US 3885179 A
An X-ray tube wherein the glow cathode coils are located outside of the direction of the vertical projection of the focal point upon the cathode body. The invention is particularly characterized in that the striking location of the vertical projection of the focal point upon the cathode body is made for the absorption of ions.
Description (OCR text may contain errors)
United States Patent 1 Friedel X-RAY TUBE  Inventor: Rudolf Friedel, Erlangen, Gemiany  Assignee: Siemens Aktiengesellschaft, Munich,
Gennany 22 Filed: Feb. 19, 1974 21 App|.No.:443,864
 Foreign Application Priority Data Feb. 28, l973 Germany 23l006l  US. Cl 313/60; 313/55  Int. Cl. H01] 35/06  Field of Search 313/60, 55, 57
 References Cited UNITED STATES PATENTS 2,691,869 3/l954 Atlee 3l3/60 [451 May 20, 1975 Primary Examiner-Rudolph V. Rolinec Assistant ExaminerDarwin R. Hostetter Attorney, Agent, or Firm-Richards & Geier  ABSTRACT An X-ray tube wherein the glow cathode coils are located outside of the direction of the vertical projection of the focal point upon the cathode body. The invention is particularly characterized in that the striking location of the vertical projection of the focal point upon the cathode body is made for the absorption of ions.
5 Claims, 7 Drawing Figures X-RAY TUBE This invention pertains to an X-ray tube wherein the glow cathode coils are located outside of the vertical projection of the focal point upon the cathode body.
The load capacity of a rotary anode in short time space is determined by the greatest permissible focal point temperature, which is limited by reaching the melting point of the anode material. However, in actual experience the maximum focal point temperature must be considerably lower to provide a stable tube operation and sufficient life duration. In addition to a few problems soluble by constructive means, such as, for example, the steaming of glass container with the resulting insufficient high tension firmness, this has substantially the following two reasons:
1. Independently from the melting point steam pressure between anode and cathode must not be so high that gas discharging takes place. A measure for this is the free passage length of electrons.
2. Anodes can be dirtied" during manufacture, further treatment and also during operation in the tube, for example, by coatings on the outer surface which have a higher steam pressure than the anode material.
Although the free passage lengths of electrons at usual maximum temperatures of the focal point of about 2,500 C or greater for tungsten anodes are substantially larger than the electrode spacing, tube instabilities can arise. Examinations have shown that responsible for that is a rear action upon the cathode. In the electronic current of the cathode atoms are ionized, for example, metal steam out of the focal point or nonremovable gases. The positive ions are then accelerated upon the heating coil of the cathode where they cause a rise in current by heating of the coil surface, secondary electron formation and reduction of space charge in front of the coil. This leads to increased steam formation, ionization, current increase, etc.
The damage thereby produced appears typically as periodic meltings of the focal point path which are produced in the stated manner in combination with the generator (inner resistance, capacities). This problem becomes substantially more important for tubes with high output since the inner resistance of the generator must be held small due to the necessity select as low as possible the empty run voltage for technical and expense reasons, and even eliminate the inner resistance in wide operational ranges.
In a known device, high load X-ray tubes have arrangements whereby electrons have the form of a hollow ray. Furthermore, a special striking surface for ions is provided in the center of the cathode for absorbing the ionic strike penetrating into the ray, instead of allowing it to strike the part emitting the electrons. However, this is a complicated and expensive solution, since it is necessary to produce a hollow electronic ray which moreover cannot be used in medical X-ray technology due to representation quality. Furthermore, the cathode must also have a special construction with a striking section for ions enclosed by the electron source or the tubular electronic path. In addition, in this construction the cathode body is located only partially in the vertical projection of the ring-shaped focal point, it also goes past it. Parts of the cathode emitting electrons are also struck by the projection, so that the abovementioned damaging effects can still take place, at least partly.
On the other hand it is known in X-ray tubes to guide the electronic ray bundle emerging from the glow cathode screw in a curved path upon a specific point of the anode and there possibly to hold it, for example, even during a rotary movement of the entire tube. Then the sscrew is located outside of the vertical projection of the focal point. However, no means are provided which would cause the ions to strike the cathode body and their absorption. The ions run without being controlled and strike, for example, the wall of the container or parts of electrodes, which is a drawback, since this may produce metal coatings which diminish the high voltage firmness of the tube.
An object of the present invention is to provide constructive means which would eliminate the abovementioned detrimental cathode back effect, which would provide greater operational safety of X-ray tubes used in medical field and which would increase their load capacity.
In the accomplishment of the objectives of the present invention it was found desirable to make the striking location of the vertical projection of the focal point upon the cathode so that it will absorb ions.
Due to this arrangement the ions strike a location of the cathode body which is located to the side of the glow coil and on which they can produce no damage. Thus is avoided an influencing of the sensitive coil, namely, an incontrolled increase of electronic emission (FIG. 2).
In a simple embodiment of the present invention the striking location of the ions is a plate which melts with difficulty and is well heat conducting, such as a plate of tungsten, having a thickness of 0.5 to 5 mm (FIGS. 3, 4, 5 and 7). The thickness of the plate is of less importance since it is only important for stability and heat capacity. It can consist of tungsten, molybdenum or tantalum. An improvement can be provided if the plate is placed into the cathode body (FIG. 4). The recess should have at least approximately the shape of the focal point. Due to this arrangement despite the high melting point and small gas pressure of tungsten, etc., the tungsten gas produced by the striking ions can not get into the range of the electronic ray or to locations which would affect the high voltage firmness of the tube.
The cathode head can be also held entirely out of the projection line of the focal point (FIG. 6). However, then it is necessary to use a complicated cathode system, for example, an electronic gun which has magnetic and electric lenses causing a sharply bundled electronic flat ray to drop upon the anode at an inclination, and to provide a separate ion receiving system.
Electrons produced by a heat coil or some other electronic emitter are deviated by an electric field to bent half-circular paths so that the outgoing direction and the striking direction of the electronic ray upon the anode differ from each other by more than 0 and a maximum of This provides that the heating coil is even more effectively protected from ionic striking.
The same effect is produced by deviating the ions by means of a permanent or electric magnet (FIG. 5). If the striking plate consists of tantalum, zirconium, titanium, etc., when the plate is heating during ionic striking in addition a getter effect is produced which is advantageous for the operation of the tube. The space in which the plate is located is enclosed by metallic walls, so that various effects, for example, the coating of walls with metal, are limited to this space and cause no dam age.
The invention will appear more clearly from the following detailed description when taken in connection with the accompanying drawing showing by way of example only, preferred embodiments of the inventive idea.
IN THE DRAWINGS FIG. 1 is a side view of an X-ray tube.
FIG. 2 is a diagrammatic section extending transversely to FIG. 1 through the cathode and anode of the tube shown therein.
FIG. 3 is a diagrammatic section showing a different cathode arrangement wherein a plate of heavy metal is provided upon the striking location of the ions.
FIG. 4 is a diagrammatic section showing a cathode arrangement wherein a plate of heavy metal is lowered into the body of the cathode.
FIG. 5 is a diagrammatic section illustrating the deviation of an ionic ray by means of a magnetic field.
FIG. 6 is a diagrammatic section showing the shooting of electrons by an electronic gun in an inclined direction upon the anode.
FIG. 7 is a diagrammatic section through a cathode arrangement wherein the electronic ray is deviated upon a curved path by 90.
FIG. 1 shows a vacuum casing l of the tube which contains the cathode device 2 and the anode device 3. The cathode device 2 located at one end of the casing includes the outer casing 4, the mounting, namely, the actual cathode body 5, and the glow coils 6 and 7 which are actuated by lines 8, 9 and 10. Electrons leaving the coils 6 and 7 strike the rotary anode 11 which is connected by the axle 12 with the rotor 13.
According to the present invention a side shifting of the coils and such a shaping of the focus milling is provided, that there is a deviation of the electronic ray 14 upon the focal point 15 of the anode 11, as shown in FIG. 2. The path 16 of ions shown by broken lines strikes the cathode body 5 at the location 17 outside of the coil 6.
According to FIG. 3 upon this location of the cathode can be placed a plate 18 consisting of tungsten which opposes a good resisting force to the striking ions.
In the construction shown in FIG. 4 the plate 19 consisting of molybdenum is arranged in a depression 20 so there is substantial space separation from the heating coil and the range of the electronic ray.
The arrangement shown in FIG. 5 corresponds to a substantial extent to that of FIG. 4. In it at the rear end of the opening 20 there is a magnetic field 21 which is used to guide the ionic ray 22 upon the plate 23 consisting of tantalum which is arranged in the striking location. The plate 23, as is also the case in the construction of FIG. 4, is located in a cover 23, shown by broken lines in FIG. 5 constituting a hollow space enclosed by metal walls, in which will be deposited the material given up by the plate 23.
The construction of FIG. 6 differs from the earlier described devices in that it does not require a special deviating device for the electronic ray 24, since this ray emerges with great acceleration from the electrode gun 25 and strikes the anode 26 in an inclined direction, so that particles emerging from the anode or ions released in space will not strike the cathode system. The striking location of ions indicated by 25 is constructed in the same manner as in embodiments shown in other figures and is connected to or separately from the gun 25 in a manner which is not illustrated.
In the construction shown in FIG. 7 the shape of the cathode body 27 produces an electric field which devi ates the electronic ray 28 from the glow coil 29 by about to the anode 30. Ions leaving he focal point of the anode 29 pass through the hole 31 and stirke the plate 32 made of tungsten which is located in a separate hollow space 33 in a manner similar to that shown in FIGS. 4 and S. The side walls of the space 33 constitute a tubular support 34 for the plate 32.
1. An X-ray tube comprising an anode body. a cathode body and at least one incandescent cathode coil located on said cathode body to project a cathode ray beam forming a focal spot on said anode body and located out of the area onto which said focal spot is projected in rectangular direction from its surface and which area is shaped to absorb ions, said area being fu rnished with a heat-conducting body of refractory material.
2. An X-ray tube according to claim 1, wherein said material is selected from the group consisting of tungsten, molybdenum and tantalum.
3. An X-ray tube according to claim 1, wherein said area is located in a hole provided in the cathode body.
4. An X-ray tube according to claim 3, wherein the bottom of said hole is provided with a heat Conducting refractory body.
5. An X-ray tube according to claim 3, wherein the hole penetrates the cathode body totally and said area is located on a heat conducting body of refractory material suppoted on the rear of the cathode body.