X-ray tube comprising a liquid-cooled anode

Abstract

The cooling side of the anode target plate of an X-ray tube is provided with a surface-increasing cooling structure. An injection device for a cooling liquid is mounted against the cooling structure such that the cooling liquid is forced to flow through ducts present in the cooling structure. At least the material of the surface of the cooling structure is preferably silver.

Description

The invention relates to an X-ray tube provided with an anode comprising an anode target plate which comprises, arranged opposite to each other, a target for an electron beam to be directed thereon and a cooling surface for giving off heat to a flowing cooling medium.

In known X-ray tubes of this kind it was found, that in spite of the cooling, the anode target plate becomes hot such that it is damaged and the service life of the tube is reduced. One of the causes thereof is known to be the presence for some time of gas bubbles on the cooling surface due to an insufficiently turbulent liquid flow. On the basis thereof, various improvements have been proposed which aim to increase the turbulence in the liquid flow.

For example, U.S. Pat. No. 2,886,723 describes an X-ray tube in which a longitudinally injected liquid flow is forced to a higher degree of turbulence by projections on the boundary walls of the flow duct. Netherlands Pat. No. 77,920 describes an X-ray tube in which, for the same reasons, a rotating disc is arranged in a transverse injected liquid flow near the cooling surface. Netherlands Pat. No. 74,278 describes an X-ray tube incorporating a transverse multiduct injection device for the cooling medium. In each of these known embodiments improved heat transfer between the cooling medium and the cooling surface is indeed realized. As a result, the anode target plate becomes less hot and the service life is prolonged.

Particularly in the case of X-ray tubes having a comparatively small target spot area, i.e., the cross-section of the electron beam at the area of the target, however, the anode target plate is still comparatively quickly damaged. It was found that this damage consists mainly in the local roughing of the target in and near the target spot. In addition to a reduced service life of the tube, this also causes a continuous reduction of the radiation efficiency of the tube.

The invention has for its object to provide an X-ray tube in which the target plate is substantially less readily damaged, also in the case of comparatively high local loading. To this end, an X-ray tube of the kind set forth according to the invention is characterized in that the anode target plate is comparatively thin, measured between the target and the cooling surface, and is provided with surface-increasing recesses at the area of the cooling surface, the flow path for the cooling medium being limited at least mainly to these recesses at these areas.

As a result of the provision of the surface-increasing structure in the anode target plate, an X-ray tube is obtained having a substantially smaller reduction in radiation efficiency and a longer service life. A contribution in this respect is made by the improved thermal contact between the anode target plate and the cooling medium, the larger cooling surface, the higher flow rate of the cooling medium at the area of the cooling surface as well as by the shorter heat-leakage path. As a result of the improved heat transfer near the cooling surface, the anode target plate may be thinner, so that it becomes less hot again. Because the anode target plate becomes less hot at the area of the target, the temperature gradients occurring at this area cause less roughening of the surface. In a preferred embodiment according to the invention, the recesses consist of adjoining isosceles pyramids which extend approximately halfway the thickness of the anode target plate. A flat boundary of an injection tube for the cooling medium which is directed towards the cooling surface is mounted against the peaks of the remaining raised portions, which are again isosceles pyramids.

Because the anode target plate also becomes less hot on the cooling surface side in an X-ray tube according to the invention, less corrosion occurs also at this side. Any corrosion occurring at this area, however, is additionally detrimental because it attacks the cooling structure. If such corrosion occurs, the improved cooling will be lost after some time and the thin anode target plate will quickly become completely unusable. So as to prevent this, in a preferred embodiment according to the invention the cooling side of the anode target plate is provided with a corrosion-resistant material. To this end in a closed cooling system use can alternatively be made of a liquid having a comparatively slight corrosion effect on the material of the cooling surface.

A few preferred embodiments according to the invention will be described in detail hereinafter with reference to the drawing.

FIG. 1 is a diagrammatic representation of a preferred embodiment of an X-ray tube according to the invention.

FIG. 2 is a diagrammatic representation of a part of the X-ray tube shown in FIG. 1 which comprises the anode target,

FIG. 3 is a diagrammatic representation of a preferred embodiment of an anode construction according to the invention.

The X-ray tube shown in FIG. 1 comprises an envelope, consisting of a glass portion 1 and a metal portion 2 which are vacuumtight connected to each other by means of the connection ring 3. The metal portion 2 comprises windows such as 4 and 5 which are vacuumtight contained in support rings 6 and 7. The portion 2 furthermore comprises a cap 8, an anode 9 with an anode target plate 10 forming part thereof. Mounted in the anode 9 is a cooling sleeve 11 with an inlet line 12 and an outlet line 13. Provided in the cooling sleeve 11 is an opening 14 for directing a cooling liquid transverse to the anode target plate 10. A sealing plate 38 is provided with a guide sleeve 39 which projects into the cooling space. The cooling sleeve 11 is preferably mounted 15 cm into the anode sleeve 9 with insertion of one or more O-rings.

Arranged opposite to the anode target plate 10 is a cathode body 16 in which an electron source is mounted, in this case a filament 17. The glass envelope 1 comprises a passage element 18 with passages 19 for connections 20 of current or voltage sources not shown.

Provided in the anode target plate 10 is a cooling structure 21 which is shown at an increased scale in FIG. 2. In addition to the anode 9, the cooling sleeve 11 and the filament 17, FIG. 2 shows an electron beam 22 and an X-ray beam 23. In this preferred embodiment the cooling structure 21 consists of isosceles pyramids 24 which are impressed in the anode target plate. The raised portions 25 also constitute isosceles pyramids. The target plate including the pyramids, has a thickness of, for example, 2 mm and the pyramids have a depth of 1 mm. An end face 26 of the cooling sleeve 11 engages the peaks of the raised portions. In this preferred embodiment, the pressure of the cooling liquid ensures that this engagement is maintained during operation. Instead of this self-adjusting construction, the cooling sleeve can alternatively be mounted against the anode target plate under spring pressure, or can form one assembly therewith. In the latter case the cooling structure consists of a duct system which is arranged between a cooling part and a target plate part. The duct system should permit lateral passage of and be in open communication with an inlet opening for the cooling medium. A cooling liquid which is pressed through the opening 14 is thus forced to flow between the raised portions. As a result, proper thermal contact between the cooling liquid and the target plate is ensured. If isosceles pyramids are used in the cooling structure, the cooling area of the cooling surface is increased exactly by a factor 2, the transverse dimension of the target being the same. In a first approximation, this results in a twice as large heat transfer to the cooling liquid. Because the cooling liquid is forced through the more or less zigzag-extending ducts between the pyramids, any gas bubbles appearing therebetween will be quickly taken along. As a result of the higher flow rate due to the narrow passage opening, the cooling liquid will become less hot and the heat transfer will be increased. The raised portions, and hence also the recesses, of the cooling structure can alternatively have a different shape, for example, the shape of half spheres, cubes, cylinders, cones etc. However, the structure should always permit lateral passage as otherwise the cooling sleeve cannot be mounted against the structure and the flow of cooling liquid will then be restricted mainly to the space to be left for this purpose. In a further preferred embodiment, the cooling structure consists of a system of preferably zigzag-extending ducts which are provided in the anode target plate, for example, by etching.

As was already stated, a minor corrosion of the cooling surface quickly has serious consequences in comparison with known X-ray tubes. On the one hand, the cooling medium must flow through narrow so readily clogging ducts, while on the other hand the peaks of the raised portions can disappear after some time due to corrosion, with the result that the liquid passage can then concentrate on a resultant free passage opening. In both cases the proper, uniform cooling is lost. So as to prevent these phenomena, a preferred embodiment of an X-ray tube according to the invention incorporates a known closed cooling system. In this system the heat taken up from the anode is given off in a heat exchanger. The choice of the cooling medium in such a system is free to a high degree. For example, a binary mixture can be used, such as water with alcohol, one component of which is subjected to an alternating phase transition during the cooling process.

In a further preferred embodiment according to the invention, the attack of the cooling structure is reduced by a suitable choice of the materials of the anode target plate at the area of the cooling surface. In addition to copper, silver is suitable material for this purpose in view of its favourable heat-conductivity and high corrosion-resistance. The cooling structure can be provided with a silver layer, for example by vapour-deposition or in a galvanic manner.

In a further preferred embodiment according to the invention, the anode target plate comprises, as is shown in FIG. 2, a comparatively thin target disc 30 and a cooling disc 31 which also serves as a support for the target disc. The cooling disc is made, for example, of silver or copper, whilst the target disc is made of one of the metals known to be used for this purpose, for example, copper molybdenum, tungsten, cobalt and the like. The target disc can be provided on the cooling disc by diffusion, but any other method is also feasible, provided that the necessary proper thermal contact between the two discs is realized.

The mutual orientation of a cooling disc 33, a cooling sleeve opening 34 and a line-like target spot 35 of a further preferred embodiment are shown in FIG. 3. Line-focus tubes of this kind are frequently used for diffraction examinations. The line-like target spot or the line focus has a width of, for example, 0.4 mm and a length of 8 mm. The cooling disc is now mounted such that the line focus encloses an angle of approximately 45.degree. with straight lines 36 along which the pyramids are arranged. The cooling sleeve opening 34 is arranged directly opposite to the line focus, with the result that the cooling medium is injected against the line focus on the cooling side. In a preferred embodiment, the cooling disc is provided with areas 37 in which no cooling structure is present. These smooth areas are arranged in the longitudinal direction of the line focus, but are situated at least a few times the width of the line focus outside the line focus. As a result of the smooth areas 37, the flow direction of the cooling medium is forced more transverse to the longitudinal direction of the line focus.

An X-ray tube according to the invention is furthermore particularly suitable for use in an X-ray fluorescopy apparatus which is equipped with a so-termed end-window tube. In such tubes the target plate is arranged at a small distance from an end face of the envelope. So as to prevent damage by dispersed electrons, the anode is positive with respect to the surroundings. Consequently, the anode target plate must be cooled with de-ionized water. This would cause additionally fast corrosion of the cooling surface. In these tubes usually no space is available for a complex cooling system at this area. The use of an X-ray tube comprising an anode target plate provided with a cooling structure according to the invention offers a favourable solution in such a case.

Claims

What is claimed is:

1. In an X-ray tube, an anode comprising:

a wall;

an anode target plate of heat conductive material having on one side thereof a target area for an electron beam and having the opposite side thereof facing said wall with an array of heat conductive projections substantially increasing the heat radiation surface thereof and extending from said plate to said wall effectively forming an interconnected system of ducts around said projections; and

means for directing cooling medium between said wall and plate forcing said medium turbulentyly through said system of ducts around said heat conductive projections, thereby cooling said target area.

2. An anode as defined in claim 1 wherein said array of projections is a regular pattern of adjoining pyramids.

3. An anode as defined in claim 1 wherein said means for directing cooling medium directs said cooling medium through said wall toward said anode target plate.

4. An anode as defined in claim 3 wherein said means for directing cooling medium includes an aperture in said wall.

5. An anode as defined in claim 4 wherein said aperture corresponds in shape and position to the target spot.

6. An anode as defined in claim 4 wherein said aperture is elongate and said wall and plate are joined along opposing edges thereof to force said cooling medium substantially in opposite directions lateral with respect to said elongate aperture, said projections being positioned in a staggered array with respect to said opposite directions to cause said cooling medium to flow in a zig zag fashion around said staggered projections.