Abstract:
A system and method are proposed for cooling the anode of an X-ray tube. A bearing shaft associated with the anode has an associated single rotating seal there around, and contains a liquid metal. A primary liquid metal flow path is used to transfer heat from the anode, and a secondary liquid metal flow path is provided to seal the single rotating seal. Accordingly, the present invention provides an effective means for containing liquid metal in the bearing shaft of an anode assembly, and using the liquid metal to cool the anode of the X-ray tube.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to X-ray tubes and particularly to a liquid metal cooled anode concept for X-ray tube application. 
     In the X-ray tube art, conventionally cooled tubes are reaching their limit in power dissipation. Future X-ray tubes will be required to operate under ever-increasing power demands and at noise levels below 55 Db. 
     Liquid metal may be capable of extracting heat from the anode portion of the X-ray tube, and also possibly allow the bearings to run more quietly. However, technical problems associated with such technology have not been effectively overcome. Furthermore, the cost of manufacture for potential designs can be prohibitive and restrictive. 
     It would be desirable, then, to be able to apply a liquid metal cooled anode concept to effectively cool X-ray tubes. It would further be desirable to apply such a liquid cooled anode concept which has the effect of increasing the power dissipation of the tube. 
     BRIEF SUMMARY OF THE INVENTION 
     Liquid cooled metal technology is used for extracting heat from the anode of an X-ray tube. 
     A system and method are provided for cooling the anode of an X-ray tube. A bearing shaft associated with the anode has an associated single rotating seal there around, and contains a liquid metal. A primary liquid metal flow path is used to transfer heat from the anode, and a secondary liquid metal flow path is provided to seal the single rotating seal. 
     Accordingly, the present invention provides an effective means for containing liquid metal in the bearing shaft of an anode assembly, and using the liquid metal to cool the anode of the X-ray tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art cross-sectional illustration of a typical X-ray tube; and 
     FIG. 2 illustrates a cross-sectional view of an X-ray tube anode assembly incorporating features of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to rotating X-ray tubes which employ a rotating anode assembly and a cathode assembly. A liquid metal cooled anode concept is proposed to effectively cool X-ray tubes. The liquid cooled anode concept has the further effect of increasing the power dissipation of the tube. This is accomplished by conducting heat directly from the target, down to the bearing shaft and into the cooling oil in the base of the shaft, the oil being on the air side of the anode. A gallium alloy allows conduction between rotating parts. The heat path from the bearing shaft across the gallium is into the conduction rod and oil via conduction and convection. 
     Referring now to the drawings, FIG. 1 illustrates a typical prior art X-ray tube  10 . The X-ray tube  10  is typically built with a rotating anode assembly  12  for the purpose of distributing the heat generated at a focal spot, and an X-ray tube cathode assembly  14  for providing a focused electron beam which is accelerated across a large anode-to-cathode vacuum gap  16  and produces X-rays upon impact with the anode. 
     Continuing with FIG. 1, an anode assembly  12  may be rotated by an induction motor typically comprising a cylindrical rotor  18  built around a cantilevered axle  20 . The cantilevered axle  20  supports a disc shaped anode target  22  connected via the hub and stud  24  to rotor  18  and cantilevered axle  20  which contains bearings facilitating rotation. The stator  26  of the induction motor includes a ferrous stator core  28  with copper windings  30  that surround the rotor  18 . The rotor  18  of the rotating anode assembly  12 , driven by the stator  26 , is at anodic potential. 
     The X-ray tube cathode assembly  14  includes a cathode cable receptacle  34  and a cathode terminal board  36  which internally connects the cathode assembly  14  to the receptacle  34 . The anode assembly  12  includes an anode cable receptacle  38  which electrically connects the anode to an anode high voltage cable (not shown). In a typical assembly, the anode assembly  12  and the cathode assembly  14  are sealed in a frame  44 , thus creating a vacuum region  47 , and are mounted in a conductive metal casing  46 . 
     Referring now to FIG. 2, an X-ray tube anode assembly provides increased power demands. Liquid metal is used to extract heat from the anode assembly of the X-ray tube, thereby effectively cooling the anode. In particular, the liquid metal, or gallium alloy, is contained in the bearing shaft  50  of the anode assembly. 
     The liquid metal cooled anode concept illustrated in FIG. 2 comprises one rotating gap seal  52  and a contact seal  54  associated with a conduction rod  56 . A channel  62  and exit ports  60  are cut through the conduction rod to facilitate the pumped flow of gallium to mitigate against leakage. A secondary gallium flow path  62  is incorporated to generate a resistance to the primary flow and help seal the rotating seal  52 . This dual gallium flow action addresses and solves for numerous of the technical difficulties associated with prior art attempts to apply liquid metal cooling to X-ray tubes. 
     Effective rotating seals can operate by using differential pressure. However, in a vacuum, such as in the X-ray tube, this cannot be easily accomplished, and sealing the gallium presents unique problems. However, advantage is taken of the rotating bearing shaft  50  to cause a differential pressure effect. This is achieved by machining a thin walled tube  64 , with helical grooves or slots  66  formed therein. This tube  64  is inserted into the hollow bore of shaft  50  to create the primary gallium flow zone  58 . Channel and exit ports are added in the stationary conduction rod  56  to create the secondary gallium flow zone  62 . The rotating seal  52  is thereby aided from leaking due to the difference in pressure across the end of the shaft caused by the channel and exit ports. The channel and exit ports supply a flow of gallium in opposition to the primary gallium flow. The primary gallium flow is created by the pumping action of the helical grooved tube  64  inserted in the hollow bore. 
     In a preferred embodiment of the present invention, liquid gallium alloy containment can be further enhanced. First, a close clearance region is established by the gap seal  52 . In this region, the radial gap between the stationary inner parts  56  and the rotating outer parts  50  is 100 micro meters or less. An anti-wet coat can extend into this region. A larger gap region  68  which is coated with a wetting agent merges with the close gap region  52 . Within the small gap, a capillary force is established at the wetted film and anti wetted film boundary. This force is can contain an inertial force of several G&#39;s acting on the liquid gallium alloy. 
     A second enhancement to contain the liquid gallium alloy is the contact seal  54 . The contact seal  54  can be employed as a secondary containment of any liquid droplets which escape the interface. This seal  54  can be formed by an annular disk of fluoro-elastomeric compound and placed just behind the rear bearing  72 . It will then run against the outer hardened surface of the tubular bearing shaft  50 . However, since the seal  54  will trap air, an air extraction feature can be added to allow for the extraction of air from behind the seal during X-ray tube processing. For example, a sintered molybdenum plug may be used, fabricated from appropriately sized molybdenum balls. This can create a porous plug through which the air can be evacuated. Preferably, the porosity is small enough that the gallium will not pass through. Although porous ceramics might be applied, ceramics are more fragile than molybdenum. 
     A third enhancement to contain the liquid gallium comprises stationary metallic shields  69  attached to the rear bearing assembly, and a metallic disk  70 . The metallic disk is a rotating wettable metallic disk. The stationary metallic shields and rotating disk can trap droplets of liquid gallium, and minimize liquid gallium contact with the rear bearing  72 . An associated gallium trap  71  can also operate to trap droplets of liquid gallium. 
     The fill level is set so that during operation, the liquid-vacuum interface is maintained in the large gap region  68 . Thus, thermal expansion of the liquid can be tolerated without large movement in the location of the interface. Under steady-state operation, the interface is stable. However, during transient operation, it is expected that liquid droplets will be detached from the interface. These droplets will be reabsorbed into the liquid pool, due to the nature and location of the wetted surfaces. All surfaces in contact with the gallium alloy can be coated with an anti-corrosion coating, if desired. Furthermore, when anode heat is transferred to shaft  50  by conduction and convection through the gallium, bearing operating temperature can be lowered. This allows for the opportunity to use noise dampening lubricants such as lead. 
     The liquid metal cooled anode concept provides the advantage of preventing gallium leakage during shipment and tube installation. Specifically, the large gap region  68  minimizes the gallium leakage during shipment of the X-ray tube. In the vertical (shipping) position, the gallium is prevented from coming into contact with the rotating seal  52  since it will have a natural tendency to flow into region  68  and be contained. 
     Gallium leakage is minimized with the application of several features. One advantageous feature is the hollow bearing shaft with the helical grooved tube inserted therein. A primary gallium flow is generated for heat extraction and a secondary gallium flow is generated to oppose the primary flow. This creates a differential pressure to allow the rotating seal to work effectively in a vacuum. A reservoir holds the gallium in the vertical position away from the rotating seal, minimizing gallium leakage. Finally, the seal is outer rotating, which contributes to a decrease in gallium leakage. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.