Abstract:
A field emission cathode has a field emitter and an extraction grid, and the field emitter and the extraction grid can be moved relative to one another. Such a field emission cathode is highly durable and exhibits a longer lifespan. An x-ray tube has a field emission cathode composed of a field emitter and an extraction grid that can be moved relative to one another. Such an x-ray tube is highly durable and exhibits a longer lifespan.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention concerns a field emission cathode as well as an x-ray tube with such a field emission cathode. 
         [0003]    2. Description of the Prior Art 
         [0004]    In x-ray tubes, thermionic emitters (advantageously made of tungsten, tantalum or rhenium) are conventionally used to generate the electron beam required for the generation of x-ray radiation. The thermionic emitter is heated to approximately 2,000° C., causing electrons to be thermionically emitted, and the emitted electrons accelerated toward an anode by an electrical potential of approximately 120 kV. X-ray radiation usable for imaging is created when the thermionically generated electrons strike the anode. Such a thermionic emitter is described in DE 27 27 907 C2, for example. Such thermionic emission has the disadvantage that switching of the heating current requires several seconds since the heating of the thermionic emitter is slow. 
         [0005]    As an alternative to the generation of free electrons by means of thermionic emission, the possibility exists to generate free electrons by field emission. By applying a voltage, electrons are extracted from a material with a high emission density, for example carbon nanotubes (CNT), and heating of this material is not necessary. The current densities that can be achieved with such a field emitter are typically less than 1 A/cm 2 , but well below the current densities of a thermionic emitter (with which current densities up to 10 A/cm 2  can be realized). The possibility to quickly switch such a field emitter (known as a “cold emitter” due to the fact that a heating is unnecessary, or only a slight heating is required) makes this technology very attractive for x-ray tubes. If the current density is increased to a few A/cm 2 , the lifespan of the field emitter is limited. In order to increase the lifespan it is known to arrange multiple emitter modules in parallel in order to distribute the total load of the field emitter among them, thus reducing the total load for the individual emitter modules, and thereby increasing the lifespan of the field emitter. The manufacture of such emitter modules is complicated and consequently is expensive. Furthermore, each emitter module must be activated individually. Therefore this concept can only be realized with technical difficulty in rotating anode x-ray tubes. 
         [0006]    In order to achieve high field strengths of greater than 1 V/μm for the electron emission, either a high voltage is required or the distance to the anode must be very short. An additional possibility is the use of an extraction grid (gate electrode) between the field emitter and the anode, that is at a positive potential relative to the electron emission layer. Given distances between approximately 100 μm and 1 mm, the aforementioned field strengths can be generated with average voltages in the range of a few kV, which can be handled easily. The extraction grid is composed of thin tungsten wires, for example, with a wire diameter of a few 10s of μms, and typically exhibits a grid spacing of 100 to 200 μm. 
         [0007]    An x-ray tube with a field emission cathode that has a field emitter and an extraction grid is known from the product brochure “Carbon Nano Tube Based Field Emission X-Ray Tubes”, for example. This product information is available at www.xintek.com/products/xray/index. 
         [0008]    A rotating anode x-ray tube and a rotary piston x-ray tube that has cold emitters as the electron source are described in DE 10 2005 049 601 A1 and in the corresponding United States Application Publication No. 2007/0086571. 
         [0009]    Field emission cathodes with an electron emission made of carbon nanotubes (CNT) are known from U.S. Pat. No. 6,553,096. An extraction grid that is at a positive potential relative to the electron emission is arranged between the field emitter and the anode. 
         [0010]    A field emitter with a rod-shaped nanostructure (“nanorods”) is disclosed in United States Application Publication No. 2007/0247048. 
       SUMMARY OF THE INVENTION 
       [0011]    An object of the present invention to provide a field emission cathode that is highly durable and has a long service life. 
         [0012]    Furthermore, it is an object of the present invention to provide an x-ray tube with a field emission cathode that is highly durable and has a long service life. 
         [0013]    The field emission cathode according to the invention has a field emitter and an extraction grid, and the field emitter and the extraction grid are moveable relative to one another (i.e., at least one is moveable relative to the other). 
         [0014]    In the field emission cathode according to the invention, the field emitter and the extraction grid are moveable relative to one another, so only the region of the field emitter over which the extraction grid is presently located emits electrons. In the field emission cathode according to the invention, the current density can be markedly increased without reducing the lifespan due to the higher load. If the available increase of the current density is not used, the field emission cathode according to the invention then exhibits a distinctly longer lifespan. The emission current in the field emission cathode according to the invention can be adjusted in a known manner through the grid voltage. In addition to a quick activation, high electron currents can be achieved. 
         [0015]    The field emission cathode according to the invention is suitable for stationary anode x-ray tubes, rotary anode x-ray tubes, rotary piston x-ray tubes and stationary anode annular tubes. 
         [0016]    In the solution according to the invention, a division of the field emitter into individual emitter modules that are individually controlled, or a structuring of the electron emission layer, is not necessary. The loading of the electron emission layer is already significantly reduced due to relative movement between the field emitter and the extraction grid since an extraction of electrons from the electron emission layer ensues only in the region of the electron emission layer over which the extraction grid is currently located. The regions in which the extraction grid is presently not located emit no electrons. The regions of the field emitter thus are not continuously exposed to the radiant heat originating from a hot focal spot on the anode. The thermal load is therefore correspondingly small. 
         [0017]    In the field emission cathode according to the invention, the relative movement of field emitter and extraction grid can be achieved according to advantageous embodiments by
       the field emitter being stationary and the extraction grid can be moved relative to the stationary field emitter, or   the extraction grid being stationary and the field emitter can be moved relative to the stationary extraction grid, or   both the field emitter and the extraction grid can be moved.       
 
         [0021]    In a preferred embodiment of the field emission cathode according to the invention, the field emitter has multiple emitter modules arranged in parallel. These emitter modules can be fashioned identically or can respectively exhibit different designs and/or be composed of different materials, corresponding to specific technical requirements. Furthermore, the emitter modules can be controlled together or (for special application cases) individually, so a temporal and/or spatial differentiation of the current flow can be realized. 
         [0022]    In a preferred embodiment of the field emission cathode according to the invention, the field emitter is fashioned as a ring and, for example, is arranged on an electrically conductive disc. Due to its rotational symmetry, this embodiment is particularly well suited for rotary anode x-ray tubes and rotary piston x-ray tubes. 
         [0023]    In principle, all materials that enable a field emission of electrons are suitable for the field emitter of the field emission cathode according to the invention. 
         [0024]    The field emitter advantageously consists of a carbon-based nanomaterial, in particular carbon nanotubes (CNT). 
         [0025]    According to one alternative, the field emitter consists of a synthetic graphite, for example graphene, graphenoid or HOPG (Highly Oriented/Ordered Pyrolytic Graphite). Graphene has a field emission comparable to CNT. 
         [0026]    According to a further alternative, the field emitter is fashioned as a metal tip emitter, advantageously with etched metal tips made of tungsten, for example. 
         [0027]    An embodiment in which the field emitter is executed as a Spindt emitter can also be advantageous for specific application fields. 
         [0028]    The x-ray tube according to the invention has a field emitter and an extraction grid that are moved relative to one another, and only the region of the field emitter over which the extraction grid is currently located emits electrons. In the x-ray tube according to the invention, the current density can clearly be increased without reducing the lifespan of the field emitter due to this higher load. If a possible increase of the current density is foregone, the field emission cathode of the x-ray tube according to the invention then exhibits a distinctly longer lifespan. In the field emission cathode of the x-ray tube according to the invention, the emission current can be adjusted in a known manner through the grid voltage. High electron currents can therefore be achieved in addition to a fast control. 
         [0029]    The x-ray tube according to the invention can be a stationary anode x-ray tube, a rotary anode x-ray tube, a rotary piston x-ray tube or a stationary anode annular tube. 
         [0030]    In the x-ray tube according to the invention, neither a division of the field emitter into individual emitter modules that are respectively to be controlled individually, nor a structuring of the electron emission layer, is necessary. The loading of the electron emission layer is already significantly reduced by the relative movement between the field emitter and the extraction grid since an extraction of electrons from the electron emission layer ensues only in the region of the electron emission layer over which the extraction grid is presently located. The regions in which the extraction grid is not presently located emit no electrons and therefore cool off. 
         [0031]    In the x-ray tube, the relative movement of field emitter and extraction grid can be achieved according to advantageous embodiments by
       the field emitter being stationary and the extraction grid can be moved relative to said field emitter or   the extraction grid is arranged stationary and the field emitter can be moved relative to said extraction grid or   both the field emitter and the extraction grid can be moved.       
 
         [0035]    In a preferred embodiment of the x-ray tube according to the invention (for example stationary anode annular tube), the field emitter is composed of multiple emitter modules arranged in parallel. These emitter modules can be fashioned identically or can exhibit respectively different designs and/or be composed of different materials, corresponding to specific technical requirements. Furthermore, the emitter modules can be controlled together or (for special application cases) individually, so a temporal and/or spatial differentiation of the current flow can be realized. 
         [0036]    In a preferred embodiment of the field emission cathode according to the invention, the field emitter is fashioned as a ring and is arranged on an electrically conductive disc. Due to its rotational symmetry, this embodiment can be realized particularly well in rotary anode x-ray tubes. 
         [0037]    In principle, all materials that enable a field emission of electrons are suitable for the field emitter of the x-ray tube according to the invention. For example, carbon-based nanomaterials are suitable, in particular carbon nanotubes (CNT) or synthetic graphite, for example graphene, graphenoid or HOPG (Highly Oriented/Ordered Pyrolytic Graphite). 
         [0038]    According to a further alternative, the field emitter of the x-ray tube according to the invention is fashioned as a metal tip emitter, advantageously with etched metal tips made of tungsten, for example. 
         [0039]    An embodiment of the x-ray tube according to the invention in which the field emitter is executed as a Spindt emitter can also be advantageous for specific application fields. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1  schematically illustrates a first embodiment of an x-ray tube according to the invention, in longitudinal section. 
           [0041]      FIG. 2  schematically illustrates a second embodiment of an x-ray tube according to the invention, in longitudinal section. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0042]    An x-ray tube  1  that is executed as a rotary anode x-ray tube is shown in  FIG. 1 . 
         [0043]    The x-ray tube  1  has a stationary vacuum housing  2  with a high-voltage side  3  that possesses an insulating body  4  made of ceramic. 
         [0044]    The x-ray tube  1  is mounted in a known manner in a radiator housing (now shown). A coolant liquid is located between the vacuum housing  2  and the radiator housing. 
         [0045]    A high voltage connection (terminal)  5  and a high voltage connection  6  are arranged in the insulating body  4 . The high voltage connection  5  is at a cathode potential U K , for example −120 kV, and is connected to an electrically conductive bearing  7  in which a shaft  8  is mounted such that it can rotate. A field emitter  9  is arranged on the shaft  8  such that it is rotationally fixed. 
         [0046]    The high voltage connection  6  is connected to a stationary extraction grid  10  at a grid potential U G  that increases the negative cathode potential U K  by an extraction potential U E  of, for example, +2 kV. In the shown exemplary embodiment, the grid potential U G  is thus −118 kV. The grid potential U G  is thus more positive by 2 kV relative to the cathode potential U K . 
         [0047]    The field emission cathode of the x-ray tube  1  thus is formed by the field emitter  9 , the extraction grid  10  and the associated high voltage connections  5  and  6 . 
         [0048]    The field emitter  9  has a field emitter ring  9   a  that, in the shown exemplary embodiment, consists of carbon nanotubes. The field emitter ring  9   a  is arranged on an electrically conductive field emitter substrate disc  9   b  that is seated in a rotationally fixed manner on the shaft  8 . The shaft  8  is furthermore mounted such that it can rotate inside a protective wall  11  and is connected with a rotary anode  13  at ground potential U M  in a mechanically rigid and electrically insulated manner via two insulating bodies  12   a  and  12   b.    
         [0049]    The shaft  8  thus has a shaft segment  8   a  that directs a voltage (cathode potential U K ) to the field emitter substrate disc  9   b  and an insulating shaft segment  8   b  following the shaft segment  8   a.    
         [0050]    Voltage flashovers (arcings) are thus reliably prevented by dividing the shaft  8  into a voltage-conducting shaft segment  8   a  and an insulating (thus voltage-free) shaft segment  8   b.    
         [0051]    The anode  13  is mounted such that it can rotate with its free end in an axially cooled ball bearing  14  (rotary anode) and is driven by a motor  15  (electromotor) upon operation of the x-ray tube  1 . Due to the rotationally fixed connection via the insulating bodies  12   a  and  12   b,  the field emitter substrate disc  9   b  is also driven by the motor  15  (common drive for field emitter  9  and rotary anode  13 ). 
         [0052]    In the x-ray tube  1  (rotary anode x-ray tube) shown in the drawing, anode  13  and field emitter  9  thus rotate in the same direction and with the same speed. 
         [0053]    The protective wall  11  has an opening  16  for an electron beam  17  generated by the field emitter ring  9   a.    
         [0054]    During rotation, upon reaching the stationary extraction grid  10  the field emitter ring  9   a  is locally activated and thus emits electrons that exit the protective wall  11  as an electron beam  17  through the opening  16  and strike the anode  13 . When the electron beam  17  strikes the anode  13 , x-ray radiation  18  is generated in a known manner that exits through an x-ray exit window  19  arranged in the vacuum housing  2 . 
         [0055]    The heating of the anode  13  that is creating upon the electron beam  17  striking the focal spot path leads to thermionic radiation of the anode  13  and to the exit of cations (positive ions) from the focal spot path. The field emitter  9  must be protected from the thermionic radiation and from the cations exiting from the anode  13 . The protective wall  11  fulfills this task. 
         [0056]    In the region of the opening  16 , the protective wall  11  has a back-scatter collector  20  on its side facing the anode  13 , for catching scattered back electrons. The loading of the anode  13  is significantly reduced by the collection of the back-scatter electrons. 
         [0057]    Due to mechanically-caused oscillations in the rotation movement of the field emitter  9 , the electron emission varies corresponding to the slightly varying field intensity between the extraction grid  10  and the field emitter  9 . This is compensated in the exemplary embodiment shown in  FIG. 1  by a dynamic adaptation of the grid potential U G . 
         [0058]    The x-ray tube shown in  FIG. 1  enables a fast modulation of the electron current and is therefore particularly suited for what are known as “dual energy” applications and dose modulations, in particular for clocked (synchronized) x-ray generation. 
         [0059]    Furthermore, the x-ray tube  1  according to  FIG. 1  exhibits a reduced extrafocal radiation since the x-ray radiation  18  is collimated near the focal spot. 
         [0060]    The x-ray tube  21  shown in  FIG. 2  is a rotary piston x-ray tube. 
         [0061]    The x-ray tube  21  has a rotating vacuum housing  22  with a high voltage side  23  that is executed as an insulated housing part  24  made of ceramic. 
         [0062]    The x-ray tube  21  furthermore has a high voltage connection [terminal]  25  and a high voltage connection  26 . 
         [0063]    The high voltage connection  25  lies at a cathode potential U K  of −120 kV, for example, and is directed via a brush  27  to a slip ring  28  that is arranged in the insulated housing part  24 . A field emitter  29  is connected with the slip ring  28  so as to be mechanically rigid and electrically conductive. The field emitter  29  has a field emitter ring  29   a  that, in the shown exemplary embodiment, consists of carbon nanotubes. The field emitter ring  29   a  is arranged on an electrically conductive field emitter substrate ring  29   b  that is connected with the slip ring  28  in an electrically conductive manner. 
         [0064]    The high voltage connection path  26  proceeds through a brush  30  to a shaft  31  and from this via an electrical conductor  42  to a stationary extraction grid  43  that is arranged on a substrate disc  32 . The substrate disc  32  of the extraction grid  43  simultaneously forms an insulating protective wall. The high voltage connection  26  is at a grid potential U G  that increases the negative cathode potential U K  by an extraction potential U E  of +2V. In the shown exemplary embodiment, the grid potential U G  thus amounts to −118 kV. The grid potential U G  is thus more positive by 2 kV relative to the cathode potential U K . 
         [0065]    The field emission cathode of the x-ray tube  21  thus is formed by the field emitter  29 , the extraction grid  43  as well as the associated high voltage connections  25  and  26 . 
         [0066]    The shaft  31  thus has a shaft segment  31   a  directing a voltage (cathode potential U K ) up to the extraction grid  43 . The extraction grid  32  is arranged on the shaft  31  such that it can move in rotation but is axially rigid, wherein the stationary position of the extraction grid  43  is achieved via an external electromagnetic field. 
         [0067]    The other end of the shaft  31  is executed as an insulating shaft segment  31   b.  The insulating shaft segment  31   b  is connected in a rotationally fixed manner with an anode  33  lying at ground potential U M . 
         [0068]    Voltage flashovers (arcings) are reliably prevented via the measure to divide the shaft  31  into a voltage-conducting shaft segment  31   a  and an insulating (thus voltage-free) shaft segment  31   b.    
         [0069]    In the x-ray tube  21  shown in  FIG. 2 , the insulating housing part  24  and the outside of the anode  33  thus form the rotating vacuum housing  22 , and the field emitter  29  is attached to the inside of the insulating housing part  24  via the slip ring  28 . The vacuum housing  22  and the field emitter  29  thus rotate in the same direction and with the same speed. 
         [0070]    The shaft  31 , which bears all parts of the vacuum housing  22  (insulating housing part  24 , anode  33 ) is driven by a motor  34  (electromotor) during operation of the x-ray tube  21 . 
         [0071]    The x-ray tube  21  is borne in a known manner in a radiator housing (not shown). A coolant liquid is located between the vacuum housing  22  and the radiator housing. Since the outside of the anode  33  forms a part of the vacuum housing  22 , the anode  33  is a directly cooled anode. 
         [0072]    The extraction grid  43  is arranged on the shaft  31  such that it can move in rotation and is axially rigid, and the stationary position of the extraction grid  43  is achieved by an external electromagnetic field that acts on a permanent magnet ring  35  that is arranged on the external circumferential side of the extraction grid  43 . The external electromagnetic field is generated on a coil arrangement  36  that is arranged outside of the vacuum housing  22 . The extraction grid  43  thus does not execute any rotation movement (in contrast to the vacuum housing  22 ). 
         [0073]    The extraction grid  43 , which is electromagnetically fixed during the operation of the x-ray tube, possesses an opening  37  for an electron beam  38  generated by the field emitter ring  29   a.    
         [0074]    During its rotation, upon reaching the stationary extraction grid  43  the field emitter ring  29   a  is locally activated and hereby emits electrons that exit as an electron beam  38  through the opening  37  of the extraction grid  43  and strike the anode  33 . When the electron beam  38  strikes the anode  33 , x-ray radiation  39  is generated in a known manner that exits through an x-ray exit window  40  arranged in the vacuum housing  22 . 
         [0075]    In the region of the opening  37 , the vacuum housing  22  has a back-scatter collector  41  on its inside for catching scattered back electrons. The loading of the anode  43  is significantly reduced by the collection of the back-scatter electrons. 
         [0076]    Furthermore, the x-ray tube  21  according to  FIG. 2  exhibits reduced extrafocal radiation since the x-ray radiation  39  is collimated near the focal spot by a corresponding geometric design of the inside of the vacuum housing  22 . 
         [0077]    The heating of the anode  33  generated when the electron beam  38  strikes the focal spot path leads to a thermionic radiation of the anode  33  as well as to the exit of cations (positive ions) from the focal spot path. The field emitter  29   a  must be protected from the thermionic radiation and from the cations exiting from the anode  33 . In the x-ray tube  21  according to  FIG. 2 , this is ensured by the extraction grid  43 . 
         [0078]    The invention is not limited to the exemplary embodiments shown in  FIGS. 1 and 2 , which respectively show x-ray tubes  1  and  21  with rotating field emitters  9  and  29  and stationary extraction grids  10  and  43 . Rather, additional advantageous embodiments of the field emission cathode according to the invention or of x-ray tubes according to the invention are possible within the scope of the invention. 
         [0079]    For example, the field emitter can be arranged stationary and the extraction grid can be movable relative to the field emitter. Furthermore, within the scope of the invention it is also possible for both the field emitter and the extraction grid to be movable. 
         [0080]    Moreover, the field emitters  9  and  29  do not necessarily have to consist of carbon nanotubes (CNT). In principle, all materials that enable a field emission of electrons are suitable for the field emitter of the field emission cathode according to the invention. 
         [0081]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.