US3790836A

US3790836A - Cooling means for electrodes

Info

Publication number: US3790836A
Application number: US00293904A
Authority: US
Inventors: M Braun
Original assignee: Individual
Current assignee: Varian Medical Systems Inc
Priority date: 1972-10-02
Filing date: 1972-10-02
Publication date: 1974-02-05
Anticipated expiration: 1991-02-05

Abstract

An electron discharge device having a movable electrode within an evacuated envelope, and means comprising retractable elements associated with the envelope and movable into engagement with the electrode for providing a thermally conductive path for dissipation of heat from the electrode outside the envelope.

Description

United States Patent 1191 1111 3,790,836 Braun 5] Feb. 5, 1974 COOLING MEANS FOR ELECTRODES Primary Examiner-J 1. K. Saalbach B l 3 f d [76] gg zz-g g 4 8? or St Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm-Harold A. Murphy; Joseph Filedi Och 1972 D. Pannone; John T. Meaney [21] Appl. No.: 293,904

[57] ABSTRACT 52 (LS. 131 313/13, 313/39, 313/46, An electron discharge device having a movable e1ec 3/ 1 31 trode within an evacuated envelope, and means com- [51] f H013 1/02 H011 7/24 H011 35/10 prising retractable elements associated with the enve- [58] Fleld of Search 313/13, 39, 40, 46 lope and movable i engagement i the electrode 0 for providing a thermally conductive path for dissipa- [56] References C'ted tion of heat from the electrode outside the envelope.

UNITED STATES PATENTS 3,644,769 2/1972 Kennedy 313/46 20 Clam, 8 Dawmg l PATENTE'D FEB 5 SHEET 2 (IF 3 BACKGROUND OF THE INVENTION Electron discharge devices such as X-ray tubes, for example, are provided with electrodes such as anode and cathode within an evacuated dielectric enclosure or envelope. At timesone of the electrodes becomes extremely hot during operation of the device. This problem is particularly illustrative inthe X-ray tube wherein the anode is bombarded'by a dense beam of electrons for generation of X-rays from the bombarded anode surface.

In attempts to solve this problem, such heated electrodes or targets have often been made to rotate so as to continuously present cool surface areas to the impinging electron beam. However, in such devices cooling of the electrodes is almost entirely by radiation. It is, therefore, desirable to operate the electrodes or targets at high'temperatures because radiated power increases with the fourth power of the temperature. Also, the heat storage capability of the target increases approximately with the limiting temperature.

Rotating targets have only partially solved heat dissipation problems, however, and in turn presented new problems relating to device designs suchas, for example, bearing failures, and material evaporation, warpage and crackage at high temperatures. It has been found that the target body should be a material of high heat storage capacity and heat transfer capability but which is usually of only low to average temperature stability.

It has also been found that when the heat storage capacity of a target is increased by replacing refractory metal'with a light weight material of high specific heat, such as, for example, beryllium, graphite, boron carbide, molybdenum, etc., there is introduced the added problem of securing and maintaining this material in efficient heat-conductive relation with another material such as tungsten in the case of an X-ray tube, for example, which is an efficient X-ray generating material. It is extremely difficult to establish a stable high temperature bond between such materials and mechanicalretention is sometimes unreliable.

SUMMARY OF THE INVENTION The above and other objections to the prior art are partially or entirely overcome in the present invention by the provision of novel means for cooling a rotatable electrode, this being achieved by the utilization of heatconductive means in the envelope wall which are movable into direct physical engagement with the electrode during nonrevolving cooling intervals. The heatconductive means provides a thermal flow path whereby the heat from the electrode is conducted exteriorly of the envelope where it may be dissipated more efficiently by radiation or by an enveloping coolant liquid.

The heat-conductive means may be one or more metal rings which are resiliently supported in the envelope wall, during rotation of the electrode, in slightly spaced relation from the rotatable target or electrode to be cooled. Adjacent the ring or rings is an arrangement of one or more electromagnets which are electrically connected into the control system of the device so as to be operated during intervals when the target is immobile whereupon the ring will engage the target and thus conduct heat away from it.

In one alternative structure, the heat-conductive means takes the form of a plurality of longitudinally movable rods, pins or fingers movable by the magnet toward and away from the electrode. In a still further modification, the heat-conductive means comprises a wire brush.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings, wherein:

FIG. 1 is an axial sectional view of an X-ray tube embodying one preferred form of the invention;

FIG. 2 is an enlarged fragmentary axial sectional view of the heat-conductive means shown in FIG. 1 in inoperative position;

FIG. 3 is a view similar to FIG. 2 showing the heatconductive means in operative position;

FIGS. 4 and 5 are views similar to FIGS. 2 and 3 respectively illustrating an alternative form of heatconductive means;

FIGS. 6 and 7 are views similar to FIGS. 2 and 3 respectively illustrating a still further modification of the heat-conductive means; and

FIG. 8 is an axial sectional view similar to FIG. 1 showing a further embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more particularly to the drawings wherein like characters of reference designate like parts throughtout the several views, the embodiment of the invention disclosed in FIG. 1 is illustratively depicted as applied to an electron discharge device of the X-ray generating type. The X-ray tube 10 includes a dielectric X-ray transparent bulb or envelope 12 having a neck portion 14 of reduced diameter extending from one end thereof. A target electrode 16 which is a part of the anode electrode of the tube is mounted for high speed rotation within the bulb 12. Details of the general construction of an X-ray tube are omitted here since such details do not in themselves form a part of the present invention. Therefore, only such details as are necessary for a clear understanding of this invention are included herein and for additional details reference may be made to any of the many X-ray tube technical publications in existence.

The target 16 is a disc of selected high thermal capacity material such as beryllium, graphite, boron carbide,

molybdenum, etc., which carries on one surface a focal track ring 18 of efficient X-ray generating material. By high thermal capacity is meant that the material has a high thermal storage capacity per unit weight in comparison to the X-ray generating focal track 18 which is preferably made as a coating of tungsten, rhenium, tungsten-rhenium alloy or other material known to efficiently generate large quantities of X-rays in response to electron bombardment. Focal track 18 may be mechanically separate from the target 16 or may be a metallurgically applied coating.

The bulb 12 supports a cathode electrode 20 which includes an electron emitting structure 22 located so as to direct a stream of high intensity electrons onto the focal track 18 of the target 16. A beam of X-rays then is generated and leaves the focal track through the wall of the envelope.

In order to continuously present cool areas of the focal track to the electron beam, and thereby prevent locallized heat buildup and consequent target damage, the target is made during an X-ray generating cycle to rotate at high speeds such as l0,000 rpm, for example.

In general practice, therefore, a tube is operated to provide only a single short burst of X-rays or a series of a few extremely short bursts, since any extended cycles will cause considerable damage if not complete destruction of the tube.

The target 16 is mounted on the end of a shaft 24 by any suitable means such as by being gripped between a nut 26 and a circumferential ledge 28 formed on the shaft. The shaft 24 is carried within a rotor structure 30 of any suitable type such as shown, for example, in U.S. Pat. No. 2,648,025. The rotor structure 30 is mounted as by a metal ring seal 32 on the end of the envelope neck portion 14 which encircles the structure 30 in slightly spaced relation as shown. Encircling the neck portion 14 of the envelope adjacent the rotor structure 30 is suitable inductive means 34 including a coil 36. Upon energization of the inductive means 34 the coil 36 acts upon the rotor structure 30 to cause the target 16 to rotate at high speed about the axis of the structure.

The present invention is provided to reduce damage to the target by conducting away during stationary periods of the target considerable amounts of the heat which is built up in the target during the X-ray generating cycles. This is achieved by disposing within the envelope wall heat-conductive means 38 which is adapted to move into physical engagement with the target 16 when the target is stationary. The means 38 actually forms a part of the envelope between the bulb 12 and the neck portion 14 and, therefore, is en: osed to the cooling atmosphere external of the tube, which atmosphere may comprise a cooling air flow, or a liquid coolant such as oil or the like.

As shown in FIG. 1, a kovar or like metal collar 40 is sealed into the lower open end of the bulb 12. One end of a cylindrical outer mounting ring 42 is affixed to the collar 40. A second metal collar 44 is sealed at one end to the adjacent end of the envelope neck portion 14 and at its other end is affixed to an inner mounting ring 46. A number of electromagnets 48 are carried between the mounting rings 46-48 at selected intervals throughout the annulus formed by the rings 46-48.

Flexibly connected between the collar 40 and a support ring 50, which is also sealed to collar 44, are one or more ringlike heat-conductive members 52, which members 52 are interconnected to one another and to the supporting structure by flexible metal members 54. It will be noted that the axial dimensions of the ring members 52 are reduced in the vicinity of the magnets 48 so as to provide recesses in which the magnets are disposed.

In the normal cooperative relationships of the parts of the invention, the target 16 is stationary and no energy is applied to the magnets 48. The flexible springlike members 54 in their unstressed condition normally urge the heat-conductive ring members into intimate physical contact with the adjacent surface of the target 16 as shown in FIG. 3. I

When an X-ray generating cycle is to occur, the control system (not shown) first operates the magnets 48 which attract the ring members 52 toward them, moving the members 52 out of contact with the target 16 so that the target may be rotated without hinderance, as shown in FIG. 2. Such movement of the members 52 will be accomplished against the tension of the resilient members 54. Thus, when-the magnets 48 are deenergized at the end of an X-ray exposure cycle, the members will return the ring members 52 into engagement with the target 16 whereupon they will function to conduct heat away from the target.

Referring to FIGS. 4 and 5, there is shown a collar 56 sealed to the end of bulb 12 and a collar 58 sealed to neck portion 14. A supporting metal annulus 60 is sealed throughout its inner edge by a resilient springlike ring 62 to an outwardly directed flange portion of the collar 58. A second flexible ring 64 seals an annular upper surface area of the annulus 60 to the first collar 56. Thus, an evacuated enclosure is completed.

The inner surface of the annulus 60 carries a multiplicity of annular arrays of rods, pins or fingers 66 which extend toward the target 18. The adjacent surface of the target 16 is provided with a series of concentric grooves 68 into which the adjacent ends of the rods 66 extend. A number of electromagnets 70 are mounted on the outer peripheral surface of collar 56 and connected either directly or through associated structure (not shown) to an apertured disc 72 which is also affixed to collar 58 for immovably interconnecting the dielectric envelope bulb and neck portions.

In the inoperative condition of the device, the target 18 is stationary and the magnets 70 are energized. The tension inherently present in the

resilient rings

62 and 64 is overcome by the magnets and the annulus 60 is pulled toward the target 16 and the ends of the rods 66 are pulled into the grooves 66. It is to be noted that sides walls of the grooves 66 are inclined so that contact of the rods with the inclined walls is insured. To provide greater surface area of contact, each rod has an inclined surface which is adapted to engage the inclined walls of the grooves.

At the beginning of an X-ray generating cycle the magnets 70 are deenergized. This allows the resilient rings 64 to return to their normal unstressed conditions, pulling the annulus 60 in a direction away from the target and withdrawing the rods from their contact with the walls of the grooves in the target, as shown in FIG. 4. The target thus may be rotated without interference. At the end of the cycle the target is again stopped and the magnets then energized. This returns the rods 66 into heat-conductive contact with the target and aids in the dissiptation of heat from the target.

Referring now to the further embodiment disclosed in FIGS. 6 and 7, a ring seal 74 connects the end of bulb 12 to a supporting ring 76. Ring 76 carries magnets 78 which are in turn connected to the outer edge of an apertured disc 80 which has its inner edge affixed to a ring seal 82 sealed to the envelope neck portion 14.

An annular heat dissipation plate 84 is sealed adjacent its inner edge to ring seal 82 by a flexible ring member 86 and is sealed near its outer edge by a second flexible ring member 88 to the ring seal 74. Energization and deenergization of the magnets 79 causes consequent movement of the heat dissipation plate 84 toward and away from the target 16.

The target 16 is provided with a number of relatively fine resilient wires 90 which form a heat brush on its surface nearest the heat dissipation plate 84.

When the target 16 is stationary and the magnets are deenergized, the heat dissipation plate 84 is pulled by the resilient ring members into engagement with the heat brush 90. Upon operation of the device, the magnets 78 will be energized to draw the heat dissipation plate 84 out of contact with the heat brush 90, as shown in FIG. 6. Upon completion of the X-ray generating cycle the targetwill become stationary and the magnets 78 will be deenergized. This will-allow members 86-83 to again pull the heat dissipation plate 04 into contact with the heat brush 90 as shown in FIG. 7. This will allow heat to be conducted from the target 16 through wires 90 into the plate 84 for dissipation into the external ambient.

In some instances, particularly where the wires 90 are of extremely small diameter, the magnets 78 are unnecessary since centrifugal force will cause the wires to bend outwardly out of contact with the plate 84. Even if magnets are used in a structure of this type, only an initial separation pulse is required at the start of the tar gets rotation to reduce friction.

It will be noted that in all of the described embodiments of the invention, the anode target is disclosed as takingthe form of a disc and the cooling device of the invention comprises members which form a part of the envelope bulb portion 12 for engaging one of the planar surfaces of the disc.

However, it is to be understood that the cooling means may be located elsewhere and that the target may be shaped in other manners to cooperate with the cooling means. An illustrativeembodiment is shown in FIG. 8 wherein a central rotatable shaft 92 supports at one end a target 94 which rotates about the axis of the shaft in the well-known manner. The shaft 92 is supported by bearings 96 in a coaxial bore 98 formed in one end of a block 100. Block 100 has an outer surface which is hermetically sealed by'a sealing ring 102 to the dielectric envelope neck portion 14.

The target 94 is provided with a skirtlike portion 104 which extends freely down into the space between the block 100 and the encircling envelope neck portion M. As shown in FIG. 3, the inner cylindrical surface of said skirt 104 carries a conductive wire brush 106 which is adapted to engage the adjacent surface of the block 100 when the target is stationary. When the target rotates, centrifugal forces will cause the flexible wires of the brush to be displaced from engagement with the blocks surface.

It will be understood that magnet means may be provided in the envelope neck portion 14 or in the block 100 to cause a desired moving engagement of a conductive member (not shown) with the brush 106 in a manner similar to that discussed in connection with FIGS. 6 and 7. It will also be understood that rings or rods and magnet means such as shown in FIGS. L5 may be employed with an anode structure as shown in FIG. 8. Also, the block 100 may be provided with conduits or passages 108 whereby a fluid coolant may be introduced to aid in dissipation of heat from the block.

With any of the described structures, cooling of the electrode is greatly increased. Assuming beryllium to be the target material, 500 grams of beryllium have an average heat storage capability of 1,300 joules per degree C temperature increase. Because of the extremely high strength of beryllium below 800C, and a vapor l0 torr at 600C, a target will store 1,300 X 570 700,000 heat units (joules) by heating the target from room temperature to 600C. In addition the mechanical strength of beryllium is high enough at 600C to allow target diameters up to 10 inches at 10,000 rpm or up to 25,000 rpm at a diameter of 4 inches.

This will give an increase of instantaneous power by dLpercent over conventional targets which rotate at 10,000 rpm and have a diameter of 4 inches.

If the target is at 600C and the tube is immersed in oil at C, and the length of the heat conducting elements 52-66-90 (copper or silver) is 1 cm, more than 2 KW per cm would be transferred even when the actual contact area is somewhat smaller. This is 120,000 heat units and surpasses the cooling down rate of all known X-ray tube anodes. Each additional square centimeter conducting area gives an additional 2 KW cooling rate. This heat transfer rate cannot, however, be considered actual heat transfer density of 2 KW/cm because it is spread out over the whole area of contacting surfaces.

Accordingly, many improvements have been achieved by the presently described invention in accordance with the objectives of this invention. However, it is to be understood that various modifications and changes in the structures shown and described may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claims. Therefore, all matter shown and described should be interpreted as illustrative and not in a limiting sense.

I claim:

ll. An electron discharge device comprising an envelope containing an electrode and means for heating said electrode, and heat-conductive means between said electrode and envelope for conducting heat from the electrode to the exterior of the envelope, said heatconductive means being movable into and out of interconnection between the envelope and electrode during cycling of the device.

2. An electron discharge device as set forth in claim 1 wherein said movable heat-conductive means comprises a portion of the envelope.

3. An electron discharge device as set forth in claim ll wherein said movable heat-conductive means is fixed to the electrode.

4. An electron discharge device as set forth in claim it further comprising magnet means carried by said envelope adjacent said heat-conductive means for moving same.

5. An electron discharge device comprising an envelope comprising a dielectric portion and a heatconductive portion, an electrode within the envelope, means for heating said electrode, and means for mov-' ing said heat-conductive portion of the envelope into and out of engagement with the electrode during cycling of the device.

6. An electron discharge device as set forth in claim 5 wherein said means comprises at least one magnet adjacent said heat-conductive portion for moving same toward or away from the electrode. I

7. An electron discharge device as set forth in claim 5 wherein said heat-conductive portion of the envelope comprises at least one metal member attached to said dielectric portion of the envelope for movement toward and away from said electrode.

8. An electron discharge device as set forth in claim 7 wherein said heat-conductive metal member is a ring flexibly attached to the dielectric portion of the envelope.

9. An electron discharge device as set forth in claim 7 wherein said heat-conductive metal member further comprises a rod, and wherein said electrode has a recess in a surface thereof adapted to be engaged by said rod.

10. An electron discharge device comprising an envelope containing an electrode and means for heating said electrode, said electrode having a surface located adjacent a wall of the envelope, said wall of the envelope including heat-conductive means movable toward and away from said electrode and shaped for broad area surface heat-conductive engagement with said electrode.

11. An electron discharge device as set forth in claim 10 wherein said surface of the electrode is substantially planar, and said heat-conductive means comprises a plurality of rings flexibly mounted in said envelope wall and simultaneously movable into and out of heatconductive contact with said surface of the electrode.

12. An electron discharge device as set forth in claim 11 wherein magnet means is attached to the envelope adjacent said rings for controlling movement of the rings relative to the electrode.

13. An electron discharge device as set forth in claim 10 wherein said surface of the electrode is provided with a plurality of recesses, and said heat-conductive means comprises a plate flexibly mounted in said envelope wall and carrying on its inner surface a plurality of pins movable upon movement of the plate into engagement with the walls of said recesses.

14. An electron discharge device as set forth in claim 13 wherein magnet means is attached to the envelope adjacent said plate for controlling movement of the plate and consequently of the pins relative to the electrode.

15. An electron discharge device comprising an envelope containing an electrode and means for heating said electrode, said electrode having a surface located adjacent a wall of the envelope, said wall including heat-conductive means, and heat-conductive members carried by said electrode movable into and out of heatconductive relation with the heat-conductive means of the wall.

16. An electron discharge device as set forth in claim 15 wherein said heat-conductive members on the electrode are flexible wires.

17. An electron discharge device as set forth in claim 16 wherein said wires have a flexibility such that they will be displaced by centrifugal force from said heatconductive means in the envelope wall when the electrode is rotated.

18. An electron discharge device as set forth in claim 15 wherein said heat-conductive means in the wall includes a plate flexibly mounted in the wall, and magnet means is carried by said wall adjacent the plate for moving said plate with respect to said heat-conductive members.

19. An electron discharge device as set forth in claim 18 wherein said heat-conductive means comprises a plurality of wirelike metal elements.

20. An X-ray tube comprising an envelope, a rotatable target supported in the envelope, a cathode in the envelope adjacent the target for heating the target, said envelope including a heat-conductive portion located in spaced adjacent relation to said target when the target is rotating and movable into engagement with said target when the target is stationary for conducting heat from the target externally of the tube.

v .v UNITED STATES "PATENT OFFICE CERTIFICATE "OF CORRECTION Patent No. 3,7903% Dated Feb. 5', v1974 Inventofls) MaTtin'BTaun .4 v

' fIt is certified that error appeafs in the above identified patent and that; said Letters Patent are hereby corrected as.shown below:

In the Title Page please insert f-A ss ignee: ifhe Machlett Laboratories Incorporated, Sprin'gdale, Conn.

a Corporation of Connecticut Signed eed sealed this 24th da of September 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. 1 0; MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-1050 (10-69) v n I useoum-oc scan-Pee l v v t e e u.s. sovzmmim' mnirms omc: 1m 0-36-33!

Claims

1. An electron discharge device comprising an envelope containing an electrode and means for heating said electrode, and heat-conductive means between said electrode and envelope for conducting heat from the electrode to the exterior of the envelope, said heat-conductive means being movable into and out of interconnection between the envelope and electrode during cycling of the device.

3. An electron discharge device as set forth in claim 1 wherein said movable heat-conductive means is fixed to the electrode.

4. An electron discHarge device as set forth in claim 1 further comprising magnet means carried by said envelope adjacent said heat-conductive means for moving same.

5. An electron discharge device comprising an envelope comprising a dielectric portion and a heat-conductive portion, an electrode within the envelope, means for heating said electrode, and means for moving said heat-conductive portion of the envelope into and out of engagement with the electrode during cycling of the device.

6. An electron discharge device as set forth in claim 5 wherein said means comprises at least one magnet adjacent said heat-conductive portion for moving same toward or away from the electrode.

11. An electron discharge device as set forth in claim 10 wherein said surface of the electrode is substantially planar, and said heat-conductive means comprises a plurality of rings flexibly mounted in said envelope wall and simultaneously movable into and out of heat-conductive contact with said surface of the electrode.

15. An electron discharge device comprising an envelope containing an electrode and means for heating said electrode, said electrode having a surface located adjacent a wall of the envelope, said wall including heat-conductive means, and heat-conductive members carried by said electrode movable into and out of heat-conductive relation with the heat-conductive means of the wall.

17. An electron discharge device as set forth in claim 16 wherein said wires have a flexibility such that they will be displaced by centrifugal force from said heat-conductive means in the envelope wall when the electrode is rotated.