Patent Publication Number: US-9905390-B2

Title: Cooling mechanism for high-brightness X-ray tube using phase change heat exchange

Description:
FIELD OF THE INVENTION 
     The present invention relates to high brightness x-ray sources. In particular, it relates to a cooling mechanism for high brightness x-ray sources. 
     BACKGROUND OF THE INVENTION 
     Conventional x-ray sources generate the x-ray by using electron beam to excite an anode to generate x-ray emissions. Almost all of the power of the electron beam (e.g. 99%) is converted to heat in the process. A specific power density of 1 W/μm 2  and a total power of 100 W are typical specifications for anodes of state of the art stationary micro-focus x-ray tubes. In micro-focus x-ray tubes, the area of the anode hit by the electron beam (the focal spot) is very small, on the order of tens of microns, to achieve a small source size for high-resolution x-ray imaging. The amount of heat generated can be spread to a volume of about 1 mm 3  of the anode by metal thermal conduction mechanism without melting the center of the anode. However, the blackbody radiation rate alone on the surface of this small volume is not enough to radiate out this power to the outside radiation absorber cooled by water or air. Heat conduction to a larger area of radiation has to go through a long metal thermal conduction pass, which cannot transfer the amount of heat without causing significant temperature rise which can melt the spot hit by the electron beam. A rotating anode allows the heat to be distributed on a much larger area to avoid melting the anode. A specific power density of 2×10 −2  W/μm 2  and a total power of 10 kW are typical specifications for a state of the art rotating anode. For the same reason, the power density cannot be further increased for desired higher x-ray brilliance. Most conventional devices apply liquid convection methods (including liquid metal and water) to cool the anode. However, liquid convection heat exchange coefficient is not high enough to transfer the amount of heat without causing significant temperature rise that can melt the spot hit by the electron beam. 
     SUMMARY OF THE INVENTION 
     In embodiments of the present invention, a phase change heat exchange mechanism is used to provide heat transfer to match the heat impedance between the small surface of heated metallic anode and the large blackbody radiation or convection heat transfer surfaces. As a consequence, these designs allow the brightness of x-ray source to increase dramatically and at the same time increase the x-ray tube lifetime significantly. 
     In some embodiment, jet boiling evaporation or thin film evaporation phase change thermal exchange methods are used as a thermal transfer mechanism to match thermal impedance of a small e-beam heated area of metallic anode and large area of radiation cooling or convection cooling surfaces without any solid or liquid connections. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides an x-ray generator which includes: a cathode for emitting an electron beam; an anode; alignment and focusing units for focusing and directing the electron beam onto the anode; a sealed x-ray tube enclosing the cathode, the anode and the alignment and focusing units; a sealed heat exchange chamber joined to the x-ray tube, wherein the anode either forms a section of a wall of the heat exchange chamber or is in thermal contact with a section of a wall of the heat exchange chamber; a metal as a liquid to vapor phase change material disposed inside the heat exchange chamber; and a delivery mechanism for delivering a liquid of the metal onto the section of the wall of the heat change chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates an x-ray generator system with a cooling mechanism according to an embodiment of the present invention. 
         FIG. 2  schematically illustrates an x-ray generator system with a cooling mechanism according to another embodiment of the present invention. 
         FIG. 3  schematically illustrates the anode used in the first or second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention provide a mechanism for cooling the anode of an x-ray tube using a phase change material to transfer heat away from a back side of the anode. Since heat exchange flux can reach above 10 7  W/m 2  in jet boiling evaporation methods using water or certain liquid metals and in thin film evaporation methods using liquid metals, these phase change heat exchange methods can be used as a thermal transfer mechanism to match thermal impedance of a small e-beam heated area of metallic anode and a large area of radiation-cooled or convection-cooled surfaces without any solid or liquid connections. 
       FIG. 1  schematically illustrates an x-ray source according to a first embodiment of the present invention, where a phase change heat exchange method is used to cool the anode in the x-ray source. The x-ray source may be a micro-focus x-ray tube. The cathode  101  emits an electron beam  102 , which is aligned by the alignment magnet unit  103 , and further focused by the electromagnet unit (objective lens)  104  onto a small area of a stationary anode  105 . As the electron beam hits the anode  105 , the anode emits an x-ray  106 A which exit the x-ray window  106  of the x-ray tube. All of the above components are enclosed in a vacuum tube (enclosure)  107 . The cathode  101  and the anode  105  are connected to appropriate electrical voltages (not shown in the drawing). 
     The spot of the anode hit by the electron beam and nearby areas of the anode will be heated up to a very high temperature (e.g. 1000 C or higher) and can dissipate the heat by radiation. The radiation energy can exit the vacuum tube through radiation transparent enclosure  107 . This energy can be dissipated by an outside radiation absorbing unit (not shown), which can further be cooled by convection methods. 
     In this embodiment, to provide enhanced cooling, the vacuum tube  107  is joined to a phase transition heat exchange chamber  109 , where the anode  105  is mounted on a common wall between the vacuum tube and the heat exchange chamber so that the back side of the anode is exposed to the interior of the heat exchange chamber. The heat flux from the back side of the anode  105 , i.e. the side facing away from the cathode, is transferred to a much larger surface of the walls of the heat exchange chamber  109  by a phase change mechanism. To accomplish this, a jet sprayer  108  located inside the heat exchange chamber  109  ejects a liquid jet  110 A onto the back side of the heated spot of the anode  105 , and the liquid evaporates on that surface to carry away the heat. The vapor then condenses back into the liquid form on the cool inside surfaces of the phase transition heat exchange chamber  109 . The condensation falls to the bottom of the chamber  109  as indicated by the arrows along the side walls, and the accumulated liquid  110  is cycled by a pump  111  to the jet sprayer  108 . The pump  111  and the related piping can be disposed inside or outside of the heat exchange chamber  109 . 
     The liquid is a liquid to vapor phase change material (L-V PCM) chosen for heat exchange suitable for high temperature applications. Suitable materials include metals such as sodium (Na), potassium (K), tin (Sn), etc., and their alloys. The enclosure  109  should be kept sealed without any other liquid or air except for the L-V PCM inside. 
     Sprayers for spraying liquid metal are known; any suitable sprayer can be used for this embodiment. Using a sprayer can ensure that a desired amount of liquid metal is delivered to the hot surface. In the example of  FIG. 1 , the anode is oriented such that its back surface is disposed horizontally at the top of the heat exchange chamber, and the sprayer is located just below the back surface of the anode. In another example, the anode may be oriented such that its back surface is vertical or near vertical. In yet another example, the back surface of the anode is located near the bottom of the heat exchange chamber, a reservoir is provided to contain the liquid PCM, and the liquid is pumped to a sprayer located above the anode. 
     Further, beside jet sprayers, other delivery mechanisms can also be used to deliver the phase change material to the anode for evaporation. For example, a falling film method may be used to form a thin film of liquid metal on the back side of the anode when it is oriented vertically or near vertically. 
     The enclosure  109  of the chamber may be cooled from the outside by convection methods not shown in the drawing, such as forced air cooling, etc. 
     The structure of the anode  105  in one implementation is shown in more detail in  FIG. 3 . The anode  105  is a piece of metal which forms a part of the common wall  105 A between the x-ray tube enclosure and the heat exchange chamber enclosure. The anode may be thinner in an area  105 A near where the electron beam hits than in other portions of the wall, in order to enhance heat transfer from the front side of the anode to the back side. In this implementation, the anode itself forms a part of the enclosure of the heat exchange chamber. Alternatively, as shown in  FIG. 3A , the anode  105  may be mounted on a metal plate  105 A that forms a part of the enclosure, and the liquid PCM is sprayed onto the back side of the plate. Heat is transferred from the anode  105  to the backside of the plate  105 B where the liquid metal is spray onto. A variation of the structure of  FIG. 3A  is that the anode  105  is mounted in an indentation of the plate  105 B. 
       FIG. 2  schematically illustrates an x-ray source according to a second embodiment of the present invention. This system is similar to the first embodiment shown in  FIG. 1 , except for an additional system of heat exchange tubes enclosed in the enclosure  209 . Like components are labeled with like numbers: The cathode  201 , electron beam  202 , alignment magnet unit  203 , electromagnet unit (objective lens)  204 , anode  205 , x-ray  206 A, vacuum tube (enclosure)  207 , jet sprayer  208 , heat exchange chamber  209 , L-V PCM  210 , PCM liquid jet  210 A, and pump  211  perform the same functions as the corresponding components in the embodiment of  FIG. 1 . 
     The heat exchange tubes  212  are provided with fluid inlet and outlet  213  and  214 , and a cooling fluid such as water circulates in the tubes. The surfaces of the tubes provide additional cool surfaces to condense the vapor of the L-V PCM inside the chamber  209 , and the heat is carried away by the cooling fluid. 
     To summarize, because the anode of an x-ray tube becomes very hot during operation, metal can be used as the liquid to vapor phase change material to transfer the heat from the anode to a larger cool surface. A sprayer may be used to spray the liquid metal onto the back side of the anode where it evaporates. This system can effectively remove heat form the small area of the back of the anode. 
     It will be apparent to those skilled in the art that various modification and variations can be made in the x-ray generator structure and related method of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents.