Patent Publication Number: US-8121259-B2

Title: Thermal energy storage and transfer assembly and method of making same

Description:
BACKGROUND OF THE INVENTION 
     Embodiments of the invention relate generally to a thermal management system, and more particularly, to a thermal energy storage and transfer assembly for gathering and dispersing radiant thermal energy and kinetic energy of electrons, such as within an electron beam generating device. 
     Electron beam generating devices, such as x-ray tubes and electron beam welders, operate in a high temperature environment. Typically, an x-ray beam generating device or x-ray tube comprises opposed electrodes, a cathode and an anode, enclosed within a cylindrical vacuum vessel. A hot cathode filament emits thermal electrons that are accelerated across a typical voltage difference of 20 kV to 200 kV and impact the target zone of the anode at high velocity. The primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. 
     However, less than 1% of the primary electron beam energy is converted into x-rays. The balance of the beam energy is contained in back scattered electrons or converted to heat. This thermal energy from the hot target is radiated to other components within the vacuum vessel of the x-ray tube. Additionally, some of the electrons back scatter from the target and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result of the high temperatures caused by this thermal energy, the x-ray tube components are subject to high thermal stresses. 
     Since the production of x-rays in a medical diagnostic x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2700° C., while the temperature in the other parts of the anode may range up to about 1800° C. 
     The excessive temperatures that build up within the x-ray tube can decrease the life of the transmissive window, as well as other x-ray tube components. Due to its close proximity to the focal spot, the x-ray transmissive window is subject to very high heat loads resulting from thermal radiation and back scattered electrons. The high heat loads cause very large and cyclic stresses in the transmissive window and can lead to premature failure of the window and its hermetic seals. 
     Some methods to address thermal loads in x-ray tubes rely on quickly dissipating thermal energy by using a circulating, coolant fluid within structures contained in the vacuum vessel. Other methods have been proposed to electromagnetically deflect back scattered electrons so that they do not impinge on the x-ray window. These approaches, however, often do not adequately minimize thermal stress on the transmissive window. 
     Therefore, it would be desirable to design an thermal energy management and transfer assembly that thermally and mechanically isolates the transmissive window in order to minimize thermal stress on the transmissive window. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with one aspect of the invention, an apparatus includes an electron collector includes a body having an internal bore formed therethrough along a first direction and a window side having an aperture formed in a first portion thereof along a second direction different from the first direction. The apparatus also includes a cover plate having a bottom surface coupled to a second portion of the first surface of the electron collector, and an x-ray transmission window coupled to the cover plate and aligned with the aperture along the second direction, wherein a recess is formed along the second direction in one of the first portion of the first surface of the electron collector and a portion of the bottom surface of the cover plate, and wherein a gap is formed between the bottom surface of the cover plate and the first surface of the electron collector. 
     In accordance with another aspect of the invention, a method of fabricating an assembly includes providing a thermal storage body having a bore formed therein in a first direction to allow an electron beam to pass therethrough and having a window side surface oriented parallel to a central axis of the bore, wherein the window side surface comprises a first portion and a second portion, and wherein an aperture is formed between the bore and the second portion of the window side surface. The method also includes coupling a first portion of a bottom surface of a cover plate to the first portion of the window side surface of the thermal storage body such that an internal pocket is formed between a second portion of the bottom surface of the cover plate and the second portion of the window side surface of the thermal storage body, and disposing an x-ray transmission window in the cover plate. 
     In accordance with another aspect of the invention, an apparatus includes a vacuum chamber, a cathode positioned within the vacuum chamber and configured to emit electrons, and an anode positioned within the vacuum chamber to receive the electrons emitted from the cathode and configured to generate a beam of x-rays from the electrons. The apparatus also includes an electron collector configured to allow passage of the beam of x-rays therethrough. The electron collector includes a collector body having an anode side, a cathode side, and a window side adjacent to the anode and cathode sides, wherein a bore is formed between the anode side and the cathode side, and wherein the window side comprises a window surface having a first portion and a second portion, the second portion having an aperture formed therein. The electron collector also includes a plate having a first surface portion and a second surface portion, wherein the first surface portion is coupled to the first portion of the window surface of the collector body, and wherein a vacuum gap is formed between the second surface portion of the plate and the second portion of the window surface of the collector body, and a window disposed in the plate and positioned to allow a portion of the beam of x-rays to pass therethrough. 
     Various other features and advantages will be made apparent from the following detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate several embodiments presently contemplated for carrying out the invention. 
       In the drawings: 
         FIG. 1  is a schematic diagram of an imaging system that can benefit from incorporation of embodiments of the invention. 
         FIG. 2  is a schematic block diagram of an imaging system that can benefit from incorporation of embodiments of the invention. 
         FIG. 3  is a perspective view of an x-ray tube assembly incorporating a electron collector assembly in accordance with an embodiment of the present invention. 
         FIG. 4  is a sectional perspective view of an x-ray tube incorporating an electron collector assembly in accordance with an embodiment of the present invention. 
         FIG. 5  is an exploded cross-sectional perspective view an electron collector assembly in accordance with an embodiment of the present invention. 
         FIG. 6  is a cross-sectional top perspective view of an electron collector assembly in accordance with an embodiment of the present invention. 
         FIG. 7  is a side plan view of an electron collector assembly in accordance with an embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of an electron collector assembly incorporated within an x-ray tube in accordance with an embodiment of the present invention. 
         FIG. 9  is a cross-sectional view of an electron collector assembly in accordance with another embodiment of the present invention. 
         FIG. 10  is a pictorial view of a CT system for use with a non-invasive package inspection system. 
     
    
    
     DETAILED DESCRIPTION 
     The operating environment of embodiments of the present invention is described with respect to a sixty-four-slice computed tomography (CT) system. However, it will be appreciated by those skilled in the art that the present invention is equally applicable for use with other multi-slice configurations. Moreover, the present invention will be described with respect to the detection and conversion of x-rays. However, one skilled in the art will further appreciate that the present invention is equally applicable for the detection and conversion of other high frequency electromagnetic energy. The present invention will be described with respect to a “third generation” CT scanner, but is equally applicable with other CT systems. 
     Referring to  FIG. 1 , a computed tomography (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a detector assembly or collimator  18  on the opposite side of the gantry  12 . Referring now to  FIG. 2 , detector assembly  18  is formed by a plurality of detectors  20  and data acquisition systems (DAS)  32 . The plurality of detectors  20  sense the projected x-rays that pass through a medical patient  22 , and DAS  32  converts the data to digital signals for subsequent processing. Each detector  20  produces an analog electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuated beam as it passes through the patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . 
     Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to an x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high speed reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 . 
     Computer  36  also receives commands and scanning parameters from an operator via console  40  that has some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus. An associated display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28  and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44  which controls a motorized table  46  to position patient  22  and gantry  12 . Particularly, table  46  moves patients  22  through a gantry opening  48  of  FIG. 1  in whole or in part. 
     Referring now to  FIG. 3 , a perspective view of the x-ray tube assembly  14  incorporating a thermal storage body or electron collector  11  is illustrated in accordance with one embodiment of the present invention. The tube assembly  14  includes a housing unit  52 , a coolant pump  54 , an anode end  56 , a cathode end  58 , and a center section  60  positioned between the anode end  56  and cathode end  58 , which contains the x-ray tube  18 . The x-ray tube  18  is enclosed in a fluid chamber  62  within lead-lined casing  64 . The chamber  62  is typically filled with fluid, such as dielectric oil, but other fluids including water or air may be utilized. The fluid circulates through housing  52  to cool the x-ray tube  18  and may insulate casing  64  from high electrical charges within the x-ray tube  18 . 
     Referring now to  FIG. 4 , a sectional perspective view of the x-ray tube  18  incorporating electron collector  11  is shown in accordance with an embodiment of the present invention. The x-ray tube  18  includes a rotating anode  80  having a target  82  and a cathode assembly  84  disposed in a vacuum within a vacuum vessel  86 . Electron collector  11  is interposed between anode  80  and cathode assembly  84 . Upon energization of the electrical circuit connecting cathode assembly  84 , a stream of electrons  90  are directed through an internal bore  92  of electron collector  11  and accelerated toward target  82 . The stream of electrons  90  strike a focal spot  94  on target  82  and produce high frequency electromagnetic waves  96 , or x-rays, and residual energy. The residual energy is absorbed by the components within x-ray tube  18  as heat. X-rays  96  are directed through the vacuum toward an x-ray transmission window  100  in electron collector apparatus  11 , which efficiently allows the passage of x-rays  96 . According to embodiments, x-ray transmission window  100  may comprise beryllium or a beryllium alloy. Alternatively, x-ray transmission window  100  may comprise a non-beryllium bearing alloy, such as stainless steel, titanium, or a titanium alloy, for example. 
     Referring now to  FIGS. 5-8 , electron collector  11  is shown according to embodiments of the invention.  FIGS. 5-8  discuss common elements and components of electron collector  11  relative to the same reference numbers. 
     Referring first to  FIG. 5 , an exploded cross-sectional perspective view of electron collector  11  is shown according to an embodiment of the invention. As illustrated, electron collector  11  includes a collector body  110  and a cover plate  112 , which has a transmission window  100  disposed therein. Collector body  110  includes an anode side  116  and a cathode side  118  opposite anode side  116 . Internal bore  92  extends through collector body  110  between anode side  116  and cathode side  118 . Collector body  110  also includes a window side  120  that faces x-ray transmission window  100 . 
     A heat exchanger enclosure or pocket  122  is defined in collector body  110  and is sized to receive a heat exchange unit or heat exchange assembly  124 , such as, for example, a fin pack for cooling collector body  110 . According to one embodiment, heat exchanger enclosure  122  is positioned within collector body  110  adjacently to window side  120  and anode side  116  of collector body  110 . However, one skilled in the art will readily recognize that heat exchanger enclosure  122  may be positioned at any location within collector body  110  wherein temperature regulation may be beneficial. Further, multiple fin packs may be positioned at various locations within collector body  110 , according to alternative cooling strategies. 
     Window side  120  of collector body  110  comprises a first portion  126  and a second portion  130 . First portion  126  of window side  120  defines an outer perimeter of window side  120 . Second portion  130  is recessed from first portion  126 , in one embodiment, in a direction toward internal bore  92  and perpendicular to a central axis  132  (shown in  FIG. 7 ) of bore  92 . The space between second portion  130  of collector body  110  and a bottom surface  134  of cover plate  112  defines a gap  136  (shown in  FIGS. 6-8 ) therebetween. 
     Referring now to  FIGS. 6 and 7 , electron collector  11  is illustrated in an assembled state.  FIG. 6  provides a perspective cross-sectional view of electron collector  11 , and  FIG. 7  is a side plan view of electron collector  11 . As illustrated, electron collector is generally cubical in shape and includes three sides  138 ,  140 ,  142  between anode side  116  and cathode side  118 . Transmission window  100  is hermetically sealed to cover plate  112  at a joint  144 , such as by vacuum brazing, diffusion bonding, friction welding, or application of a hermetic-capable adhesive seal, for example. Likewise, a joint  146  hermetically seals cover plate  112  to collector body  110 . Together, joints  144 ,  146  serve to maintain a vacuum within vacuum vessel  86 . 
       FIG. 8  is a cross-sectional view of electron collector  11  coupled to vacuum vessel  86 . A joint  148  hermetically seals collector body  110  to vacuum vessel  86 . As shown, anode side  116  of collector body  110  includes an anode receiving area  150  within which anode  80  may be positioned adjacently to electron collector  11 . Cathode assembly  84 , electron collector  11 , and anode  80  are arranged such that stream of electrons  90  strikes focal spot  94  and a portion of resulting x-rays  96  are directed toward x-ray transmission window  100 . While stream of electrons  90  is illustrated as being offset from central axis  132  of bore  92 , one skilled in the art will readily recognize that cathode assembly  84  may be configured to direct stream of electrons  90  along central axis  132  or any other desired projection line within bore  92 . 
     Also shown in  FIG. 8  is an aperture  152 , which extends from internal bore  92  to window side  120  of collector body  110  in a direction perpendicular to central axis  132  of bore  92 . Aperture  152  is positioned within collector body  110  to be aligned with x-ray transmission window  100  such that x-rays  96  from focal spot  94  pass therethrough. 
       FIG. 9  is a cross-sectional view of electron collector  11  according to another embodiment of the invention. As illustrated, bottom surface  134  of cover plate  112  includes a first portion  156  that is coupled to first portion  126  of window side  120  of collector body  110 . A second portion  158  of bottom surface  134  is recessed from first portion  156  in a direction away from internal bore  92  and toward x-ray transmission window  100 . The space between collector body  110  and second portion  158  of bottom surface  134  defines a gap  160  therebetween. Aperture  152  is aligned with second portion  158  of bottom surface  134  of cover plate  112 . 
     According to one embodiment, heat exchange element  124  is positioned within heat exchanger enclosure  122 , which is positioned at anode side  116  of electron collector body  110 . However, one skilled in the art will recognize that heat exchange element  124  may be positioned at alternative locations based on a desired cooling characteristic. 
     In operation, the resulting separation between window  100  and collector body  110  together with the vacuum present within vacuum vessel  86  thermally isolate window  100  from the high temperatures present within collector body  110 . That is, the geometry of electron collector  11  is such that the conductive heat transfer path between window  100  and collector body  110  is sufficiently long enough to effectively thermally isolate window  100  and associated joint  144  from any areas of high temperature within collector body  110 . Further, due to the two-piece construction of electron collector  11  (i.e., cover plate  112  coupled to collector body  110 ), mechanical stresses resulting from any temperature difference between collector body  110 , cover plate  112 , and window  100  are experienced primarily in joint  146  between collector body  110  and cover plate  112 , rather than in joint  144  between window  100  and cover plate  112 . Thus, joint  144  of transmission window  100  is effectively mechanically isolated from the non-symmetric heat load and associated thermal growth of collector body  110 , thereby reducing the plastic strain in joint  144 . 
     Referring now to  FIG. 10 , package/baggage inspection system  200  includes a rotatable gantry  202  having an opening  204  therein through which packages or pieces of baggage may pass. The rotatable gantry  202  houses a high frequency electromagnetic energy source  206  as well as a detector assembly  208 . A conveyor system  210  is also provided and includes a conveyor belt  212  supported by structure  214  to automatically and continuously pass packages or baggage pieces  216  through opening  204  to be scanned. Objects  216  are fed through opening  204  by conveyor belt  212 , imaging data is then acquired, and the conveyor belt  212  removes the packages  216  from opening  204  in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages  216  for explosives, knives, guns, contraband, etc. 
     Therefore, in accordance with one embodiment, an apparatus includes an electron collector includes a body having an internal bore formed therethrough along a first direction and a window side having an aperture formed in a first portion thereof along a second direction different from the first direction. The apparatus also includes a cover plate having a bottom surface coupled to a second portion of the first surface of the electron collector, and an x-ray transmission window coupled to the cover plate and aligned with the aperture along the second direction, wherein a recess is formed along the second direction in one of the first portion of the first surface of the electron collector and a portion of the bottom surface of the cover plate, and wherein a gap is formed between the bottom surface of the cover plate and the first surface of the electron collector. 
     In accordance with another embodiment, a method of fabricating an assembly includes providing a thermal storage body having a bore formed therein in a first direction to allow an electron beam to pass therethrough and having a window side surface oriented parallel to a central axis of the bore, wherein the window side surface comprises a first portion and a second portion, and wherein an aperture is formed between the bore and the second portion of the window side surface. The method also includes coupling a first portion of a bottom surface of a cover plate to the first portion of the window side surface of the thermal storage body such that an internal pocket is formed between a second portion of the bottom surface of the cover plate and the second portion of the window side surface of the thermal storage body, and disposing an x-ray transmission window in the cover plate. 
     In accordance with yet another embodiment, an apparatus includes a vacuum chamber, a cathode positioned within the vacuum chamber and configured to emit electrons, and an anode positioned within the vacuum chamber to receive the electrons emitted from the cathode and configured to generate a beam of x-rays from the electrons. The apparatus also includes an electron collector configured to allow passage of the beam of x-rays therethrough. The electron collector includes a collector body having an anode side, a cathode side, and a window side adjacent to the anode and cathode sides, wherein a bore is formed between the anode side and the cathode side, and wherein the window side comprises a window surface having a first portion and a second portion, the second portion having an aperture formed therein. The electron collector also includes a plate having a first surface portion and a second surface portion, wherein the first surface portion is coupled to the first portion of the window surface of the collector body, and wherein a vacuum gap is formed between the second surface portion of the plate and the second portion of the window surface of the collector body, and a window disposed in the plate and positioned to allow a portion of the beam of x-rays to pass therethrough. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.