Abstract:
A cathode assembly ( 18 ) for an x-ray tube ( 1 ) includes a base ( 60 ) to which a filament ( 70 ) is mounted. A pair of deflectors ( 82, 84 ) are carried by the base for deflecting a beam (A) of electrons generated by the filament. Metal tubes ( 130, 132 ) are mounted in bores ( 106 ) of insulator blocks ( 104, 105 ). Metalized ends ( 150 ) of the insulator blocks are brazed into bores ( 122 ) in the base. A rod ( 130, 132 ) attached to the deflector is slid into the tube and the deflector&#39;s position and alignment are gauged and accurately set. The rod and tube are crimped to set the deflector position then welded.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention pertains to the vacuum tube arts, and in particular to an x-ray tube cathode cup structure for deflecting a focal spot of a beam of electrons. It finds particular application in conjunction with rotating anode x-ray tubes for CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in the generation of radiation and in vacuum tubes for other applications.  
           [0002]    Conventional x-ray tubes include a vacuum enclosure and a source of a beam of electrons in the form of a cathode. The cathode includes a heated filament which emits electrons. The impact of the electron beam on the anode causes a beam of x-radiation to be emitted from the x-ray tube, typically through a beryllium window. A trend toward shorter x-ray exposure times in radiography has placed an emphasis on having a greater intensity of radiation and hence higher electron currents. Increasing the intensity can cause overheating of the x-ray tube anode. An electrical bias voltage is applied to the beam of electrons in order to control, to some extent, the size of the focal spot.  
           [0003]    One way to control the size of the focal spot of the electrons on the anode more closely is to mount the cathode filament within a cathode focusing or support cup member. Such a system is shown in U.S. Pat. No. 4,689,809. A cathode cup is split into two portions, surrounding the filament. The portions are biased equal to or negative with respect to the filament. The biased cup reduces unwanted “wings,” or diffused areas, appearing as part of the x-ray focal spot.  
           [0004]    Other cathode cup and filament arrangements for controlling the size and shape of the electron focal spot on the tube anode are discussed in U.S. Pat. Nos. 4,685,118, 5,224,143, and 5,065,420.  
           [0005]    To minimize the power requirements of the focussing system and to maintain accurate positioning of the filament relative to the deflectors, it is desirable to mount both the deflectors and the filament to the same support. Cathode cups thus typically include a base or arm portion which supports the filament and a pair of deflectors. The deflectors are mechanically mounted to the base, but are electrically insulated from it. This is achieved through the use of ceramic insulators which are brazed to both the base and the deflectors in the form of a sandwich. The ceramic insulators include central bores through which a bolt is received for maintaining alignment of the components during brazing. To avoid shorting, the bolt is electrically isolated from the base. Such a cathode cup design is difficult to assemble, difficult to align, and is susceptible to shorting. This can occur if the material used to braze the ceramic insulator to the base or the deflector flows into the insulator bore that receives the bolt. Shorting can also occur due to natural plating of the ceramic insulator with metal vapor from the filament.  
           [0006]    The present invention provides a new and improved x-ray tube and method which overcomes the above-referenced problems and others.  
         SUMMARY OF THE INVENTION  
         [0007]    In accordance with one aspect of the present invention, a cathode assembly is provided. The assembly includes a base. A filament is mounted to the base for delivering a stream of electrons. A deflector is carried by the base for focusing the stream of electrons. An insulator electrically insulates the deflector from the base. The insulator defines a bore. A rod is connected with the deflector adjacent a first end of the rod. The rod is received within the insulator bore.  
           [0008]    In accordance with another aspect of the present invention, an x ray tube is provided. The x-ray tube includes an envelope which encloses an evacuated chamber. A cathode assembly is disposed within the chamber for providing a source of electrons. The cathode assembly includes a base supported in the envelope. A filament is mounted to the base for providing the electrons. A deflector is carried by the base for focusing the electrons into a beam. An insulator electrically insulates the deflector from the base. The insulator defines a bore. A rod is connected with the deflector adjacent a first end of the rod, the rod being received within the insulator bore. An anode is disposed within the chamber and positioned to be struck by the electrons and generate x-rays.  
           [0009]    In accordance with another aspect of the present invention, a method of assembling a cathode assembly is provided. The method includes attaching at least one rod to at least one deflector and attaching a metal tube in an insulator to define a bore for receiving the rod. The insulator is attached to a base. A filament assembly is attached to the base. The method further includes sliding the rod into the tube to mount the deflector to the base and attaching the rod to the tube.  
           [0010]    One advantage of at least one embodiment of the present invention is that a cathode cup is electrically isolated from a filament.  
           [0011]    Another advantage of at least one embodiment of the present invention is that deflectors of a cathode cup are readily aligned with a filament.  
           [0012]    Another advantage of at least one embodiment of the present invention is that components of a cathode cup are accurately aligned.  
           [0013]    Another advantage of at least one embodiment of the present invention is that deposition of vaporized filament material on to insulators which space the deflectors from a base assembly is minimized by reducing the line of sight between the filament and the insulators.  
           [0014]    Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention.  
         [0016]    [0016]FIG. 1 is a schematic sectional view of a rotating anode x-ray tube according to the present invention;  
         [0017]    [0017]FIG. 2 is a side view of a cathode assembly of the x-ray tube of FIG. 1;  
         [0018]    [0018]FIG. 3 is a front perspective view of the cathode assembly of FIG. 2;  
         [0019]    [0019]FIG. 4 is a top view of the cathode assembly of FIG. 2;  
         [0020]    [0020]FIG. 5 is a sectional view of the cathode assembly through line B-B of FIG. 4; and  
         [0021]    [0021]FIG. 6 is an exploded perspective view of the cathode assembly of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    With reference to FIG. 1, a rotating anode x-ray tube  1  of the type used in medical diagnostic systems for providing a beam of x-ray radiation is shown. The tube includes an anode  10  which is rotatably mounted in an evacuated chamber  12 , defined by an envelope or frame  14 . A heated element cathode assembly  18  supplies and focuses an electron beam A. The cathode is biased, relative to the anode  10  such that the electron beam flows to the anode and strikes a target area  20  of the anode. A portion of the beam striking the target area is converted to x-rays B, which are emitted from the x-ray tube through a window  22  in the envelope. The cathode assembly includes a cathode cup or head  24 , which is supported in the envelope by an arm  26  of a cathode support assembly  28 .  
         [0023]    The target  20  of the anode is connected to a shaft  40 , which is supported by bearings  42  in a neck portion  46  of the evacuated envelope  14  and driven by an induction motor  48 . The induction motor includes a stator  50 , outside the envelope, which rotates a rotor  52  connected to the shaft relative to a stationary bearing housing  54 . The anode is rotated at high speed during operation of the tube. It is to be appreciated that the invention is also applicable to stationary anode x-ray tubes, rotating cathode tubes, and other electrode vacuum tubes.  
         [0024]    With reference now to FIGS.  2 - 6 , the cathode head  24  includes a base  60 , which may be integrally formed with the arm  26  or mounted thereto, for example, with bolts  62  or other suitable attachment members threaded through holes  64  in the base (FIG. 4). A filament  66  is supported by the base. FIG. 3 shows two filament supports  67 ,  67 ′ received through corresponding bores  68 ,  68 ′, which extend axially through the base such that an electron-emitting portion or tip  70  of the filament is spaced from the base. The filament may be fixed in this position by brazing the filament supports  67 ,  67 ′ to the bore or by other means, such as threading a threaded portion of the filament supports  67 ,  67 ′ to corresponding threads in the respective bore. It will be appreciated that two or more filaments may be used in place of the single filament shown, if desired. The filament is connected by conductors  74  to a suitable power source  76  outside the envelope (FIG. 3). Although a wire filament is illustrated, it is to be understood that other electron sources are also contemplated, including thin film filaments, and the like.  
         [0025]    Deflectors  80 ,  82  are carried by the base  60  in a manner which electrically insulates the deflectors from the base. Two deflectors are shown in FIG. 3, although a single deflector, or more than two deflectors, could alternatively be used. The deflectors are positioned in close proximity to the filament tip  70  for deflecting and/or focussing the beam of electrons emitted by the filament. This allows the size and location of a focal spot  86  on the target (FIG. 1) to be controlled and adjusted.  
         [0026]    As shown in FIG. 3, the deflectors  80 ,  82  are generally mirror images of each other and are positioned on opposite sides of the filament tip  70 . Each deflector has an upper surface  90  and lower surface  92  (the terms “upper” and “lower” being used with reference to the orientation shown in FIG. 3, the upper surface being closer to the base  60 ). A side wall  94  of the deflector projects inwardly, towards the filament, in the region of the filament tip  70 , thus providing a relatively narrow gap  96  between the two deflectors in the region of the filament tip.  
         [0027]    The deflectors  80 ,  82  may be formed from molybdenum, or other suitable temperature resistant, electrically conductive material. The base  60  may also be formed from molybdenum, or may be formed from less expensive, easier to machine materials, such as nickel, since it does not need to withstand as high temperatures as the deflector.  
         [0028]    With particular reference to FIGS. 4 and 6, the deflectors  80 ,  82  are spaced and insulated from the base by insulators  98 ,  100 ,  98 ′,  100 ′. As shown in FIG. 4, four insulators are employed, two for each deflector. For stability, it is preferable to use two (or more) insulators for each deflector, spaced longitudinally from each other, although it will be appreciated that a single insulator may be used. For ease of reference, the cathode will be described with reference to two deflectors, each having two insulators. As shown in phantom in FIG. 4, the filament tip  70  extends between forward and rear posts  67 ,  67 ′ along a line which is generally coincident with the longitudinal axis of the base  60  and perpendicular to a line B-B between the forward pair of insulators  98 ,  100  and is equally spaced from each insulator  98 ,  100 ,  98 ′,  100 ′ at its closest point thereto.  
         [0029]    As best shown in FIGS. 5 and 6, each insulator  98 ,  100 ,  98 ′,  100 ′ comprises a cylindrical block  104 ,  105 , each with a central axial bore  106 . A first, lower portion  110  of each block  104  is received within a correspondingly shaped cylindrical socket  112  in the deflector  80 ,  82 . It will be understood that different shaped insulator blocks may be used, such as rectangular blocks and a corresponding shaped socket in the defector provided. As will be appreciated, two sockets are formed in each deflector to receive corresponding insulator blocks, a total of four sockets in all. Each socket extends partway into the deflector, preferably, about half way.  
         [0030]    The socket  112  has a slightly larger diameter than the corresponding block  104 ,  105 , such that a gap  116  spaces the insulator from the deflector adjacent a cylindrical side  118  and preferably also a base  119  of the insulator. The gap  116  is preferably about 70-100 microns in width, such that a space is maintained between the insulator  104 ,  105 ,  104 ′,  105 ′ and the deflector  80 ,  82 . This reduces the risk of shorting out. In service, insulators sometimes become coated with a plating layer formed by evaporation of filament material. Leaving a gap between the insulator and the deflector allows for a fairly thick layer of plating material to accumulate without resulting in shorting out.  
         [0031]    A second (upper in FIG. 6) portion  120  of each insulator block  104 ,  105  is received within a cylindrical passageway  122  in the base (four passageways are shown in FIG. 4). The passageway  122  is chamfered to create a smaller diameter portion  124  at the upper end thereof with a shoulder  126  for providing an upper stop for the insulator block  104 ,  105 .  
         [0032]    The insulator blocks  104 ,  105  are formed from an electrically insulating material, such as alumina. For example, 94% purity or 99% purity alumina may be used, such as AD 94, AL 500, or equivalent purity. Al 2 O 3  meeting ASTM Standard D2442 Type 4 is an exemplary insulating material. For effective electrical insulation of the deflector from the base (and the filament), the insulators preferably provide a resistance of at least 720 giga-ohm.  
         [0033]    A pair of rods  130 ,  130 ′,  132 ,  132 ′, formed from an electrically conductive material, such as niobium, are mounted to each deflector  80 ,  82  (i.e., four rods in total) and are received through the corresponding bore  106  of the insulator blocks  104 ,  105 . The rods  130 ,  130 ′,  132 ,  132 ′ are electrically connected to a respective bias supply  134 ,  135  by suitable wiring  136  (FIG. 3). One bias supply is preferably provided for each deflector. The rod is electrically insulated from the base  60  by the corresponding insulator block  104 ,  105  and by a gap  138  at the upper end  124  of the insulator bore.  
         [0034]    The rods  130 ,  130 ′,  132 ,  132 ′ provide an electrically conductive path to the respective deflector  80 ,  82  for biasing the deflector to an appropriate voltage for deflecting or focusing the electron beam. For example, as the two deflectors  80 ,  82  both become more negative, relative to the filament, the size of the focal spot is reduced. When they become sufficiently negative, the electron beam is turned off. If one deflector is more negative than the other, the focal spot moves away from the more negative part. This latter result can be achieved by biasing only one of the deflectors and having the other deflector at the same potential as the filament. Because of the close proximity of the deflectors to the filament, a small bias is able to deflect or focus the beam. The two bias supplies  134 ,  135  may be computer controlled to permit automatic control of the width and positioning of the focal spot to a multiplicity of locations.  
         [0035]    Each rod  130 ,  130 ′,  132 ,  132  is preferably brazed to the deflector prior to insertion of the rod in the corresponding insulator block bore  106 . As shown in FIG. 6, each deflector has a central hole  140  machined in the base of each socket  112 , and shaped to receive one end  142  of the respective rod  130 ,  130 ′,  132 ,  132 . To attach the rod to the deflector, the rod is positioned in the hole  140 , together with a small piece of a suitable braze material, and the assembly heated to an appropriate temperature to braze the two components  130 ,  80  together. Other methods of attaching the rod  130 ,  132  to the deflector  80 ,  82  are also contemplated.  
         [0036]    Each of the insulator blocks  104 ,  105  preferably has a cylindrical tube  146 ,  147 ,  146 ′,  147 ′ mounted axially in the central bore  106  for receiving the corresponding rod. Although only two tubes  146 ,  147  and two blocks are shown in the view of FIG. 6, it will be appreciated that a tube is provided for each insulator block. Thus, for this embodiment, four tubes  146 ,  147 ,  146 ′,  147 ′ are employed, as shown in FIG. 4. Each passageway, insulator bock bore, and corresponding tube and rod are preferably concentrically arranged, as shown in FIG. 4. As shown in FIG. 5, the tube  146 ,  147  has an upper end which extends beyond the upper end of the insulator block, when installed, and is preferably of sufficient length to extend above the base  60  when the insulator block  104 ,  105  is located in the base. At a lower end, the tube  146 ,  147 , when installed, is preferably flush with the base  119  of the insulator block, or may be slightly set back within the block.  
         [0037]    The tube  146 ,  147  has an axially extending bore  148  therethrough with an internal diameter which is only slightly larger than the diameter of the corresponding rod  130 ,  132  so that the rod fits snugly in the tube bore. For example, the rod  130 ,  132  may have an OD of 0.100 cm+0.000/−0.018 and the corresponding tube  146 ,  147  an ID of 0.104 cm+0.025/−0.000. The tube is preferably formed from a material which is readily welded to the rod, for example, by laser welding. Exemplary materials for forming the tube include nickel and Kovar™. The tube  146 ,  147  is attached to the insulator block  104 ,  105  by brazing the two parts together, for example, by heating the tube and block with a suitable braze material between them. The quantity of braze material used should be sufficient to attach the parts firmly, without overflowing significantly at ends of the insulator block. This step is preferably carried out prior to inserting the insulator block into the base passageway  122 .  
         [0038]    The insulators are brazed to the cup base  60  by heating the base and insulator, together with a suitable brazing material. The brazing material is preferably positioned in the shelf region. The brazing material can be the same type as is used to attach the tube to the insulator block and the rod to the deflector. However, since the brazing is preferably carried out in three separate steps (rod to deflector, tube to block, and block to base), the brazing material for each of the three joints can be a different material which is compatible with the parts to be joined and heated to an appropriate temperature for the respective braze material to melt.  
         [0039]    To provide a suitable surface for brazing, the insulator block preferably has a very thin surface coating  150  of a metallizing material, such as a molybdenum-manganese or tungsten-manganese composite material (shown exaggerated in the thickness in FIG. 6). The coating may be deposited on the block by suitable deposition techniques to a thickness of about 5-20 microns. Preferably, the metallizing layer extends over only a portion of the outer surface of the blocks, such as at the upper end of the block in the region where the braze material will be applied, to minimize risk of shorting between the base and the deflector.  
         [0040]    The insulator tubes  146 ,  147  are welded or otherwise attached to the rods  130 ,  132 , for example, by laser welding. This step is preferably carried out after the insulators  104 ,  105  have been brazed into the base. This allows the deflectors to be properly aligned with the filament. The length of the rods  130 ,  132  is preferably selected such that, when the deflectors are correctly positioned, the rods are level with or protrude by a small amount from the upper ends of their respective tubes  146 ,  147 .  
         [0041]    To ensure alignment of the filament tip  70  with the deflectors, the filament posts  67  are preferably seated in the base before inserting the rods into the tubes. The filament posts are welded or otherwise fixed into the respective bore  68 . The rods are then inserted into their respective tubes. A gauge (not shown) of the appropriate thickness is then inserted between the deflector and the base to determine an appropriate gap  152  between the deflector and the base. The base and deflector are pushed towards each other (the rods sliding in their respective tubes) until the base and deflector contact the gauge.  
         [0042]    Prior to laser or otherwise welding the tubes to the rods, the respective tubes  146 ,  147  and rods  130 ,  132  are optionally crimped together to hold the desired set position. The two deflectors are preferably positioned so that the filament is approximately halfway between top and bottom surfaces of the deflector. This minimizes the risk of metallization of the insulator by material evaporating from the filament and avoids a “line of sight” being created in which material from the filament can travel in a straight line to the insulator. As can be seen from FIG. 5, the deflectors are positioned such that material evaporating from the filament tip  70  will be inhibited from traveling directly towards the insulator blocks, the closest direct paths x and y to the insulators  98 ,  100  taking the material to the base  60 , rather than to the insulator.  
         [0043]    A preferred method of assembling the cathode is thus as follows:  
         [0044]    a) braze the rods  130 ,  132  to the deflectors  80 ,  82 ,  
         [0045]    b) braze the tubes  146 ,  147  to the insulator blocks  104 ,  105 ,  
         [0046]    c) braze the insulator blocks to the base  60 ,  
         [0047]    d) set the filament  66  into the base,  
         [0048]    e) set the deflector height with a gauge and crimp the tubes  146 ,  147  to the rods  130 ,  132 .  
         [0049]    f) weld the tubes  146 ,  147  to the rods  130 ,  132 .  
         [0050]    As will be appreciated, step b) may alternatively be carried out before or concurrently with step a) and steps a), b), and/or c) may be carried out after step d).  
         [0051]    Assembling the components stepwise, with three separate brazing steps a), b), c), and a welding step f), rather than brazing the insulator to the base and to the deflector in a single brazing operation, minimizes tolerance stackups due to improper alignment of the three components. The deflectors are easily aligned with respect to the filament tip, simply by sliding the rods up and down in their respective tubes. Having two (or more) tubes which fit snugly to the corresponding rods and thus guide their movement ensures that the deflector remains parallel with the base as it is being positioned.  
         [0052]    The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.