US2108573A

US2108573A - X-ray tube

Info

Publication number: US2108573A
Application number: US745335A
Authority: US
Inventors: Alfter Josef
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1933-09-30
Filing date: 1934-09-24
Publication date: 1938-02-15
Anticipated expiration: 1955-02-15
Also published as: NL40878C; GB424822A; FR778963A

Description

R E T F L A i X-RAY TUBE Filed Sept. 24, 1934 2 Sheets-Sheet l /fwfn/ TOR JOSLF LFTER Fe. 15, 138. J. ALFTER 2,193,573

' X-RAY TUBE y Filedv sept. 24, 1934 2 Sheets-Sheet 2 Patented Feb. 15, 1938 Fries X-RAY TUBE Josef Alfter, Eindhoven, Netherlands, assigner to N. V. Philips Gloeilampenfabrieken, Eindhoven,

Netherlands Application September 24, 1934, Serial No. 745,335 In Germany September 30, 1933 13 Claims.

The present invention relates to novel means to increase the power output of an X-ray tube Without increasing the dimensions ofthe tube or reducing the sharpness of theV X-ray picture. More particularly my invention relates to X-'ray tubes in which the cathode-ray beamy successively strikes changing portions of the anode target.

I shall describe my invention. in connection with so-called rotating anode X-ray tubes. However, it should be Well understood that it also applies to X-ray tubes in which the cathode-ray beam is caused by other means Ato strike successively changing portions `of the target. For instance, the anode may be given other than rotary movement, or the-anode and other members of the tube including the envelope, may be moved with regard to a stationary cathode, or again, both the anode andthe cathode may be both rotating and the cathode-ray beam itself is moved electrically or magnetically relatively to the electrodes to successively strike different portions of the target.

The discharge current passing through the X-ray tube, i. e., the load of the tube, is usually limited by the maximum permissible temperature of the target, i. e., the temperature which the target material can withstand without injury. By exposing successive contiguous portions of the target to the cathode-ray beam, as is the case in rotating anode tubes, a given load causes a smaller increase in temperature of the target surface than it does in stationary anode tubes having the same focal area, or in other words, a higher load can be obtained With a rotating anode tube for a maximum, permissible target temperature than in a stationary anode tube, this being a known advantage of the rotating anode tubes.

In prior rotary anode tubes the density of the cathode-ray beam Where it strikes the target, i. e., in the focal spot, Was substantially uniform and thus the target was subjected to a uniform specific load (load per unit target surface area). In other words, the specic current or current density of the focal spot Was the same throughout the area of the focal spot. With such a uniform specific load of the focal area, the maximum speciiic load to which the target'can be subjected and thus the total load of the target, is limited by the requirement that the temperature, which target portion assumes at the pointy/'here it leaves thn cathode-ray beam, falls below the maximum permissible target temperature. Such uniform loading of the target in the focal area, however, does not fully utilize the inherent load capacity of the tube, for reasons stated hereafter.

As a given target portion enters-the cathode- (Cl. Z50-35) ray beam, it is practically cold, whereas during its passage through the cathode ray beam, its temperature increases, until it assumes its maX- imum temperature when leaving'the cathode ray beam, this maximum temperature being normally close to the maximum permissible target temperature.

A novel method of loading a stationary target of an X-ray tube to attain a higher load capacity of the tube during a given exposure time, has been disclosed in the copending application of A. Bouwers and W. I-I. Boldingh Serial No. 653,538, led January 25, 1933, now Patent No. 2,054,493, dated September l5, 1936. According to the invention of said application, at the beginning of the exposure, the tube is given a higher load, this load being decreased `during the exposure in such a manner that, during the major portion of the exposure, the target assumes a temperature closely approximating the maximum permissible temperature.

In my present invention a somewhat similar principle is applied to rotating anode tubes, as Well as in general to X-ray tubes in which the cathode-ray beam successively strikes changing portions of the target. Whereas in said prior application the total load of the target and of the tube varies as the exposure` proceeds and the specific load over the focal spot is uniform for any given instant, in my invention the total load of the tube and of the focal spot remains constant during the Whole exposure, Whereas the specific load over the focal spot is not uniform, but different for different portions of the focal spot.

According to my invention, I provide a cathoderay beam of uneven density and of such character that, when a given target portion enters the cathode-ray beam (the region of the focal spot) or in other Words When such a target portion assumes a position in which it begins to form part of the focal spot, the density of the cathode-ray beam is higher than when this target portion leaves this region, and linearly or otherwise decreases Within this region.

For the `sake of simplicity I shall refer hereafter in both the speciiication and the claims to the entrance and exit side or edge of the various members.

For instance by entrance side of the cathode, I shall refer to that side of the cathode at which a given target portion enters into the cathoderay beam, and by exit side of the cathode I shall refer to that side of the cathode at which a given target portion leaves the cathode-ray beam. The same applies to terms such as entrance and exit edge of a target portion or of the focal spot, as used hereafter such terms broadly applying also to such tubes which are not of the movable anode type, but fall within the above defined scope of the invention.

Thus the specific load of a target portion is a maximum at its entrance, and a minimum at its exit, and thus the current density over the focal spot is nonuniform, and decreases towards the exit side, for instance in the case of rotating anode tubes in the direction of the rotation of the anode. By this arrangement a target portion, while it forms part of the focal spot, assumes its end temperature more rapidly than has been the case in tubes using the same specific load throughout the whole width of the focal spot, without this end temperature exceeding the maximum permissible target temperature. The higher density of the cathode-ray beam, or higher electron concentration, in the initial load period of a given target portion results in an increase of the total load of the tube and in a correspondingly greater discharge current; at the same time the total load and the discharge current remain constant during the whole exposure.

My invention makes it possible to increase initially the specic load at the entrance edge of the focal spot to twice or more the value of the specific load at the exit edge of the focal spot, the latter specic load being determined by the maximum permissible temperature of the target. The result is that the total load or output of the tube can be considerably increased for instance by about 30% and more, over that of tubes having the same dimensions but having a uniform specific load over the whole focal area, without any danger of injury to the target.

Theoretically, the specific load which can be applied to the entering edge of a target portion could be made infinite, but practical considerations limit it to a value which, however, as stated, can still be several times as large as the specific load at the exit edge of this portion, whereas the latter specific load corresponds to the maximum secic load permissible in present-day tubes.

To obtain the desired non-uniform cathode-ray distribution various constructional embodiments may be used. For instance, the cathode filament may be given a higher temperature on the entrance side than on the exit side, or the reflecting properties of the focusing device may be made greater at the entrance than at the exit side.

Again, the area of the electron-emitting surface of the cathode may be made greater cn the entrance side than on the exit side or an asymmetrical meshed screen, acting as a grid-which may be given an auxiliary potential, and which is placed between the cathode and the anode-may cause a greater density cathode-ray beam to strike the target at the entrance edge than at the exit edge.

In order that the invention. may be clearly understood and more readily carried into eifect, several embodiments thereof will be described more fully with reference to the accompanying drawings. The invention is, however, not limited to the shown examples, as various further embodiments may suggest themselves to those skilled in the art without departing from the spirit of my invention.

Figure 1 is a schematic view of a rotating anode X-ray tube to which my invention can be applied.

Fig. 2 is a cross-section of the electrodes of the X-ray tube of Fig. 1 taken along the lines II-II,

showing schematically an embodiment of my invention in which a higher cathode emitting temperature is obtained at the entrance side than at the exit side.

Fig. 3 is a cross-section of the electrodes, showing schematically an embodiment in which unequal temperature distribution of the cathode is obtained, by making the lament thinner at the entrance side than. at the exit side.

Figs. 4 and 5 are top Views of cathode filament constructions giving higher cathode temperatures at the entrance side than at the exit side.

Fig. 6 is a cross-section view of the electrodes, showing schematically an embodiment of my invention in Which a meshed grid is used to decrease the density of the cathode-ray beam at the exit side.

Fig. 7 is a schematic top view of a cathode lament having a greater emitting surface at the entrance side than at the exit side.

Fig. 8 is a top view of a fllamentary cathode formed of two helices of different diameter and providing a larger electron-emitting surface on the entrance side than on the exit side.

Fig. 9 is a cross section of the electrodes when ipivsin a cathode lament structure as shown in Fig. 10 is a cross section through the electrodes showing two coiled filament wires one of which serves primarily to heat the entrance side of the other.

Fig. 11 is a diagram illustrating graphically the advantages obtained with my invention.

Referring to Figure 1, there is shown schematically a rotating anode X-ray tube on hand of which my invention will be explained. Various constructions for such a tube may be used, for instance that of U. S. Patent #1,893,759 to Albert Bouwers. The tube comprises a sealed envelope 20 in which is rotatably mounted an anode 2l driven, for instance electromagnetically by a magnetic stator 22 provided outside of the tube envelope.

While as a rule in rotating anode tubes, the anode is given a conical target surface, for the sake of simplicity a cylindrical anode 2| having a flat circular target surface I will be assumed. The cathode 23 comprises a focusing device 3 and a lament 2 which is disposed eccentrically with respect to the anode to oppose one side of the target surface.

Referring now to Figure 2, which shows schematically the electrodes in cross section taken along the line II-II of Fig. 1, I represents a portion of the usually ground target surface of the anode 2l and is made as a rule of a refractory metal such as tungsten. The anode in this, as Well as in all of the other figures, is assumed to move toward the right as indicated by the arrow. The focusing device 3, is provided with a recess 25 into which is placed the coiled filament 2. The focal spot is formed on that portion of the target surface I which, at any given instant, opposes, the cathode and is subjected to the cathode-ray beam indicated by dotted lines 50.

To obtain a large specific load at the entrance edge of the focal spot and a decreasing specic load thereof towards the exit edge, the recess 25 of the focusing device is made asymmetrical with regard to the filament 2. Thereby a larger amount of electrons, or a higher density cathode-ray beam is directed toward the entrance edge of the focal spot than towards its exit edge. This effect can be increased by providing the wall ofthe recess at the entrance side with a highly thermally reflecting surface of metal` as shown at 4, whereas at the exit side the surface of the recess wall is not provided with .such a substance or may be given a special, poorly refleeting surface. Such difference in the reflecting capacity of the entrance and exit side of the recess 25 may altogether suflice to obtain the desired changing characteristic of the specific load along the width of the focal spot, even with a symmetrical recess as shown in the drawings by dotted line' 5l.

'In Fig. 3 the cavityV of the focusing device 6 is symmetrical, and the desired asymmetry is ob- Y tained, by reducing at the entrance side, the cross section of the elliptically wound filament 5, this being .-achieved, for instance, by progressively etchingthe filament towards the entrance side. 'Ihevtapered' thickness of the filament 5 toward the entrance side, results in a higher temperature and greater electron emission at the entrance side, and a gradual decrease of same towards the exit side.

The same effect can be obtained by other asymmetrical arrangements attained either by the asymmetry ofthe filament or of cooperating members. I l

, Forinstance in Fig. 4 there is shown an electron-,emitting cathode constructed as a harp, the cords 21, 28, 29 and 30 `of which decrease in diameter from left toward the right. The` two ends of the individual cords are clamped by ccnductive members l and 8 respectively, which members are connected to the current leads 3|v and 32 respectively. As the cords 21 to 30 are in parallel connection, as long as they are of identical material, the density of the current passing through the individual cords is the same for all of the cords. However, as the heavier cords have a? smaller heat-radiating surface per unit of volume than have the smaller cords, they assume a higher temperature. Thus the electron emission of the individual cords will decrease from the left toward the right. /In Fig. 5 an arrangement somewhat similar to that of Fig. 4 is used. However, instead of connecting the cords in parallel, they are connected in series,` or a single wire having a thickness which increases in steps from the left to the right is used. The clamps 9 and I0 are in thiscase of insulating material and the conductors 33 and V34 are connected to the two ends of the series combination. As a common current passes through the series combination, the current density of the thin portions will be larger than that of the thick portions, withV the result that a higher electron emission will take place on the left of the cathode than on itsright side.

` Referring to Fig. 6, both the incandescent filament II and the focusing device 35 surrounding same, are symmetrically arranged with regard to the focal spot the asymmetry being obtained by providing between the cathode I I and the target surface I a grid I2, shown schematically and consisting, for instance, of meshed wire having decreasing openings toward the exit side.

The grid. I2 may be connected to the cathode or may be insulated therefrom, in the latter case preferably having a small negative potential with respect to the cathode to increase its blocking action.

The above construction again results in a higher electron emission and loading of the target at its entrance than at its exit.

By proper proportioning and spacing of the various elements used in the aforedescribed embodiments it is possible to achieve the above set forth purpose of the invention, and to obtain a target temperature, which approaches the maximum permissible target temperature during the major part ofthe passage of a target portion across the cathode-ray beam.

The integral of the excess specific loads above that which exists at the exit edge of the focal spot, represents the gain in load obtainable with my invention. This can be Vmore clearly explained with reference to the diagram shown in Fig. 11.

The portion of the diagram falling above the abscissa axis N-W gives the specific load ofthe focal spot along the width of the focalspot; whereas the portion below the abscissa axis gives the temperatures of the focal spot along the width of the focal spot;

For a uniform-specific load alongthe width of the focal spot, which corresponds tothe known practice, the specific load at any point of the focal spot will have the value represented by OA.

The temperature of the focal spot increases ,v towards its exit edge according Vto the curve t1 from a low entrance Value-assumed for the sake of simplicity as being zero--to the exit value CE (orVOD), the latter value being equal or slightly below the maximum permissible target temperature.

As a target portion passes the exit edge of the focal spot no further heating thereof takes place and the temperature of this target portion after it leaves the focal spot region, drops in accordance with the curve EF.

It thus appears that in the present day rotat-` able anode tubes, in which the specific load is the same over the entire focal spot area, as shown by line b1, the total load 'of the focal spot and thus the total load of the tube can be represented by the rectangle OABC. Y

If Vthe temperature of a target portion upon entering the focal spot region could be increased instantaneously substantially to the maximum permissible target temperature and maintained at this temperature throughout, its passage through the focal spot region, thus following the line t2 of Fig. il, a load indicated by the line bz would result. l Thus a very large increase of the total load of the target and thus of the tube could be obtained.

However, such ideal condition, because of the Vinertia of the various parts and of other factors,

Vembodiments of the invention discussed heretofore and hereafter, whereby the temperature of a target portion passing through the cathode-ray beam, i.'e., through the focal spot region, is represented by a curve falling between t1 and t2, for instance by the curve t3 shown in the drawings. The specific load curve of the focal spot area, corresponding to the temperature curve tais b3. The hatched area falling between the curve bc and the line In, represents the actualgain obtained in the total load of the tube.l

In an arrangement as shown in Fig. 'l the change of the specific load from the left to the right is practically linear, thus would make, in Fig. li,-the line b3 practically a straight line. The cathode shown in Fig. 7 comprises a flat filament being looped back and forth and having largeprimary loopsi and small secondary loops i3, the secondary yloops I3 being provided only at the left side of the filament (assuming again the anode to move toward the right).

Due to the provision of the secondary loops I3 on the left side of the lament the density of the discharge current on the left side is considerably greater than on its right and decreases substantially linearly towards the right.

Instead of using a single wire filament, the emitting surface of which is increased on one side by additional loops, the filament may consist of two or more helically-Wound wires, with one or more helices of smaller diameter located eccentrically with regard to the main helix of larger diameter. Such an arrangement is illustrated in Fig. 8 in which the filament consists f two helices I4 and I5 having the same pitch and a common tangent I6 on the left side of the filament. The helix I4 has turns of smaller diameter and is preferably of thinner wire than is the helix I5, and its axis falls within helix I5. Thus the individual turns of the two helices, are alternately interposed without their touching each other. Again, on the left side of the filament, the electron emission is greater than on the right and the decrease of electron emission toward the right is gradual, and has a substantial linear character.

Fig. 9 shows in cross section the anode and cathode with a cathode filament of the type shown in Fig. 8.

It is also possible to bring the diiierent portions of a symmetrically-formed cathode filament to unequal electron-emitting temperatures by providing a heater which heats the entrance side of the cathode filament. For instance, as shown in Fig. 10, the recess 4I! of the focusing device IEI, besides containing the cathode filament Ill, also contains a heater filament I9 placed to the left and slightly below the cathode filament I3. The filament I9 may serve only to increase the temperature of the filament I8 on its left side and thereby increase its electron emission on this side, or if desired the filament I9 may also contribute to the electron emission of the cathode. However, the filament is screened from the anode at 43, so that only such cathode beams may pass from the filament I9, which fall within the cathode ray beam of filament I8, thus a widening of the focal spot is prevented.

While I have described my invention on hand of specific embodiments and in a specific application, I do not wish to be limited thereto and various modifications of my invention are possible.

For instance, while I have described my invention in connection with a tube having an anode rotated by exterior means within a stationary envelope and in opposition to a stationary cathode, I wish it to be understood that my invention also applies to any other tube in which the cathode-ray beam successively impinges upon changing portions of the anode, and in the claims when referring to a rotating or movable anode such cases are also to be included. Also further constructional embodiments may suggest themselves to those skilled in the art.

Therefore, I desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is:-

1. In an X-ray tube, an anode structure having a target portion adapted to be rotated during the operation of the tube, a cathode structure having an electron-emitting filament extending substantially normally to the direction of movement of said target portion, said filament having a higher electron emission at one side than at the other side as taken in the direction of movement of the target portion, and means to continuously rotate said target portion in the direction in which the temperature of the iilament decreases.

2. In an X-ray tube, an anode structure having a target portion adapted to be rotated during the operation of the tube, means to rotate said target portion, a line focus cathode structure having an electron-emitting filament extending substantially normally to the direction of movement of said target portion and comprising a plurality of members arranged side by side as taken in the direction of movement of said target portion in the vicinity of the filament, each successive member as taken from side to side of the filament having a lower resistance than the preceding member.

3. An X-ray tube comprising an envelope, an anode structure having a target portion adapted tol be rotated during the operation of the tube, a line focus cathode structure having an electron-emitting iilament extending substantially normally to the direction of rotation of said target portion and comprising a plurality of members of the same material arranged side by side, each successive member as taken from side to side of the filament having a smaller cross-sectional area than the preceding member, and means to continuously rotate said target portion in a direction corresponding to the decrease in temperature of the lament.

4. An X-ray tube comprising an envelope, an anode structure having a target portion adapted to be rotated during the operation of the tube, a line focus cathode structure having an electron-emitting element extending substantially normally to the direction of movement of Said target portion and comprising a plurality of series-connected members of the same material arranged side by side, each successive member as taken from side to side of the filament having a smaller cross-sectional area than the preceding member, and means to continuously rotate said target portion in a direction corresponding to the increase in cross-section of the members.

5. An X-ray tube comprising an envelope, an anode structure having a target portion adapted to be rotated during the operation of the tube, means to continuously rotate said target portion, a line focus cathode structure having means including an electron-emitting element to direct a cathode-ray beam towards said target portion, and means including a grid member interposed between said element and said target portion for causing the number of electrons moving toward the target portion to decrease in the direction of the movement of the target portion.

6. An X-ray tube having an envelope, an anode structure having a target portion adapted to be moved during the operation of the tube, a cathode structure comprising means including an electron-emitting element to direct the cathoderay beam toward said target portion, and means cooperating with said target portion to continuously move same, the electron-emitting area of said element decreasing in the direction of movement of the target.

'7. An X-ray tube comprising an envelope, an anode structure having a target portion adapted to be moved during the operation of the tube, a cathode structure having an electron-emitting, looped filament and means cooperating therei ing disposed at one side of the electron-emitting lament to produceV a cathode-ray beam Whose density decreases from one side to the oppositeV side, and means to continuously rotate said target portion in such a direction that the target portion passes through the cathode-ray beam in the direction of decreasing density, each ele- `l1nent of the target during its passage through the beam being at substantially the maximum permissible temperature of the target material.

8. An X-ray tube comprising an envelope, an anode structure having a target portion adapted to be rotated during the operation of the tube, a line focus cathode structure comprising an electron-emitting member and means cooperating therewith to direct a cathode ray beam toward said target portion, said member comprising an outer wire helix and an inner Wire helix having a smaller diameter than that of the outer helix, said inner helix being eccentrically disposed Within said outer helix, and means cooperating with said target portion to continuously rotate same in such a direction that the target `portion approaches the electron-emitting member from the side to which the inner helix is eccentrically disposed.

9. In an X-ray tube, an anode structure having a target, means including anelectron-emitting member for producing and for projecting upon said target a cathode-ray beam Whose electron density in the vicinity of vthe target decreases from one side of the beam to the opposite side, and means for continuouslychanging the relative positions of the target and the beam, the relative motion, with respect to the cathoderay beam, ofthe target portion Within the beam being in the direction of decreasing electron density.

10. In an X-ray tube, an anode structure having a target, means including an electron-emitting member for producing on said target a focal spot Whose specic load decreases from side to side, and means for continuously changing the relative positions of the target and focal spot, the relative movement, With respect to the focal spot, of the target portion at the focal spot being substantially in the direction of decreasing specific load.

Y 11. In an X-ray tube, an anode structure having a target, means including an electron-emitting member for producing upon said target a focal spot Whose specific load at one edge is more than double the specific load at the opposite edge, and means for continuously changing the relative positions of the target and the focal spot, the relative movement, with respect to the focal spot, of the target portion at the focal spot being from the highly loaded edge toward the opposite edge.

12. In an X-ray tube,- a cathode structure having an electron-emitting element, an .anode structure having a target spaced Vfrom said element to expose a surface area of theA target to the electron emission of said element, and means to continually change the relative positions of said target and element, the electron-emissivity of said element `decreasing in a direction corresponding to the relative motion, with respect to said element, of the exposed portion of the target.

13. In an X-ray tube, a cathode structure having an electron-emitting element, an anode structure having a target spaced from said element to expose a surface area of the target to the electron emission of said element, and means Vto continuallyV change the relative positions of said target and element, the temperature of said element during operation decreasing in a direction corresponding to the relative motion, with respect to said element, of the exposed portion of the target.

JOSEF ALFTER.