisingian evaeuatcdmetallic envelope having first andsecond ends and an axis extending thcrebetween: means forming a high frequency generating structure within saidenvelope-and disposed about said axis in a reference plane perpendicular thereto; means positioned within said envelope' for emitting electrons in said reference plane and a toroidal permanent magnet having an axis of: symmetry and first and second poles at opposite ends thereof, said toroidal permanent magnet surrounding said envelope with saidfirst and second poles contiguous-with said first and second ends, respectively, of said 'envelopewliercby a magnetic field is applied to the emitted electrons transverse to their direction or" travel.

6. In a high frequency electron discharge device wherein'the magnetron effect of a magnetic field upon electrons moving ina path at right angles to the field is our 'ployedto generate'microwave signals, the combination comprisingran evacuated envelope; first means forming with said envelope 9. high frequency generating structure 5 in a predetermined reference plane within said envelope; second means spaced from said first means for emitting electrons in said predetermined reference plane and toward said high frequency generating structure; and a toroidal permanent magnet having an axis of symmetry and first and second poles at opposite ends thereof, said toroidal permanent magnet surrounding at least the portion of said envelope including said high frequency generating structure and being positioned with said axis of symmetry perpendicular to said reference plane; said 10 first and second poles being contiguous with the external surface of said envelope for generating a static magnetic References Cited in the file of this patent UNITED STATES PATENTS

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