Patent Publication Number: US-5291538-A

Title: X-ray tube with ferrite core filament transformer

Description:
This is a continuation of application Ser. No. 07/817,296, filed Jan. 6, 1992 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the x-ray tube art. It finds particular application in conjunction with high power x-ray tubes for use with CT scanners and the like and will be described with particular reference thereto. It will be appreciated, however, that the invention will also have other applications. 
     Typically, a high power x-ray tube includes a cathode filament through which a current of about 5 amps is passed at a voltage sufficient to provide about 75 watts of power. This current heats the filament sufficiently that it is caused to emit a cloud of electrons, i.e. thermionic emission. A high potential on the order of 100 kV is applied between the cathode and the anode. This potential causes the electrons to flow between the cathode and the anode through the evacuated region in the interior of the envelope. Generally, this electron beam or current is on the order of 10-500 mA. The electron beam impinges on the anode generating x-rays and producing extreme heating as a byproduct. In high energy x-ray tubes, the anode is rotated at high speeds such that the electron beam does not dwell on only a small area of the anode causing thermal deformation. Each spot on the anode which is heated by the electron beam cools substantially during one rotation of the anode before it is again heated by the electron beam. Larger diameter anodes have a larger circumference, hence provide greater thermal loading. In most conventional rotating anode x-ray tubes, the envelope and the cathode remain stationary while the anode rotates inside the envelope. Heat from the anode is dissipated by thermal radiation through the vacuum to the exterior of the envelope. 
     High power x-ray tubes have been proposed in which the anode and vacuum envelope rotate, while the cathode filament inside the envelope remains stationary. This configuration permits a coolant fluid to be circulated to the anode to provide a direct thermal connection between the anode and the exterior of the envelope. See for example, U.S. Pat. Nos. 4,788,705 and 4,878,235. One of the difficulties with this configuration is providing electrical energy to the stationary cathode within the rotating vacuum envelope. Conveying 5 amps of power into an evacuated envelope without degrading the vacuum can be achieved by using an air core coil or an air core transformer as illustrated by the above-referenced patents. One drawback of the air core coil or transformer configurations is that any vibration of the cathode structure induces changes in the magnetic flux linking the external primary and the internal secondary. These vibration induced changes in the flux linkage cause corresponding variations in the filament current, leading to erratic filament emission. Another drawback to these patents is that the air core coil or transformer operates at about 13.56 MHz which corresponds to a skin depth in copper of about 0.024 mm. Because the electrical current is constrained to such a shallow skin depth, problems arise in the design of the low-resistance leads to the filament, as well as to localized hot spots on the filament itself. Additionally, when multiple secondary turns are provided, wire insulation systems present serious problems with respect to vacuum outgasing and particles. 
     The present invention provides a new and improved technique for transferring electrical power to the filament of an x-ray tube in which there is relative rotational movement between the envelope and the cathode. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an x-ray tube is provided in which an evacuated envelope and a filament contained therein undergo relative rotational movement. A ferrite core transformer conveys electrical power from an AC source across the envelope to the filament disposed in the interior of the envelope. 
     In accordance with a more limited aspect of the present invention, the ferrite core of a primary winding disposed outside the envelope is of a significantly larger cross-section than the ferrite core of a secondary winding disposed within the envelope. 
     In accordance with another aspect of the present invention, the secondary winding is not coated with electrical insulation. Rather, the secondary winding is wound in grooves of an insulative bobbin, which insulative bobbin electrically insulates the uninsulated turns. 
     In accordance with another aspect of the present invention, the cathode and an anode are held at a relatively high potential difference. The primary and secondary windings of the ferrite core transformer are held substantially at the potential of the cathode. An isolation transformer is provided between the primary winding and an AC current source to isolate the ferrite core transformer from other circuitry. 
     In accordance with another aspect of the present invention, the primary winding is connected with a relatively low frequency AC source, in the KHz range. 
     In accordance with another aspect of the present invention, a plurality of filaments are provided, each connected with a different secondary winding. 
     In accordance with another aspect of the present invention, a plurality of filaments are connected with a common secondary winding. Switching means controllable from exterior to the envelope are provided for selecting which of the filaments receives electrical potential from the secondary winding 150. 
     One advantage of the present invention resides in its stability. 
     Another advantage of the present invention resides in its simplicity. 
     The present invention is also more cost efficient than the prior art. 
     Still further advantages of the present invention will be come 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 
     The invention may take form in various components and arrangements of components, and in various steps and arrangement of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     FIG. 1 is a longitudinal cross-section of an x-ray tube in accordance with the present invention; 
     FIG. 2 is a transverse sectional view through sections 2--2 of the filament transformer assembly of FIG. 1; 
     FIG. 3 is an exploded view illustrating the secondary winding of one of the ferrite core transformers of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, an x-ray tube includes a anode A and a cathode assembly B. An evacuated envelope C is evacuated such that an electron beam 12 passing from the cathode to the anode passes through a vacuum. A rotating means D enables the anode A and the envelope c to undergo rotational movement relative to the cathode assembly B. 
     The anode A has a beveled, annular anode surface 10 which is bombarded by the electron beam 12 from the cathode assembly B to generate a beam 14 of x-rays. The entire anode may be machined from a single piece of tungsten. Alternatively, the beveled, peripheral anode path 10 may be an annular strip of tungsten which is connected to a highly thermally conductive disk or plate. Typically, the anode and envelope are immersed in an oil-based dielectric fluid which is circulated to a cooling means. In order to keep the face of the anode surface 10 cool, portions of the anode between the cooling fluid should be highly thermally conductive. 
     The anode A forms one end of the vacuum envelope C. A ceramic cylinder 20 is connected between the anode A and an opposite or cathode end plate 22. At least an annular portion of the cylinder 20 closely adjacent to the anode is x-ray transparent to provide a window from which the x-ray beam 14 is emitted. Preferably, the cylinder 20 is constructed at least in part of a dielectric material such that a high voltage differential can be maintained between anode A and the end plate 22. In the preferred embodiment, the end plate 22 is biased to the potential of the cathode assembly B, generally about 100 kV or more negative than the anode. 
     The rotation means D includes stationary mounting portions 30, 32. A first bearing 34 interconnects the first stationary portion 30 and the end plate 22. A second bearing 36 interconnects the second stationary portion 32 and the anode A. A motor 38 rotates the anode and envelope combination relative to the stationary portions 30, 32. An isolation drive coupler 39 electrically isolates the motor 38 from the anode A. A greaseless bearing 40 is mounted between the cathode assembly B and the envelope C to enable the envelope and the cathode to rotate relative to each other. A means 42 holds the cathode assembly B stationary relative to the rotating envelope C. In the preferred embodiment, the means 42 includes an array of magnets represented here by a pair of magnets 44, 46. Magnet 44 is mounted to the cathode assembly and magnet 46 is mounted to a stationary structure outside of the envelope C. The magnets are mounted with opposite poles towards each other such that the stationary magnet 46 holds magnet 44 and the cathode assembly stationary as the envelope C and the anode A rotate. 
     The cathode assembly B includes a cathode mounting plate 50 which is mounted on an outer race of the cathode bearing 40. The cathode plate supports a first or larger thermionic filament means $2 and a second or smaller thermionic filament means $4. One of the large and small filaments selectively receives sufficient electric current that it is heated to a temperature at which electrons are emitted. Optionally, additional coils, plates, or other electronics (not shown) may be mounted adjacent the filaments to focus the beam 12. The filaments and any focusing electronics are connected with a ferrite core transformer means 60 for communicating electrical power from an AC electrical power supply 62 exterior to the envelope C to the cathode filaments in the evacuated interior of the envelope. 
     With continued reference to FIG. 1 and further reference to FIGS. 2 and 3, the ferrite core transformer means 60 includes a secondary 64 interior to the envelope and a primary 66 exterior to the envelope. The interior secondary 64 includes a generally U-shaped ferrite core 70 having pole faces 72, 74 which are shaped for close, noninterfering conformity with the circularly cylindrical shape of the cylinder 20. The ferrite core material, a nickel-zinc/magnesium-zinc alloy, is vacuum compatible to temperatures up to about 500° C. A ceramic bobbin 76 is disposed around a central portion of the ferrite core 70. The bobbin 76 defines a spiral groove 78 which are separated by a spiral divider 80. An uninsulated copper wire 82 is wound in the groove 78. The width of the divider wall so is selected relative to the dielectric properties of the ceramic bobbin 76 such that the current carried by the secondary winding 82, on the order of 5 amps in the preferred embodiment, does not arc. In the embodiment illustrated in FIG. 3, the bobbin is shown as being constructed in two halves. Alternately, the ferrite core 70 may be constructed in multiple parts to permit receipt of a single piece, cylindrical bobbin. As another alternative, because vacuum is a relatively good electrical insulator, the bobbin surface may define wire winding guides rather than the complete divide wall 80. The winding guides, e.g. dielectric pins assist in configuring the windings with sufficient spacing to prevent arcing. Optionally, larger diameter bobbins may be mounted over prior layers of wire windings wrapped about smaller diameter bobbins to obtain multiple layers of wire windings. 
     The primary 66 includes a generally U-shaped ferrous core member 90 around which a primary wire winding 92 is wrapped. The ferrous core member 90 is substantially larger in diameter than the ferrous core member 70 of the secondary. The flux coupling efficiency between the primary and secondary is relatively low, on the order of 20%. Accordingly, the primary is configured to generate about five times the flux that would saturate the secondary before it saturates. This enables the primary to be driven up to the point of saturation of the secondary before it saturates. Moreover, having larger diameter pole faces simplifies aligning of the primary and secondary. Alternatively, the pole faces of the primary core are tapered 94 to focus the magnetic flux towards a smaller face 96 which is more similar in size to the secondary pole faces 72, 74. 
     To accommodate multiple filaments and focusing plates or electronics, additional secondarys 64&#39; are provided. The primary 66 can be rotated from secondary to secondary to assure that only a single filament is powered at a time. Additional filaments may be mounted at regular angular intervals around plate 50 to provide backup filaments should one filament burn out. As these filaments are rotated to the operating position, the corresponding secondary is rotated concurrently into alignment with the primary. As yet another alternative, a switching means 100 may interconnect a plurality of filaments to a common secondary winding. The switching means 100, such as reed switches, tuned filters, or the like are controllable from exterior of the vacuum envelope 20 to connect a selected cathode filament(s) to the secondary winding. In another embodiment, two or more separate primary windings, disposed outside of the envelope, are magnetically coupled to a like number of separate secondary windings disposed within the envelope. The secondary windings are each operatively connected to separate cathode filaments disposed within the common cathode assembly B, such as filaments 52 and 54. In this manner, an alternate means is provided for actuating one or more cathode filaments simultaneously or independently. 
     A high voltage source 110 applies a high voltage across the anode A and cathode B. Typically, the high voltage is on the order of 150 kV. The secondary 64 which is mounted to the cathode assembly plate 50 has substantially the same potential as the cathode. An isolation transformer 112 is provided between the primary 66 and the AC source 62 in order to permit voltage isolation of the primary from other associated circuitry. This enables the primary and secondary both to be biased to the cathode potential for optimum transformer performance. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alternations 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.