Abstract:
Apparatus for connecting an HV cable to the cathode of an X-ray tube is provided with a housing disposed for attachment to the X-ray tube, and a quantity of epoxy or other electric insulating material contained within the housing. The epoxy serves to insulate the exposed end portions of the HV cable conductors, which extend beyond the cable insulation for insertion into the X-ray tube casing. The connector apparatus further includes a heat transfer device, such as a heat pipe, which extends long the cable within the connector housing. A quantity of working fluid contained in the heat transfer device is disposed for bi-directional movement along the device to transfer heat from a first location within the insulating material to a second location proximate to the housing.

Description:
BACKGROUND OF THE INVENTION 
     The invention disclosed and claimed herein generally pertains to improved apparatus for connecting a high voltage (HV) electric cable to an X-ray tube. More particularly, the invention pertains to apparatus of the above type which effectively transfers heat through the connector apparatus, so that heat generated in the X-ray tube is not trapped in a region proximate to the connector. Even more particularly, the invention pertains to apparatus of the above type which employs an elongated heat transfer device, such as a heat pipe or the like, to enhance heat dissipation with respect to the connector apparatus. 
     In a rotating anode X-ray tube, a beam of electrons is directed through a vacuum and across very high voltage, on the order of 100 kilovolts, from a cathode to a focal spot position on an anode. X-rays are produced as electrons strike the anode, comprising a tungsten target track, which is rotated at high speed. However, the conversion efficiency of X-ray tubes is quite low, typically less than 1% of the total power input. The remainder, in excess of 99% of the input electron beam power, is converted to thermal energy or heat. Accordingly, heat removal, or other effective procedure for managing heat, tends to be a major concern in X-ray tube design. 
     In a common arrangement, an HV electric power cable is employed to provide the requisite 100 kilovolt potential difference between the cathode and anode, in order to produce X-rays as stated above. One end of the cable is connected to a power source of sufficiently high voltage, and the other end is connected into the tube, for connection to the cathode, by means of an HV connector assembly. The connector assembly generally comprises structure for holding the end of the cable in place with respect to the tube, so that the end portion of the cable conductors, which may comprise either a single conductor or a number of conductors, can be joined to a tube. Accordingly, the connector assembly further comprises a quantity of HV insulation placed to surround any exposed portion of the cable conductors which lie outside the tube. The HV insulation is joined to the X-ray tube and is comparatively thick, in view of the high voltage of the cable conductors. 
     Generally, good high voltage insulating materials, such as epoxy, also tend to be very poor thermal conductors. This can create a very undesirable situation, if an HV connector assembly of the prior art is directly attached to an X-ray tube, such as across an end thereof. As stated above, a great deal of heat is generated in the X-ray tube, as an undesired byproduct of X-ray production. Some of this heat is directed against the connector insulation material, which has a comparatively large area in contact with the tube. Because of its poor thermal conductive properties, this insulator serves as a heat barrier, so that a substantial amount of heat tends to accumulate proximate to the connector. As a result, the temperature limits of the connector insulation may be readily exceeded, so that the steady state performance of the X-ray tube must be limited. 
     In one previous arrangement for dealing with this constraint, a reservoir of cooling oil is placed between the HV connector and structure inserted into the tube to support the cathode. However, this arrangement requires that the oil serve as a dielectric. In another arrangement, cooling oil is circulated through the HV connector. This arrangement, however, requires a completely separate oil circuit, provided with tubing and a circulation pump. Thus, this approach can significantly increase cost. In a third prior art arrangement, a good thermal conductor is placed in the electrical insulation of the HV connector to enhance heat flow. However, such thermal conductors can compromise or degrade dielectric characteristics, and have tended to diminish the electrical insulating capabilities of the HV connector assembly. 
     SUMMARY OF THE INVENTION 
     The invention provides apparatus for connecting a high voltage electric cable to an X-ray tube, wherein the apparatus may be attached directly to the tube, such as to the outer surface of the tube casing. The apparatus effectively insulates any exposed portions of the HV cable conductors, and at the same time readily dissipates heat from regions proximate or adjacent to the connector apparatus. The apparatus generally comprises a housing joined to the X-ray tube, and a quantity of selected electric insulating material contained within the housing, the insulating material being traversed by a portion of the HV cable. The apparatus further comprises an elongated heat transfer device positioned within the insulating material to extend along the traversing portion of the cable, in closely spaced relationship therewith. A quantity of selected working fluid is sealably contained in the heat transfer device, the working fluid being disposed for bidirectional movement along the device to transfer heat from a first location within the insulating material to a second location which is proximate to or outside of the housing. By placing the heat transfer device along the cable, and more particularly along the electric conductors thereof, the transfer device does not cut across voltage potential lines, and therefore will not interfere with the electrical insulating requirements of the HV connector. 
     Preferably, the heat transfer device comprises a conduit segment of selected length, the conduit segment having an inner wall in adjacent relationship with a sealed interior space. A selected porous material is attached to the inner wall and configured to define a passage through the sealed interior space that extends along the length of the conduit segment, the porous material being selected in relation to the working fluid so that the fluid, when in liquid form, is disposed for movement through the porous material by means of capillary action. When the first location is at a selectively higher temperature than the second location, fluid proximate to the first location is vaporized into gaseous form, moved along the passage by means of convection to the second location, and then condensed into liquid form. 
     In one useful embodiment, the conduit segment is placed or positioned with respect to the cable so that the electrical conductors of the cable extend through the center of the conduit segment, along the axis thereof. The conduit segment comprises a selected electrically conductive material. A sleeve, likewise formed of electrically conductive material, is positioned within the conduit segment, in coaxial relationship therewith, between the sealed interior space and the conductors of the cable. 
     In a second useful embodiment, the apparatus is provided with a sleeve of selected electrically conductive material which is placed around the cable conductors. The conduit segment comprises one of a plurality of substantially identical conduit segments which are positioned around the outer surface of the sleeve, in abutting relationship therewith and equally spaced apart from one another. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view with a section broken away, showing an X-ray tube provided with a simplified embodiment of the invention. 
     FIG. 2 is a partial sectional view showing the embodiment of FIG. 1 in greater detail. 
     FIG. 3 is a sectional view taken along lines  3 — 3  of FIG.  2 . 
     FIG. 4 is a perspective view showing a heat transfer device, with a section broken away, which may be adapted for the embodiment of FIG.  1 . 
     FIG. 5 is a partial sectional view showing a second embodiment of the invention. 
     FIG. 6 is a partial sectional view taken along lines  6 — 6  of FIG.  5 . 
     FIG. 7 is a sectional view taken along lines  7 — 7  of FIG.  6 . 
     FIG. 8 is a sectional view showing a modification of the embodiment shown in FIG.  5 . 
     FIG. 9 is a sectional view showing a third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown an X-ray tube  10 . In accordance with conventional practice, tube  10  generally includes a metal housing  12  which supports other X-ray tube components including a cathode  14 , and also provides a protective vacuum enclosure therefor. Cathode  14  directs a high energy beam of electrons  16  onto a target track  18  of an anode  20 , which consists of a refractory metal disk and is continually rotated by means of a conventional mounting and drive mechanism  22 . Target track  18  has an annular or ring-shaped configuration and typically comprises a tungsten based alloy integrally bonded to the anode disk  20 . As anode  20  rotates, the electron beam from cathode  14  impinges upon a continually changing portion of target track  18  to generate X-rays, at a focal spot position  24 . A beam of X-rays  26  generated thereby is projected from the anode focal spot through an X-ray transmissive window  27  provided in the side of housing  12 . 
     In order to produce X-rays as described above, there must be a potential difference on the order of 100 kilovolts between cathode  14  and anode  20 . In a monopolar tube arrangement this is achieved by connecting the anode to a ground (not shown), and applying power at the required 100 kilovolt range to cathode  14  through an electric cable  28 . Because of the high voltage carried by cable  28 , it is necessary to use an HV connector  30  in coupling the cable to cathode  14 . The connector  30  and its interconnection with cable  28  is shown in greater detail in FIG.  2 . 
     Referring to FIG. 2, there is shown HV connector  30  provided with a housing  32 , which is usefully formed of aluminum and is joined to tube housing  12 , such as at an end thereof. FIG. 2 further shows HV cable  28  comprising electric conductor or conductors  34  positioned along the center of the cable, and a layer of HV insulation  36  surrounding conductors  34 . As stated above, there may be a single solid conductor  34  or a number of conductors, as shown in FIG.  2 . Conductors  34  usefully comprise copper, and insulator  36  usefully comprises a material such as EP rubber. Such material provides HV cable  28  with flexibility, and at the same time provides sufficient insulation for the high voltage electric power carried thereby. 
     Referring further to FIG. 2, there is shown cable  28  inserted into HV connector  30 , through an aperture in connector housing  32 . Conductors  34  extend beyond the end of insulation layer  36 , and as shown by FIG. 1 are directed through tube housing  12  and mated with an electric coupling element  38 , joined to cathode  14 . Coupling element  38  and cathode  14  are supported in place by insulating structure  40 , inserted into the end of tube  10  and formed of ceramic material or the like. 
     In order to insulate the exposed end portion of conductors  34 , that is, the portion extending between the end of EPR insulator  36  and ceramic insert  40  within tube  10 , FIG. 2 shows HV connector housing  32  filled with electrical insulating material such as epoxy  42 . However, as is well known in the art, substantial amounts of heat are generated by operation of X-ray tube  10 . Some of this heat is directed toward insert  40  and HV connector  30 , as illustrated by the leftward-directed arrows of FIG.  2 . While ceramic insert  40  is a comparatively good thermal conductor, the epoxy insulation  42  of connector  30  is a very poor thermal conductor. Accordingly, the epoxy  42  acts as a thermal barrier. 
     In order to dissipate heat projected toward connector  30  from within the tube  10 , and to prevent such heat from raising the temperature of connector  30  to an unacceptable level, FIGS. 2 and 3 show an elongated heat transfer device  44  placed within cable  28  and connector  30 . The heat transfer device  44  comprises a heat pipe or like device of extremely high thermal conductivity, as described hereinafter in further detail in connection with FIG.  4 . FIGS. 2 and 3 show heat transfer device  44  positioned in closely spaced relationship with conductors  34 , and extending along a portion of the length thereof. More particularly, FIG. 2 shows transfer device  44  having an end  44   a  positioned close to insert  40 , and thus close to the heat received therethrough, and further shows the opposing end  44   b  of device  44  extending outward from connector housing  32 . If a location along the transfer device  44  is at a different temperature than another location, the device  44  will operate to rapidly transfer heat from the location of higher temperature to the location of lower temperature. Thus, device  44  readily serves to transfer excessive heat from regions proximate to its end  44   a , close to insert  40 , to its opposing end  44   b . Even though opposing end  44   b  is within insulating layer  36 , it lies outside the epoxy layer  42  of connector  30 , so that heat can readily be dissipated therefrom into housing  32 , and radiated by the housing into the surrounding air. The transfer of heat by device  44  is passive. Thus, the heat transfer device  44  of connector  30  provides simple and effective cooling, while maintaining essential electrical characteristics required for the connector. 
     To illustrate operation of a heat transfer device, FIG. 4 shows a heat transfer device  46  comprising a length of copper tubing or conduit, which is tightly closed or sealed at its ends to form a vacuum tight vessel. Device  46  is similar or identical to heat transfer device  44  of FIG. 2, except that device  44  is provided with an angled bend along its length whereas device  46  has a linear configuration. The vacuum tight vessel of heat transfer device  46  is evacuated and partially filled with a working fluid  52 , such as water, and is usefully of circular cross section. FIG. 4 further shows a porous metal wicking structure  50 , which is joined to the inner wall or surface  48   a  of copper conduit  48 . Wicking structure  50  is usefully formed of a porous material, such as a material comprising small copper pellets or beads which are sintered together. Wick structure  50  is configured to surround or define a passage  54  which extends along the length of transfer device  46 . 
     By providing a heat transfer device with the construction shown in FIG. 4, such device is enabled to transfer heat by respective evaporation and condensation of working fluid  52 . More particularly, if point  46   a  along device  46  is at a higher temperature than a location  46   b  spaced apart therefrom, heat is inputted through conduit  48  into the interior thereof, proximate to location  46   a . As a result, fluid  52  is vaporized in passage  54  proximate to location  46   a . This creates a pressure gradient in passage  54 , between a region proximate to location  46   a  and a cooler region proximate to location  46   b . This pressure gradient forces the vaporized fluid to flow along passage  54  to the cooler region, where it condenses to a liquid and gives up its latent heat of vaporization. The working fluid  52 , now in liquid form, then flows in the opposite direction along device  46 , back toward location  46   a , through the porous wick structure  50 . Such fluid motion is caused by capillary action in the wick structure, or by gravity if device  46  is oriented to decline downwardly from location  46   b  to location  46   a . Usefully, a heat transfer device  44  or  46  comprises a device which is similar to a product sold by Thermacore Inc. and referred to commercially thereby as a heat pipe. Devices of such type may have an effective thermal conductivity which exceeds the thermal conductivity of copper by more than 10 3  times. 
     Referring to FIG. 5, there is shown an alternative embodiment of the invention, comprising an HV connector  56 , which for reasons set forth hereinafter significantly reduces the electric field, in comparison with the previously described embodiment. The embodiment of FIG. 5 also enhances uniformity of the electric field, that is, causes the field to be less non-uniform. Connector  56 , in like manner with connector  30 , is provided with an aluminum housing  32  filled with a layer of epoxy  42 , and cable  28  is passed through connector  56 , from a location outside the connector into X-ray tube  10 . Connector  56  is also provided with a heat transfer device  60  extending along a portion of the cable  28 . As best shown by FIGS. 6 and 7, transfer device  60  comprises a sealed copper conduit  58  of circular cross-section and a porous wick structure  62  joined thereto, similar to conduit  48  and wick structure  50 , respectively, of heat transfer device  46  described above. Wick structure  62  defines a passage  64  along transfer device  60  which contains water or other working fluid  66 . However, the diameter of heat transfer device  60  is substantially greater than the diameter of transfer device  44 , whereby device  60  can be positioned around cable conductors  34  rather than placed alongside them. More particularly, conduit  58  of device  60 , as shown by FIGS. 6 and 7, is positioned in coaxial relationship with cable  28 , so that cable conductors  34  extend through the center of conduit  58 , proximate to the axis thereof. 
     Referring further to FIGS. 6 and 7, there is shown heat transfer device  60  provided with a cylindrical sleeve  68 , formed of copper or the like, which extends along conduit  58  in coaxial relationship. Sleeve  68  is placed around conductors  34  in closely spaced relationship, and its ends (not shown) are seably joined to corresponding ends (not shown) of conduit  58 . Accordingly, passage  64  through transfer device  60  comprises a sealed interior space which is separated from conductors  34  by the sleeve  68 . 
     FIGS. 6 and 7 further show the space between conductors  34  and the inner surface of sleeve  68  filled with a material  70 . In one embodiment, material  70  comprises metal powder filled epoxy or other conductive material. In such embodiment sleeve  68  and conduit  58  of heat transfer device  60  are electrically connected to the cable conductors  34 , and are thus at the same voltage U, such as 100 KV. As is known by those of skill in the art, the electric field of a conductive cylinder is inversely proportional to the cylinder radius R. Accordingly, by electrically connecting conduit  58  to conductors  34 , the electric field around transfer device  60  will be determined by the radius of conduit  58  rather than the radius of conductors  34 . Since the radius of conduit  58  is substantially greater, the electric field will be significantly reduced. Moreover, the circular cross-section of conduit  58  provides a much more uniform E-field than the generally elliptical or irregular shaped cross-section of the cable conductors  34  and heat transfer device  44 . 
     In an alternative embodiment, the material  70  shown in FIGS. 6 and 7 comprises an epoxy which principally serves as an insulator, but is also selected to have a conductivity which is slightly greater than the conductivity of insulation layer  36  surrounding heat transfer device  60 , as shown in FIGS. 5 and 6. As a result, there will be a first voltage potential between cable conductors  34  and conduit  58  of device  60 , and a second voltage potential between conduit  58  and the outer surface of insulating layer  36 . For example, by judicious selection of the conductivity of material  70 , the first voltage potential could be on the order of 20 KV, and the second voltage potential could be on the order of 80 KV. Such configuration provides a graded insulating system, from conductors  34  through device  60  to the outer edge of insulating layer  36 , to optimize the overall electric field distribution inside connector  56 . 
     Referring further to FIG. 5, there is shown an end of heat transfer device  60  proximate to aluminum housing  32 , rather than extending outward therethrough. This arrangement enables device  60  to readily transfer heat from the interior of connector  56  to connector housing  32 , which effectively dissipates heat into the surrounding air. Proper termination of the end, and maintenance of a sufficient high voltage clearance between the end of device  60  and housing  32 , are necessary to provide an acceptable design margin. In another arrangement, the bend or elbow in device  60  may be eliminated. 
     Referring to FIG. 8, there is shown an alternative construction for heat transfer device  60 . Instead of a wick structure  62  surrounding a passage  64 , a wick structure  72  is provided which extends from conduit  58  to sleeve  68 . A number of passages  74  are formed through the wick structure, equally spaced around the sleeve  68 , to carry vaporized working fluid as described above. 
     Referring to FIG. 9, there is shown a heat transfer arrangement  76 , which may be used in connector  56  instead of the heat transfer device  60  described above. Transfer arrangement  76  comprises a sleeve  78 , formed of copper or other conductive material, which is positioned around and extends along the conductors  34  within connector  56 , in coaxial relationship therewith. A number of heat transfer devices  80 , each similar to transfer device  44 , are equally spaced around the inner surface of sleeve  78 . While not shown, each of the transfer devices  80  is bended or angled as necessary to extend along sleeve  78 , in generally parallel relationship with the axis thereof. FIG. 9 further shows notches  82  formed in sleeve  78 , to accommodate respective transfer devices  80 . By placing the transfer devices  80  around conductors  34  in the symmetrical arrangement shown in FIG. 9, desirable electric field effects are achieved, similar to those described above in connection with transfer device  60 . However, the configuration of FIG. 9 should have significantly less cost. Referring further to FIG. 9, there is shown the space between conductors  34  and sleeve  78  filled with material  70  as described above. 
     Obviously, many other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the disclosed concept, the invention may be practiced otherwise than as has been specifically described.