FIELD OF THE INVENTION 
       [0001]    The present invention relates to an X-ray tube for generating X-radiation and a method for manufacturing an X-ray tube, and an X-ray system for diagnostic use comprising an X-ray tube and in particular to a method for manufacturing an X-ray system, which comprises an X-ray tube. 
       BACKGROUND OF THE INVENTION 
       [0002]    A rotating anode X-ray tube generates X-radiation in a diagnostic system, wherein the anode of the X-ray tube heats up upon operation and cools during exposure and afterwards. 
         [0003]    The thermal heat flow and thermal cycling causes thermo mechanical distortion of the tube components. Therefore, the tube components have to be designed such that reliable operation is guaranteed under all specified conditions. 
         [0004]    Many modern high performance X-ray tubes use hydrodynamic bearings to support the rotating anode and to dissipate the heat from the anode by direct conduction cooling towards an external cooling fluid. The loading capacity of these hydrodynamic bearings is a strong function of the gap size between the active surfaces of the rotating and stationary bearing members. The gap size is typically in the range of only 5 to 20 um, while the range of bearing diameters is typically 2 to 10 cm, its length 5 cm to 15 cm. So the gap is of relatively small size. Given a certain speed of rotation, large gaps as well as low viscosity of the bearing fluid (hot liquid metal) both cut down the loading capacity (bearing stiffness). 
       SUMMARY OF THE INVENTION 
       [0005]    Therefore, it would be desirable to provide an improved device and method for stabilising the gap of the bearing. These needs may be met by the subject matter according to one of the independent claims. Advantageous embodiments of the present invention are described in the dependent claims. 
         [0006]    According to the invention the size of the bearing gap is stabilized against thermo mechanical distortion using controlled matching expansion of the bearing members. This can be achieved by using at least some parts of the members materials of different thermal expansion coefficients c th . (E.g. the material of the bearing member which is at lower temperature during operation is selected to have a higher c th  compared to the material of the member at higher temperature). Another solution is to use mechanical piston-like force generation e.g. by hydraulic means. The advantages are e.g. a reduction of friction losses in cold state and a prevention of rotation instability in hot state. 
         [0007]    According to a first aspect of the invention an X-ray tube for generating X-radiation is proposed. The X-ray tube for generating X-radiation comprises a rotary structure, which comprises a rotating anode, a stationary structure for rotatably supporting the rotary structure, a bearing, which is arranged between the rotary structure and the stationary structure, wherein the bearing comprises a gap between the rotary structure and the stationary structure, means for stabilising the dimensions of the gap with respect to distortions because of thermo-mechanical causes. 
         [0008]    The X-ray tube and the anode will be heated during operation by electron beam, which impinges on the target to generate X-ray. Therefore, a circulating cooling fluid system is arranged to compensate and to stabilise the temperature of the tube. There are regions of different temperature within the tube. Different temperatures lead to different expansion of the gap of bearing between the stationary part of the tube and the rotary part of the tube. In case the key gap dimensions vary locally (especially in case of different sizes of the cross-section) problems may arise during operation of the X-ray tube. Therefore, the tube according to the invention has means for compensating the above mentioned effect, which results in approximately constant key dimensions of the gap of bearing. 
         [0009]    According to a second aspect of the invention it is provided a method for manufacturing the tube according to one of the claims  1  to  12 , wherein means for stabilising the dimensions of the gap are arranged. 
         [0010]    According to a third aspect of the invention it is proposed an X-ray system for diagnostic use comprising the tube according to one of the claims  1  to  12 , wherein the X-ray system is adapted to stabilise the dimensions of the gap. 
         [0011]    According to a fourth aspect of the invention it is proposed a method for manufacturing the X-ray system according to claim  14 , wherein means for stabilising the dimensions of the gap are arranged in such a way that the X-ray system is adapted to stabilise the dimensions of the gap. 
         [0012]    Further embodiments are incorporated in the dependent claims. 
         [0013]    According to the present invention it is provided an X-ray tube, wherein the tube comprises a wall as a mechanical limitation for the gap, wherein the means for stabilising comprise an inlay, which is inserted in the wall, wherein the inlay has a different thermal expansion coefficient with respect to at least a part of the wall. 
         [0014]    There are regions of different temperature because of the arrangement of a heat source (the anode) and a heat sink (the circulating cooling fluid). Therefore, the expansion of the material can also be different. This could result in a deformation of the gap. In order to avoid this effect it is proposed to arrange material, which expands little, at sites, which are hot and to arrange material, which expands in a higher degree, at sites, which are relatively cold. This can be done by inserting inlays into the tube. 
         [0015]    According to an exemplary embodiment it is provided a tube, wherein the inlay is arranged adjacent to the gap. This is advantageously because in this case the effect of the inlays on the gap can be enhanced. 
         [0016]    According to another exemplary embodiment it is provided a tube, wherein the inlay has a large thermal expansion coefficient, wherein the inlay is arranged in a relatively cold surrounding. 
         [0017]    According to an exemplary embodiment it is provided a tube, wherein the inlay has a small thermal expansion coefficient, wherein the inlay is arranged in a relatively hot surrounding. 
         [0018]    According to another exemplary embodiment it is provided a tube, wherein the inlay comprises a sandwich structure of different materials, wherein materials with a close thermal expansion coefficient compared to the thermal expansion coefficient of the wall will be arranged adjacent to the wall, wherein materials with a thermal expansion coefficient, which is substantially different compared to the thermal expansion coefficient of the wall will be arranged far away to the wall. 
         [0019]    According to an exemplary embodiment it is provided a tube, wherein the inlay is adapted to stabilise the dimensions of the gap because of an appropriate shape. The inlay could have a shape which is adapted to the gap. In this case the shape of the inlay improves the stabilising character of the inlay in order to stabilise the dimensions of the gap. 
         [0020]    According to a further exemplary embodiment it is provided a tube, wherein the wall is adapted to be deformed by means for deforming for stabilising the dimensions of the gap. The stationary part of the X-ray tube comprises a bearing axis. 
         [0021]    This axis has to be hollow in order to contain the circulating cooling fluid system. In case the walls of the bearing axis are thin enough it is possible to deform these walls in order to compensate deformations of the bearing gap. 
         [0022]    According to an exemplary embodiment it is provided a tube, wherein the means for deforming comprise a lever for applying a mechanical force on the wall. 
         [0023]    According to another exemplary embodiment it is provided a tube, wherein the means for deforming comprise means for applying fluid pressure on the wall. 
         [0024]    According to a further exemplary embodiment it is provided a tube, wherein the wall has a thickness of about 1 to 20 mm. 
         [0025]    According to an exemplary embodiment it is provided a tube, wherein the means for stabilising comprise a channel for directing the flow of heat, wherein the channel is arranged in such a way that the deformation of the gap is uniform. 
         [0026]    It should be noted that the above features may also be combined. The combination of the above features may also lead to synergetic effects, even if not explicitly described in detail. 
         [0027]    These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Exemplary embodiments of the present invention will be described in the following with reference to the following drawings. 
           [0029]      FIG. 1 . shows an X-ray tube in a diagnostic X-ray system, 
           [0030]      FIG. 2 . shows an X-ray tube, 
           [0031]      FIG. 3 . shows a cross-sectional view of an X-ray tube, 
           [0032]      FIG. 4 . shows a cross-sectional view of an X-ray tube, 
           [0033]      FIG. 5 . shows a cross-sectional view of an X-ray tube, 
           [0034]      FIG. 6 . shows a cross-sectional view of an X-ray tube with deformed bearing gaps, 
           [0035]      FIG. 7 . shows a cross-sectional view of an X-ray tube with deformed bearing gaps, 
           [0036]      FIG. 8 . shows a cross-sectional view of an X-ray tube with inlays, 
           [0037]      FIG. 9 . shows a cross-sectional view of an X-ray tube with inlays, 
           [0038]      FIG. 10 . shows a cross-sectional view of an X-ray tube with a piston-type mechanical expansion device, 
           [0039]      FIG. 11 . shows a cross-sectional view of an X-ray tube comprising a device for hydraulic expansion of the bearing axis, 
           [0040]      FIG. 12 . shows a cross-sectional view of an X-ray tube comprising channels for heat conduction. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0041]      FIG. 1  depicts a typical X-ray tube  102 , wherein the rotating anode X-ray tube  102  generates X-radiation  103  in a diagnostic X-ray system. During the X-radiation is generated by the rotating anode X-ray tube  102  the anode of the X-ray tube  102  heats up upon operation and cools down afterwards. These thermal cycling causes thermo-mechanical distortion of the X-ray tube components. Therefore, the tube components have to be designed such that reliable operation is guaranteed under all specified conditions. It is also shown a more detailed illustration of the tube  101 . 
         [0042]      FIG. 2  depicts a bearing gap  201 , which is filled with liquid metal, a hollow bearing axis  202 , which is fixed to support the X-ray tube, a rotating bearing sleeve  204 , a channel for the circulating cooling fluid  203 , and a rotating anode  205 . 
         [0043]      FIG. 3  depicts a cross-sectional view of an X-ray tube. It is shown the rotating anode  305 , the rotating bearing sleeve  303 , the radial bearing  307 , the axial bearing  306  and the circulating cooling fluid  304 . Further, it is depicted the hollow bearing axis  302 , which is fixed to the tube support. 
         [0044]      FIG. 4  depicts an X-ray tube with a circulating cooling fluid  405 , the bearing gap  401  and the anode  404 . It is shown that there is the mechanical force of the gravity  406 , which could result in deformation of the mechanical arrangement of the X-ray tube. There is also depicted a part of the rotary part  403  of the tube and a part of the stationary part  402  of the tube, wherein the stationary part of the tube  402  comprises the hollow bearing axis. 
         [0045]      FIG. 5  depicts the result of thermo-mechanical deformation because of a hot anode  504 , wherein there is a heat flux  506 ,  508 . This heat flux  506 ,  508  leads through the bearing gaps  507  and  509 . The heat results in large expansion of the rotating bearing member because of high temperature at the sites  510 ,  509 . Further, the heat leads to small expansion of the stationary bearing axis because of moderate temperatures at the sites  508 ,  511 . The different dimension of expansion at the sites  507 ,  509  and  508 ,  511  leads to the consequence of increased gap sizes, which results in reduced loading capacity of the bearing at the sites  507 ,  509 . 
         [0046]    Summarizing the above mentioned it can be stated that the heating of the anode  504  causes thermal gradients inside the hydrodynamic bearing. Unequal expansion of its members may cause a significant distortion of the gap size and negatively affect bearing stability and loading capacity. Low viscosity of the heated bearing fluid adds negatively to this. Usually, the bearing members are of the same material. By design, they may be shaped such, that the bearing is stable under all thermal conditions. But usually, this results in an unusable loading capacity and excessive friction losses at cold state. 
         [0047]      FIG. 6  depicts stabilised gaps  601 , wherein the size is kept approximately constant. This can be achieved by choosing material with a large coefficient of thermal expansion at the sites  611 ,  610  and by arranging material with a small coefficient of thermal expansion at the sites  605 ,  611 . The varying of the coefficient of thermal expansion compensates the different temperatures. Therefore, the effect of stabilising the gap of the bearing  607 ,  609  is achieved.  FIG. 6  shows the heat flux  606 ,  612 , which starts from the hot anode  604  and runs through the rotary part  603  of the X-ray tube to the stationary part  602  of the X-ray tube. The tube will be cooled by the circulating liquid fluid  608 . 
         [0048]      FIG. 7  depicts an embodiment of the invention, wherein the stabilising of the gaps is achieved by implementing inlays  707 ,  708  at the sites where the heat flux  706 ,  709  is passing through. The inlays  707 ,  708  are arranged in the neighbourhood of the border between the rotary part  703  of the X-ray tube and the stationary part  702  of the X-ray tube in such a way, that the dimensions of the gap  701  will be stabilised efficiently. The X-ray tube will be cooled by the circulating cooling fluid  705  in order to compensate the heating because of the anode  704 . 
         [0049]    The inlays  707 ,  708  in the bearing members  702 ,  703  can be used for compensation. Upon heating, they expand differently from the bulk and maintain the gap size. There could be different embodiments with the help of the inlays, e.g. using inlays with a large (compared to the bulk material) c th  on the cold side, using inlays with a small c th  on the hot side. Further, both embodiments can be combined. 
         [0050]    For optimal shaping of the gap  701 , the form of the inlays  707 ,  708  can be matched with the local heat flux pattern. With the help of this principle radial and axial bearings can be stabilized. A further option could be for chemical stability against the bearing fluid, to cover the inlays  707 ,  708  e.g. with the bulk material. 
         [0051]      FIG. 8  depicts the X-ray tube, wherein the heat flux  806 ,  809 , which starts from the anode  804  passes through the rotary part of the tube  803 , the gap  801  and the stationary part of the tube  802 . The compensation of the unequal expansion of the gap  801 , because of the cold side because of the circulating cooling fluid  805  and the hot anode  804  will be achieved by arranging inlays  807 ,  808 . One embodiment can be to use a sandwich structure of the inlays  807 ,  808  in order to match bulk and inlay material. 
         [0052]    The effect of using the compensation inlays  807 ,  808 , which consist of sandwich structures of different materials and forms, is to avoid cracking caused by residual intrinsic stress from the manufacturing process (e.g. brazing or Plasma Vapor Deposition). The different materials may be ordered by their thermal expansion coefficient and/or their mutual adhesion. Those having characteristics close to the bulk bearing material may be located closest to the latter. 
         [0053]      FIG. 9  depicts the heat flux  906 ,  909 , which starts from the heat source, the anode  904 , and leads to the heat sink, the circulating cooling fluid  905 . The heat flux is passing through the rotary part  903  of the tube, the gap  901  to the stationary part  902  of the tube. According to this embodiment there are inlays  907 ,  908 , which are formed for maximal bearing stability such that the shapes of the active bearing surfaces and gap  901  are optimally formed upon heating. 
         [0054]    Therefore, the compensation inlays  907 ,  908  may be formed such that upon heating the bearing gap  901  is formed locally in a desired way. When hot, the gap  901  may get a minimal size in those areas where the bearing is loaded most. E.g. to handle gyroscopic forces, this is needed at the outer edges of the set of radial bearings. 
         [0055]      FIG. 10  depicts the arrangement of the tube with the anode  1004 , the rotary part  1003  of the tube, the stationary part  1002  of the tube, the gap  1001  between the rotary part  1003  and the stationary part  1002 . There is also shown the heat flux  1006 ,  1010 . Within the stationary part  1002  of the tube there is arranged a piston-type mechanical expansion device, wherein there is expansion upon pressing. The levers  1007 ,  1008  are controlled by the thermal expansion device  1005  with the help of the piston  1009 . 
         [0056]    According to this embodiment of the invention the inner hollow axis  1002  may be expanded also mechanically. The actuated piston  1009  pushes levers  1007 ,  1008 , which push out the inner surface of the hollow axis  1002 . The force on the piston  1009  may be generated through a device  1005  which expands upon rising temperature. (material with large c th ). This may serve as an automatic expansion control. The piston  1009  may also be driven by hydrodynamic pressure of the cooling fluid, e.g. using an aperture. The aperture would be attached to the piston  1009 . The amount of oil flow controls the pressure drop across the aperture and with it the force on the piston  1009 . According to the invention mechanical and thermal compensation may also be combined. 
         [0057]      FIG. 11  depicts the arrangement of the tube with the hot anode  1104 , the rotary part  1103  of the tube, the stationary part  1102  of the tube and the gap  1101 . It is also shown the heat flux  1106 ,  1108 . In order to stabilize the dimensions of the gap  1101  the hollow axis  1102  is filled with a fluid with the pressure P fluid . This pressure P fluid  is achieved by using a hydraulic pump  1107 , which supplies the fluid through the channel  1105  to the hollow axis  1102 . 
         [0058]    A static fluid pressure P fluid  can be applied to the bearing axis  1102 . When the inner wall of the axis  1102  is thin enough (ca. 1 mm), this pressure P fluid  can drive the expansion of the inner axis  1102 . The local thickness of the wall is chosen such, that the local expansion optimally matches the thermal expansion of the outer rotating bearing member. Usually the inner surface of the bearing axis  1102  is cooled with a circulating fluid, driven by fluid pump  1107 . The heat is then dissipated to the ambient by an external heat exchanger. The static pressure P fluid  can also be applied in such a case. The whole fluid circuit is then put under this static pressure P fluid  in addition to the dynamic pressure generated by the driving pump  1107 . Usually the fluid will be fluent (water, oil), but the invention comprises also other forms of fluids (air under pressure). 
         [0059]      FIG. 12  depicts an embodiment of the invention, wherein the heat flux  1206 ,  1212  is directed from the anode  1204  to the rotary part  1203  of the tube, wherein the heat flux  1206 ,  1212  can be divided in e.g. two parts  1211 ,  1207 , which get through the gap  1201  and arrive at the stationary part  1202  of the tube. This leads to the effect that the heat is no more focused on single spots. It is also shown radial bearings  1209 ,  1208  within the cooling channel. 
         [0060]    This embodiment leads to the effect that the heat conduction will be channeled through the anode  1204  in such a way that there is only uniform bearing gap deformation. The pattern is achieved through shaping of the parts and/or selection of materials. Shaft cooling is done in such a way to prevent non-uniform gap deformation, i.e. the gap  1201  may be distorted, but symmetrically in the radial bearings  1209 ,  1208 , such that both radial bearings  1209 ,  1208  still have the same stiffness. 
         [0061]    It should be noted that the term ‘comprising’ does not exclude other elements or steps and the ‘a’ or ‘an’ does not exclude a plurality. Also elements described in association with the different embodiments may be combined. 
         [0062]    It should be noted that the reference signs in the claims shall not be construed as limiting the scope of the claims. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           101  X-ray tube, 
           102  X-ray tube, 
           103  X-radiation, 
           201  bearing gap, 
           202  hollow bearing axis, 
           203  circulating cooling fluid, 
           204  rotating bearing sleeve, 
           205  rotating anode, 
           301  gap, 
           302  hollow bearing axis, 
           303  rotating bearing sleeve, 
           304  circulating cooling fluid, 
           305  rotating anode, 
           306  axial bearing 
           307  radial bearing, 
           401  gap, 
           402  rotary part, 
           403  stationary part, 
           404  anode, 
           405  circulating cooling fluid, 
           406  gravity, 
           501  gap, 
           502  rotary part, 
           503  stationary part, 
           504  anode, 
           505  circulating cooling fluid, 
           506  heat flux, 
           507  bearing gap, 
           508  heat flux, 
           509  site, 
           510  site, 
           511  site, 
           601  gap, 
           602  stationary part, 
           603  rotary part, 
           604  anode, 
           605  site, 
           606  heat flux, 
           607  bearing gap, 
           608  circulating cooling fluid, 
           609  bearing gap, 
           610  site, 
           611  site, 
           612  heat flux, 
           701  gap, 
           702  stationary part, 
           703  rotary part, 
           704  anode, 
           705  circulating cooling fluid, 
           706  heat flux, 
           707  inlay 
           708  inlay, 
           709  heat flux, 
           801  gap, 
           802  stationary part, 
           803  rotary part, 
           804  anode, 
           805  circulating cooling fluid, 
           806  heat flux, 
           807  inlay, 
           808  inlay, 
           809  heat flux, 
           901  gap, 
           902  stationary part, 
           903  rotary part, 
           904  anode, 
           905  circulating cooling fluid, 
           906  heat flux, 
           907  inlay, 
           908  inlay, 
           909  heat flux, 
           1001  gap, 
           1002  stationary part, 
           1003  rotary part, 
           1004  anode, 
           1005  thermal expansion device, 
           1006  heat flux, 
           1007  lever, 
           1008  lever, 
           1009  piston, 
           1010  heat flux, 
           1101  gap, 
           1102  stationary part, 
           1103  rotary part, 
           1104  anode, 
           1105  channel, 
           1106  heat flux, 
           1107  hydraulic pump, 
           1201  gap, 
           1202  stationary part, 
           1203  rotary part, 
           1204  anode, 
           1205  cooling channel, 
           1206  heat flux, 
           1207  heat flux, 
           1208  radial bearing, 
           1209  radial bearing, 
           1210  heat flux, 
           1211  heat flux, 
           1212  heat flux.
The invention describes an X-ray tube for generating X-radiation, wherein the tube comprises a rotary structure, which comprises a rotating anode, a stationary structure for rotatably supporting the rotary structure, a hydrodynamic bearing, which is arranged between the rotary structure and the stationary structure, wherein the bearing comprises a gap between the rotary structure and the stationary structure, means for stabilising the dimensions of the gap with respect to distortions because of thermo-mechanical causes. A further aspect, which is described, is a method for manufacturing the tube according to the invention, wherein means for stabilising the dimensions of the gap are arranged. It is also described an X-ray system for diagnostic use comprising the tube according to the invention, wherein the X-ray system is adapted to stabilise the dimensions of the gap. Another aspect of the specification is a method for manufacturing the X-ray system.