US3842305A

US3842305A - X-ray tube anode target

Info

Publication number: US3842305A
Application number: US00429611A
Authority: US
Inventors: M Braun
Original assignee: Machlett Laboratories Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1973-01-03
Filing date: 1974-01-02
Publication date: 1974-10-15
Anticipated expiration: 1991-10-15

Abstract

An X-ray tube having a rotating anode comprising a target which is provided with improved heat dissipation or compensation characteristics by means of an X-ray generating member and an associated heat sink of high thermal storage capacity material which during operation of the tube more closely engages the X-ray generating member for providing efficient thermal conductivity therebetween.

Description

United States Patent [191 Braun [4 1 Oct. 15,1974

1 1 X-RAY TUBE ANODE TARGET [75] Inventor: Martin Braun, Stamford, Conn.

[73] Assignee: The Machlett Laboratories Incorporated, Stamford, Conn.

[22] Filed: Jan. 2, 1974 [21] Appl. N0.: 429,611

Related [1.8. Application Data [63] Continuation of Ser. No. 320,804, Jan. 3, 1973.

[52] US. Cl. 313/60, 313/330 [51] Int. Cl. H013 35/10 [58] Field of Search 313/60, 330

[56] References Cited UNITED STATES vPATENTS FOREIGN PATENTS OR APPIJCATIONS 536,661 10/1931 Germany 313/60 654,089 12/1937 Germany ..313/60 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm-Harold A. Murphy; Joseph D. Pannone; John T. Meaney [5 7 ABSTRACT An X-ray tube having .a rotating anode comprising a target which is provided with improved heat dissipation or compensation characteristics by means of an X-ray generating member and an associated heat sink of high thermal storage capacity material which during operation of the tube more closely engages the X;ray generating member for providing efficient thermal conductivity therebetween.

18 Claims, 10 Drawing Figures PATEmm-w 1 51914 3.842.305

SHEET 2 0F 4 g 50 F/G 4 5 E In ef z\ 5 Grumman 1IIIT-1- L1I TEMPERATURE C CONSTANT- F RATIO Kpsi/ DENSITY or CONSTANT 1 X-RAY TUBE ANODE TARGET This is a continuation of application Ser. No. 320,804, filed Jan. 3, 1973, now abandoned.

BACKGROUND OF THE INVENTION In the X-ray field new techniques have become commonplace which require that an X-ray generator or tube be operated at higher power levels. it has been found that where once it was merely necessary to rotate an X-ray generating target to constantly present cooler areas to an impinging electron beam, it is now necessary to provide more exotic means for dissipating heat from the target.

Heat dissipation has been improved in some cases by increasing the rotational speed of the anode, and by judicious choice of material from which the target is made. For example, molybdenum has often been-used as a replacement for the commonly used tungsten since it was found to provide better thermal storage capacity, and to be lighter weight which permitted'higher rotational speeds to be used. However, in order to obtain the desired efficient X-ray generation it became necessary to coat the focal track with a thin layer of tungsten or tungsten-rhenium alloy. This presented problems due to deformation or separation of the coating from the target body.

A later improvement in target structures is disclosed in pending US. Pat. application Ser. No. 42,375, filed June 1, 1970, and owned by the assignee of the present invention, wherein a multi-piece target is described which comprises an annular focal track ring of refractory material such as tungsten which is sandwiched between discs or members of lighter weight material having higher thermal storage capacity than the ring. In this structure heat built up in the ring is quickly transferred to the discs which have the capacity for storing this heat without damage for extended periods of time during a series of exposure cycles. During and after the exposure cycles the heat is dissipated by radiation. This has enabled the X-ray tube containing the target to be operated for lengthier periods of time and at higher power levels than theretofore.

With such a target structure the heat transfer from ring to discs is intended to be by conductivity. However, at high power levels with generation of extremely high temperatures, non-symmetrical heat input and differences in expansion coefficients sometimes cause deformation of the geometry of the parts. For example, the ring sometimes becomes slightly warped and this tends to produce small gaps between the parts and, therefore, to introduce some deterioration in the transfer of heat by conduction between the parts. While transfer of heat by radiation through these gaps then takes place, overall heat transfer may be reduced. In fact, in some cases where spacing between the parts exist the disc material, instead of functioning as a heat sink, functions more as a heat shield. Therefore, it is essential that the ring and discs be maintained in good heat conductive relation.

Probably the best way a contact between the two materials can be made is by brazing. However, because of high temperature stresses the brazed joint easily breaks apart. If mechanical fastening is used, the contacting surfaces very often wont stay in contact for the reasons pointed out above in the discussion of the invention described in the aforementioned copending application.

Some of these difficulties have been overcome by the structure disclosed in pending US. Pat. application Ser. No. 240,713, filed Apr. 3, 1972, by the present inventor. In this structure the anode comprises a target ring of refractory material having heat sink material located on its inner periphery and retained there in such a manner that during operation of the tube the heat sink material will tend to more closely engage the target ring either through the influence of centrifugal force or by thermal expansion. This enables the heat generated in the target ring to be efficiently transferred by conduction into the heat sink from which the heat is eventually removed by radiative dissipation.

SUMMARY OF THE INVENTION Additional disadvantages sometimes found in anodes of X-ray tubes operated at high power levels are overcome by the present invention-wherein the anode comprises a cylindrical or cup-shaped member of heat sink material having the refractory X-ray generating target located on its inner periphery and retained there in such a manner that during operation of the tube the target will tend to more closely engage the heat sink member either through the influence of centrifugal force or by thermal expansion. This will enable the heat generated in the target to be efficiently conducted into the heat sink which has the capability of storing the heat in large quantities without damage for relatively lengthy exposure cycles. During and after the exposure cycles,

the heat is removed from the sink by radiative dissipation.

The target is an annulus of tungsten, rheniumtungsten, molybdenum or niobium or the like having an X-ray generating surface disposed to be revolved through the path of an electron beam as the target rota'tes about its axis. The heat sink member may be supported by suitable means such as a disc or cone mounted on the anode arbor or shaft. The target, which lies on the heat sink member may be an annular arrangement of refractory members of tungsten or the like which are disposed upon, closely adjacent or actually within the inner periphery of the heat sink member.

The heat sink member, which is of higher heat capacity than the target material, may be substantially cupshaped with the target being disposed on the inclined side walls of the cup. The member may, however, be substantially cylindrical. In either case, the resultant X-ray beam passes out through an end of the tube envelope instead of through the side thereof as in conventional tubes.

Centrifugal force acting upon the refractory target urges it outwardly and tends to insure close physical engagement of the target with the heat sink member. Thus, heat will be efficiently transferred by conduction from the target to the heat sink member.

Examples of suitable heat sink materials for this purpose are beryllia, molybdenum, graphite, boron carwill cause it to deform. Accordingly, at least a portion of the target is preferably embedded into the surface of the heat sink.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will become apparent from the following description taken in connection with the accompanying drawings, wherein:

FIG. 1 is a front elevational view of a schematic illustration of an X-ray device embodying the invention;

FIG. 2 is a side elevational view similar to FIG. 1;

FIG. 3 is an enlarged axial sectional view taken approximately through the center of the device shown in FIG. 2;

FIG. 4 is a graph illustrating tensile strengths of certain high temperature materials usable as target materials and heat sinks;

FIG. 5 is a graph illustrating the relative strengths of the materials shown in FIG. 4 in rotating systems;

FIG. 6 is an enlarged fragmentary sectional view illustrating a modified target-heat sink structure;

FIG. 7 is an enlarged fragmentary sectional view of a further modified target-heat sink structure;

FIG. 8 is a front elevational view of a schematic illustration of another embodiment of the invention;

FIG. 9 is a side elevational view similar to FIG. 7; and

F IG. 10 is an enlarged axial sectional view taken approximately through the center of the device shown in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, FIGS. 1 and 2 are illustrations of a novel X-ray device embodying the invention, the X-ray tube and anode being shown in phantom for illustrative purposes. Internal structures are clearly shown in FIG. 3. The X-ray device comprises a substantially cylindrical housing 10 having electrical cable horns l2 and 14 at one end and an X-ray beam exit port 16 in the opposite end.

Within the housing 10 is an X-ray tube of the rotating anode type which embodies a dielectric envelope 18 in which is supported an anode 20 and a cathode 22. The cathode 22 includes a supporting cylinder 24 one end of which is sealed to a reentrant end portion 26 of the envelope. On the inner end of cylinder 24 is mounted one end of a transversely extending angled support bracket 28, in the free end of which is located a cathode head 30. The cathode head 30 contains an electron-emitting filament to which a suitable electrical potential is applied through leads 32 extending externally of the tube through cylinder 24 to a cable termination 33 in horn 14.

The opposite end of the envelope 18 carries the anode 20 which includes a target assembly 34 mounted on one end of a rotor shaft 36 extending from a rotor 38 rotatably located in a neck portion 40 of the envelope. The rotor carries a skirt 42 bolted thereto, and the assembly is adapted to rotate rapidly when the tube is mounted in suitable inductive means 44 surrounding the neck 40 when the inductive means is energized.

The tube is mounted firmly in the housing 10 by any suitable means such as, for example, by being bolted to a number of supporting bosses, lugs or similar fixed projections 46 which extend inwardly from the inner side walls of the housing 10. For the purpose, the inductive means 44 is secured within a retaining ring 48 which is in turn attached to lugs 46 by bolts 50. A mechanical shock absorbing ring 47 may be used to assist in restraining the tube from undesired lateral movement within the envelope.

The lower end of rotor 38 is bolted or otherwise secured to the adjacent end of the envelope neck portion 40 with suitable means 52 being provided through the neck portion 40 to connect the rotor 38, and consequently the anode, to a second cable termination 54 mounted in the second horn 12 via a suitable conductor 56.

In accordance with this invention, the anode target assembly 34 comprises a cup-shaped heat sink member 58 having a transversely extending base portion with a substantially cylindrical upstanding circumferential wall 60 having an inclined inner surface 62 directed inwardly and upwardly. Surface 62 has affixed to it or in it an annular ribbon-like target member 64 which extends throughout the circumference of the surface 62. The exposed surface of the target 64 is directed toward the cathode head 30 so that an electron beam from the cathode may be made to impinge upon the target. As is well known in X-ray tubes, the anode is rotated so as to move the target in a circular path through the electron beam. As a result, X-rays are generated in the target and pass out through the dielectric envelope wall.

The housing 10 is provided with an exit port structure 16 including a frame 65 containing an X-ray transparent window 66 throughwhich the X-rays pass out of the housing as a restricted X-ray beam. The anode target member 64 is in the form of a ring made of suitable high atomic number material, such refractory materials as tungsten or tungsten-rhenium or molybdenum being particularly suitable.

The target of conventional X-ray tubes usually comprises the. entire anode disc or is a metallurgically deposited coating upon a suitable backing of high thermal capacity material. For example, the entire target of conventional X-ray tubes may be made of tungsten, or a target backing of tungsten, graphite, molybdenum or the like may carry on its surface a focal target ring of a deposited or metallurgically bonded material such as tungsten or tungsten-rhenium alloy.

It has been found that solid targets of tungsten or molybdenum do not have satisfactory thermal characteristics and, when bombarded by high density electrons, become damaged by the resulting severe mechanical stresses. It has also been found that a coating of X-ray generating material upon the surface of such a solid backing will not prove satisfactory when a tube is operated at high power levels since the metallurgical bond between the coating and backing will not withstand the stresses resulting from the thermal shock of the impinging electron beam. Furthermore, it is difficult to obtain a thermal expansion match of the coating with suitable backing materials over a full operating temperature range which may extend from room temperature to approximately 3000C.

In accordance with this invention, the target 64 is made as a member separate from the heat sink member 58, and is preferably made up of a number of separate elements. With the improved heat transfer characteristics achieved by this invention, the target member 64 may be a ring which lines the adjacent surface 62 of the heat sink wall portion 60. Such a ring may be closely fitted onto the surface 62 or, preferably, will be disposed within a closely interfitting groove or channel in surface 62.

The important factor achieved is that this construction utilizes natural centrifugal forces available with rotating systems to improve the heat-conductive relation between the X-ray generating metal and the heat sink material. Therefore, when the target is bombarded by the electrons from the cathode, the resultant heat generated within the target material will easily be conducted into the higher heat capacity material of the heat sink.

The wall 60 of the heat sink 58 is relatively thick in order to provide suitable mass for efficient heat collection. The target ring 64, however, need be only of a width corresponding to or slightly larger than the length of the focal spot formed at the area of impingement of the electron beam. The base portion of the heat sink 58 is relatively thin to aid in preventing any substantial amounts of heat from passing from the wall 60 into the shaft 36 and associated bearing structure (not shown).

In one embodiment of the invention the target ring 64 is made up of several separate elements located in closely adjacent side-by-side relation. As shown in FIG. 6, for example, these elements may comprisea number of elongated pinor rod-like elements 68 which are disposed within a slot formed throughout the inner circumference of the wall 60. The elements 68 extend parallel in the direction of the width of the slot and are held in place by lips 70-71 which overlie the end portions ofthe elements 68. The active surface of the target ring, therefore, will be recessed beneath surface 62 an amount which corresponds to the thickness of the lips 70-71. The elements 68 will, of course, preferably be slightly wider at their upper ends so as to compensate for the inclined geometry.

It will be apparent that centrifugal forces will insure firm physical engagement of the target elements 68 with the wall 60 of the heat sink member 58. As rotational speeds are increased, the centrifugal forces pressing the elements against the heat sink will correspondingly increase. Also, when the elements 68 become hotter they become more ductile, and consequently will more snuggly contact the heat sink.

It is important that the heat sink be constructed of a material which not only has higher thermal storage capacity per unit weight than the material of the target but also has inherent strength sufficient to withstand deformation at the high temperatures which are employed. Referring to FIG. 4, the tensile strength of several materials has been shown. It will be noted that boron carbide is superior in strength to the other materials illustrated, at slightly above I000C having a tensile strength of about 58 kpsi. and at 2000C having about 32 kpsi. Pyrolytic graphite also exhibits excellent strength characteristics, as does graphite.

Along with tensile strength, however, must be considered the heat transfer characteristics of the selected material. The crucial material constant is k VK/k, where K is the heat diffusivity and k the heat conductivity. The temperature increase of the material on the surface from power input at that surface is directly proportional to k. This constant is 0.75 for pyrolytic graphite, 1.0 for tungsten, and 2.1 for boron carbide.

Thus, a typical cineradiographic series exposure will raise the average target temperature after 3.1 seconds to about 650C for about 100 pm thick tungsten or pyrolytic graphite, to about 900C for pure tungsten, to 1400C for 1 mm thick tungsten on boron carbide, and to about 1800C for 100 pm thick tungsten on boron carbide.

In another embodiment, a three composite target structure is shown in FIG. 7 wherein the anode is supported by a base portion or disc 74 on the shaft 36 and carries at its periphery a wallportion 60a which is a ring of boron carbide or the like. The inner annular surface of the ring 60a is slotted throughout its length to receive therein a ring 76 of pyrolytic graphite. Covering the exposed surface of the ring 76 is a target ring 78 of X-ray generating refractory material such as tungsten, rhenium-tungsten, molybdenum or the like. This structure thus combines the positive properties of the component materials. The graphite ring may be divided into a number of pieces to avoid expansion mismatch and to assure better heat contact when centrifugal forces are applied by rotation of the structure. The target ring 78 may be retained inplace as by providing it with an outwardly directed flange 80 at one end which is inserted between the rings 76 and 60a.

The heat storage capability of both boron carbide and graphite is'excellent compared to the refractory metals. For a heat storage capability of about 500,000 heat units, about 300 grams of either of these two materials is needed, while about 300 grams of molybdenum provides less than 100,000 heat units storage capability.

It has been found that with a structure and materials as described above, the average temperature of a target can be raised to about l700C and the cooling rate will be increased substantially. At l700C, radiated power is three times that at l200C. For a target as shown in FIG. 3 at l700C would have radiated power of 12 KW, which is considerably higher than from conventional rotating anodes.

Because of the exceptionally high strength of boron carbide or pyrolytic graphite, higher rotating speeds may be employed over those of conventional rotating anodes. An increase by a factor of two to 20,000 rpm will increase the instantaneous power capability by about 40% and still leave a large safetyfactor with respect to strength. The light weight of these target materials facilitates the increase of speed without increasing bearing wear.

An advantage of cylindrically shaped anodes is that the centrifugal forces are not directed towards the patient and not in the direction of the X-ray beam, as in conventional X-ray tubes. A safety ring can easily be applied around the whole target without the need for a hole for the emerging X-rays, as would be necessary in conventional targets.

The inside configuration of the present construction does not permit secondary electrons to easily strike the glass walls of the envelope as they do in existing tubes. This helps to avoid high voltage breakdown problems.

Additionally, unwanted radiation is reduced considerably, because there is only X-ray generating material at the focal track while the rest is low Z number material. Even those X-rays clue to secondary electrons striking the focal track at other places than where the primary beam strikes it are reduced because most of the focal track is hidden and does not look at the output part, as distinguished from conventional targets, where large portions of the target are seen from the outside.

Referring now to FIGS. 8, 9 and 10, there is shown a modification of this invention wherein a large diameter anode target structure 80 is mounted on a shaft 82 for rotation in the same manner as described thereinbefore in the description of the device shown in FIG. 3. The structure 80 comprises an inverted cup-shaped target member having a base portion 84 of suitable rigid supporting material. Depending from the outer edge of portion 84 is a ring 86 of suitable material such as molybdenum, graphite, boron carbide or tungsten. The inner surface 88 of ring 86 is inclined as shown and may be slotted to receive a target ring 90 of refractory X-ray generating material such as tungsten, rhenium-tungsten or the like. The ring 90 may be located within the slot or may be disposed suitably upon surface 88, as desired.

The cathode structure 92 includes a head 94 which is located within the annulus defined by the target ring 90 so as to direct electrons thereon. The surface of ring 90 is inclined as surface 88 and thus X-rays will pass upwardly through the base portion 84 and out of the tube through the envelope wall. Therefore, the base portion or support disc 84 will preferably be comprised of a material which is efficiently transparent to X-rays, or may be of a material which is selected to filter out certain portions of the X-rays emitted from ring 90.

The cathode 92 is suitably supported on a bracket 96 secured to a terminal structure 98. The tube is mounted in a housing 100 ofa novel relatively flat configuration, as shown in FIGS. 8 and 9, which has spaced

horns

102 and 104 for receiving cable terminals (not shown). The cathode and anode structure of the tube are connected to the cable terminals in a conventional manner.

This target structure, being of large diameter, achieves extremely high heat radiation; for example, 25 KW at l500C or 7OKW at 2000C. This, therefore, makes heat storage capability less important and the tube may be run continuously. If no increase in instantaneous ratings is required, a rotational speed of 3500 rpm is sufficient to arrive at ratings comparable to the 10,000 rpm ratings of conventional tubes. Obviously, this will cause less bearing wear, decrease noise, and simplify motor controls.

From the foregoing it will be apparent that all of the objectives and advantages of this invention have been achieved by the structures shown and described wherein efficient transfer of heat by conductivity from a target member into an associated heat sink member is achieved. it is to be understood, however, that 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 is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. An X-ray tube comprising an envelope, an anode structure rotatably supported in one end of the envelope, a cathode structure supported in the opposite end of the envelope, said anode structure including an axially extending shaft, a cup-shaped target carried by said shaft for rotation therewith, said target comprising a connecting portion secured to the shaft, a wall portion extending axially from the periphery of the connecting portion, and a focal track ring disposed on the inner surface of said wall portion, the cathode structure including an electron-emitting device disposed to emit an electron beam toward said focal track ring, said wall portion being of a material having heat storage capacity per unit weight greater than that of the material from which the focal track ring is formed, and an intermediate ring disposed between said wall portion and said focal track ring.

2. An X-ray tube as set forth in claim 1 wherein said intermediate ring is pyrolytic graphite.

3. An X-ray tube as set forth in claim 2 wherein said intermediate ring is comprised of several separate elements disposed in end-to-end relation throughout an annulus.

4. An X-ray tube as set forth in claim 1 wherein said wall portion is pyrolytic graphite.

5. An X-ray tube as set forth in claim 1 wherein said wall portion is boron carbide.

6. An X-ray tube as set forth in claim 1 wherein the inner surface of said wall portion is provided with an annular groove extending throughout the annular length thereof encircling the axis of the tube, and said focal track ring comprises a plurality of substantially axially extending rod-like members disposed in side-byside relation within the groove throughout its length.

'7. A target for rotating anode X-ray tubes comprising an annular body portion adapted to be rotated about its axis, and a focal track ring disposed on the inner surface of said'body portion, said ring comprising a number of elements located in side-by-side relation and adapted to be urged toward said surface by centrifugal force during rotation of the body portion about its axis.

8. A target as set forth in claim 7 wherein said elements are pinlike members extending in a direction substantially longitudinally of the target.

9. A target as set forth in claim 8 wherein said target includes means for retaining said members on said surface while permitting freedom of limited movement of the members toward said surface when urged by centrifugal force.

10. A target as set forth in claim 8 wherein said inner surface of the target is provided with an annular groove wherein said elements are located and with retaining means overlying respective opposite ends of the members.

11. A target as set forth in claim 7 wherein said elements are located in an annular groove in said inner surface and are movable toward the bottom of the groove upon subjection to centrifugal force, and a focal track ring is disposed in overlying relation to said elements.

12. A target as set forth in claim 11 wherein said elements are pyrolytic graphite.

l3. An X-ray tube comprising an envelope, an anode structure rotatably supported in the envelope, a cathode structure supported within the envelope in spaced relation to the anode structure, said anode structure including an axially extending shaft, a target supported by said shaft for rotation therewith, and a connecting portion connecting said target to the shaft, the target comprising an annular portion extending from the periphery of the connecting portion in a direction substantially coaxially of the shaft, and a focal track ring disposed on the inner surface of said wall portion, said ring comprising a number of elements located in sideby-side relation and adapted to be urged toward said surface by centrifugal force during rotation of the target.

14. An X-ray tube as set forth in claim 13 wherein said elements are pinlike members extending in a direction substantially longitudinally of the anode structure.

15. An X-ray tube as set forth in claim 14 wherein said target includes means for retaining said members on said surface while permitting freedom of limited movement of the members toward said surface when urged by centrifugal force.

16. An X-ray tube as set forth in claim 14 wherein said elements are pyrolytic graphite.

Claims

1. An X-ray tube comprising an envelope, an anode structure rotatably supported in one end of the envelope, a cathode structure supported in the opposite end of the envelope, said anode structure including an axially extending shaft, a cupshaped target carried by said shaft for rotation therewith, said target comprising a connecting portion secured to the shaft, a wall portion extending axially from the periphery of the connecting portion, and a focal track ring disposed on the inner surface of said wall portion, the cathode structure including an electron-emitting device disposed to emit an electron beam toward said focal track ring, said wall portion being of a material having heat storage capacity per unit weight greater than that of the material from which the focal track ring is formed, and an intermediate ring disposed between said wall portion and said focal track ring.

6. An X-ray tube as set forth in claim 1 wherein the inner surface of said wall portion is provided with an annular groove extending throughout the annular length thereof encircling the axis of the tube, and said focal track ring comprises a plurality of substantially axially extending rod-like members disposed in side-by-side relation within the groove throughout its length.

7. A target for rotating anode X-ray tubes comprising an annular body portion adapted to be rotated about its axis, and a focal track ring disposed on the inner surface of said body portion, said ring comprising a number of elements located in side-by-side relation and adapted to be urged toward said surface by centrifugal force during rotation of the body portion about its axis.

13. An X-ray tube comprising an envelope, an anode structure rotatably supported in the envelope, a cathode structure supported within the envelope in spaced relation to the anode structure, said anode structure including an axially extending shaft, a target supported by said shaft for rotation therewith, and a connecting portion connecting said target to the shaft, the target comprising an annular portion extending from the periphery of the connecting portion in a direction subStantially coaxially of the shaft, and a focal track ring disposed on the inner surface of said wall portion, said ring comprising a number of elements located in side-by-side relation and adapted to be urged toward said surface by centrifugal force during rotation of the target.

16. An X-ray tube as set forth in claim 14 wherein said inner surface of the target is provided with an annular groove wherein said elements are located and with retaining means overlying respective opposite ends of the elements.

17. An X-ray tube as set forth in claim 15 wherein said elements are located in an annular groove in said inner surface and are movable toward the bottom of the groove upon subjection to centrifugal force, and a focal track ring is disposed in overlying relation to said elements.

18. An X-ray tube as set forth in claim 17 wherein said elements are pyrolytic graphite.