Abstract:
Methods for connecting electrical potential to an extractor cup at the cathode of a miniature x-ray tube are disclosed. The various connection schemes are designed to form a rugged and conveniently manufacturable connection between the metal extractor cup and one side of the cathode filament, so that the extractor cup shapes the path of electrons as desired en route to the anode of the tube. Some of the disclosed connections involve evaporation of conductive metal or other materials off the filament when the filament is first activated. Others involve applying a paste or paint conductive precursor directly to a base to connect a post and the extractor, the paste being heat-cured after the completion of assembly. Others involve a fine wire or spring strip from one filament post to the walls of the extractor cup. Other schemes include welded or brazed wires or foil, crimping, pinching, swaging and other connections, all made inside the tube enclosure.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention concerns construction of miniature x-ray tubes. In particular the invention is directed at an efficient and rugged connection of a high voltage cathode filament lead to an extractor cup which helps shape the path of electrons from the cathode in such an x-ray tube.  
         [0002]     Miniature x-ray tubes, generally of the size and configuration contemplated in this invention, are shown in Xoft Microtube U.S. Pat. No. 6,319,188, and also in U.S. Pat. Nos. 5,854,822 and 5,621,780. Also, Xoft Microtube pending application Ser. No. 10/397,498 describes a cathode assembly with a cathode manufactured by MEMS technology and discloses a means of forming an extractor cup and electrically connecting the extractor cup to high voltage.  
         [0003]     As is known, an extractor cup is usually needed to help focus and direct the stream of electrons leaving a cathode en route to the anode in an x-ray tube, and the need for focusing this electron beam typically becomes more acute in the case of miniature x-ray tubes. However, the connection of an extractor cup to high voltage, in a rugged, reliable and feasibly manufacturable manner, presents something of a challenge. There are problems of reliably connecting a conductor to one end of a cathode filament or a wire lead to the cathode; it is not feasible simply to extend a conductor wire through the tube wall to the exterior, because of sealing problems and because of the requirement to isolate this HV from the tube exterior which is at ground potential; and in miniature size, which may be down to about  1  mm in tube diameter, the options are limited in making secure high voltage connections in proper alignment, to withstand high temperature, without causing the tube to fail ultimately through arcing and while still obtaining a rugged and reliable connection of the extractor cup to a base of the cathode and secure connection of the cathode itself to the base.  
       SUMMARY OF THE INVENTION  
       [0004]     The invention encompasses various means for making secure and rugged connections of an extractor cup to high voltage at the cathode of a miniature x-ray tube.  
         [0005]     The various connection schemes are designed to form a rugged and conveniently manufacturable connection between the metal extractor cup and one side of the cathode filament, so that the extractor cup shapes the path of electrons as desired en route to the anode of the tube. Some connections of the invention involve evaporation of conductive metal or other materials off the filament when the filament is first activated. Some involve direct liquid application of conductive metal as a paste or paint. Others involve a fine wire or spring strip from one filament post to the walls of the extractor cup, or a direct contact of one filament post with the extractor wall. Other schemes include welded or brazed wires or foil, crimping, pinching, swaging and other connections, including shifting of a conductive member after initial assembly, all made inside the tube enclosure.  
         [0006]     In one preferred embodiment of the invention, a miniature x-ray tube has an extractor cup generally surrounding a cathode filament, the two ends of the cathode filament being connected in a low voltage cathode heater circuit, and the filament being at high voltage opposing the anode of the tube. The cathode filament is supported on posts from a cathode base, at least one of the posts being conductive. The filament is pre-coated with a conductive metal such as gold which will flash off or evaporate from the filament when the filament is initially energized in the heater circuit and heated. When the cathode filament is heated, the conductive metal is coated onto all adjacent surfaces, including the base. A small shield or shadowing device is mounted on one of the filament posts to shadow an area of the base adjacent to the one post from receiving the coating. This forms an electrical connection between the other filament post and the base surface, and between the base surface and the wall of the extractor cup, thereby connecting high voltage to the extractor cup. The one filament post referenced above remains insulated from the other post, so as not to create a short in the low voltage heater circuit.  
         [0007]     In a variation of the above, the cathode filament is pre-coated with a semiconductor material that will flash off or evaporate when heated. The shield is not included on either post, and the semiconductor material is evaporated onto the base along both posts and onto the extractor cup. The semiconductor material has a sufficiently high resistance as not to interfere with the low voltage circuit of the cathode filament so that current flow to heat the cathode is largely unaffected. This method also has the advantage of draining extraneous charge buildup from the extractor cup due to electrons striking the extractor.  
         [0008]     In other preferred embodiments a spring strip, wire, conductive whisker or conductive foil is placed inside the tube to connect one of the cathode filament posts to a conductive surface of the extractor. In one scheme a spring strip or springy sheet of foil or whisker is spot welded onto one of the filament posts, extending to the walls of the extractor cup to from a connection which will be robust even during thermal expansion. In another scheme a foil sheet is placed against a glass preform which comprises the base of the cathode assembly, engaging around or against one of the filament posts and also against a wall of the extractor. A braze alloy that melts below about 900° C. may be used, for the case where glassing temperature is about 950° C. During the thermal cycle for the glass preform, the braze material will melt and create an electrical bath between the one filament post and the extractor.  
         [0009]     In other connection methods a wire or whisker is crimped together with the cathode filament at one end, into the filament post, and this wire extends into contact with the conductive surface of the extractor cup. This can be done with a braze alloy on the end of the wire and with the wire contacting the internal diameter of the extractor cup. The temperature to which the tube is raised during assembly will equal or exceed the melting temperature of the braze alloy to provide a permanent bond of the wire or whisker with the extractor wall. In another arrangement the end of the wire that extends from the filament post hangs over the edge of the insulating base on which the posts are mounted, and when the extractor ring is assembled down onto the insulating base, the end of the wire is pinched between the edge of the preform and the wall of the extractor cup, deforming and swaging the wire to form a good connection. For this purpose the wire is advantageously formed of platinum or other soft metal. The connection is made permanent when the preform is heated.  
         [0010]     In another type of connection the filament of the cathode extends between a single post and the wall of the extractor cup, with that wall being connected to another lead at the base of the extractor, so that the extractor serves as part of one filament lead. A further scheme has two filament posts, one being longer and placed so as to make contact with a top edge of the extractor cup, near its opening, on assembly of the extractor to the base. In another method a cathode assembly has two posts or pins supporting the cathode filament, and the filament is secured to these pins or posts such that after being crimped to one of the posts, the filament extends beyond that post and makes contact with the extractor wall.  
         [0011]     In a different embodiment, the cathode filament is supported between coaxial conductors which extend up into the extractor cup. The external coaxial conductor is conductive, and in one type of connection the extractor cup, all of conductive material, has a bottom or base with a hole which on assembly slides down over the outer coaxial conductor and makes electrical contact. Other connection schemes involving the coaxial filament leads include a conductive metal strip extending radially from the outer coaxial conductor to the extractor wall; use of wires or spring wires which contact the exterior coaxial conductor and extend to the extractor wall; and the use of spring clips that engage between the outer coaxial conductor lead and the extractor wall.  
         [0012]     It is therefore among the objects of the invention to provide rugged and reliable high voltage connections from a cathode filament to a surrounding extractor cup, in a manner that can be reliably manufactured in a miniature x-ray tube. These and other objects, advantages, and features of the invention will be apparent from the following description of preferred embodiments, considered along with the drawings. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic view in perspective and partially cut away, showing a portion of a x-ray tube with a cathode and an extractor cup and showing a means of connecting high voltage to the extractor involving use of a wire connected to the cathode filament.  
         [0014]      FIG. 2  is a view similar to  FIG. 1 , showing a different connection arrangement involving a filament-connected wire.  
         [0015]      FIGS. 3 and 4  are simplified schematic views showing further embodiments of cathode/extractor connections, in this case involving a filament support post directly contacting the extractor cup.  
         [0016]      FIG. 5  is a simplified schematic view showing a cathode filament with a tail end directly contacting an extractor cup wall.  
         [0017]      FIG. 6  is a simplified schematic view showing a cathode filament supported between a single pin or post and the wall of an extractor cup.  
         [0018]      FIG. 7  is a simplified schematic view showing another connection arrangement in which a metal film, i.e. a paint or paste, is applied as a connecting conductor and later heat-cured.  
         [0019]      FIG. 8  is a schematic view in perspective showing a connection technique involving conductive metal evaporated from the cathode filament onto a base surface to make the needed connection with a portion of the base shadowed by a shield.  
         [0020]      FIG. 9  is a view similar to  FIG. 8 , but showing use of a different evaporative material, without any shield.  
         [0021]      FIG. 10  is a schematic view showing a flat piece of conductive foil which can be used to connect a filament post to an extractor wall, the foil being cured by heating.  
         [0022]      FIG. 11  is a schematic sectional view showing one connection scheme wherein the cathode filament leads ate coaxial conductors.  
         [0023]      FIG. 11A  is a sectional view of the-arrangement shown in  FIG. 11 .  
         [0024]      FIG. 12  is a view similar to  FIG. 11 , but showing a different means of connection.  
         [0025]      FIGS. 12A and 12B  are schematic sectional views of the arrangement shown in  FIG. 12 , and of a variation.  
         [0026]      FIG. 13  is another view similar to  FIG. 11 , but showing a further connection arrangement, in this case including spring clips as conductors.  
         [0027]      FIG. 13A  is sectional view illustrating the arrangement of  FIG. 13 .  
         [0028]      FIGS. 14 and 14 A are sectional views showing further connection arrangements involving wires, for a cathode assembly having a coaxial lead generally as in  FIG. 11 .  
         [0029]      FIG. 15  is a simplified schematic cross-sectional view through an extractor cup and cathode assembly, showing the use of a spring clip or spring wire as a connecting conductor, with a dual-filament post assembly.  
         [0030]      FIG. 16  is a schematic view in elevation showing the dual filament posts and the spring clip of  FIG. 15 .  
         [0031]      FIGS. 17, 18 , and  19  are schematic sectional and sectional elevation views showing another connection scheme involving rotation of a conductive member to make the needed electrical contact after initial assembly and prior to final firing.  
         [0032]      FIGS. 20 and 21  relate to another scheme for making the electrical contact, in this case with an elongated crimp of the cathode filament supporting posts, with  FIG. 21  showing a tool for such a crimping operation.  
         [0033]      FIG. 22  is a view similar to  FIG. 1 , but showing a variation wherein a third HV wire connects to the extractor, permitting a bias to be introduced. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0034]      FIG. 1  shows a portion of a miniature x-ray tube  10 , including a tube envelope  12  and a cathode assembly  14 . Within the cathode assembly are a base  16 , typically a glass preform, a pair of cathode filament supports posts or pins  18  and  20 , a cathode filament  22 , and an extractor cup  24 . The filament support posts or pins  18  and  20  preferably extend up through openings in the base  16 , being connected below the base to conductors which run through a flexible cable which may be part of a catheter. These posts, and the cathode filament  22 , are in a low voltage cathode heater circuit, and high voltage potential is also supplied to the entire cathode so that electrons from the cathode will flow toward the anode (not shown) at the other end of the x-ray tube  10 . Thus the two cathode posts or pins  18  and  20  are both at high potential, but different by the small amount of the low voltage circuit.  
         [0035]     The extractor cup  24  should be at similar high voltage potential to the cathode filament  22 , its purpose being to repel electrons so as to shape the stream of electrons flowing toward the anode, something like a lens acting on light.  FIG. 1  shows one arrangement for connecting the preferably metal extractor cup  24  to the high potential of one side of the filament  22 . In this case a “whisker” of wire  26 , which may be Kovar, is attached to one-of end of the filament within the post  20 , which may be accomplished by crimping the tubular post end  28  over both the filament end and the wire  26  end.  
         [0036]     The whisker of wire  26  in a preferred embodiment has a small amount of braze alloy at its outer end  26   a , and this outer end contacts the extractor cup&#39;s inner wall. The braze alloy may be attached to the wire by resistance welding, mechanical attachment or pre-melting. Its purpose is to secure the end  26   a  of the wire permanently to the inner wall of the extractor cup  24 . Thus, the temperature encountered during assembly of the tube  10  must equal or exceed the melting temperature of the alloy in order to provide the desired bond. The alloy melting temperature must be above the temperatures encountered during operation of the x-ray tube  10 .  
         [0037]     The advantage of this connection method is in establishing a very robust electrical connection that will not fail during device operation.  
         [0038]     In a variation of the above connection method, the braze alloy is omitted. The wire  26  is springy and remains springy under operation temperature, maintaining firm contact with the inner extractor wall under all temperatures encountered.  
         [0039]      FIG. 2  shows another variation of the filament-attached wire scheme shown in  FIG. 1 . In this form of connection, a wire  30 , preferably of platinum or other soft conductive metal, is again co-crimped together with the cathode filament  22  at the upper end  28  of one filament support post  20 , which may be of the material Kovar. In a preferred embodiment the wire  30  has a diameter of about 0.002 inch. The other end of the soft wire  30  is laid down over the edge of the glass preform base  16  as shown in  FIG. 2 . The extractor cup  24  has a bore or rim  32  which is just slightly larger than the glass preform  16  at the bottom, and when the extractor cup is pressed down over this glass preform, a firm electrical connection is made with the interior metal or metalized surface of the extractor cup. This assembly pinches and swages the soft wire  30 .  
         [0040]     When the glass preform is heated and partially melted, this locks the extractor  24  in place and assures a continued electrical connection.  
         [0041]     To prevent severing of the wire  30 , the glass preform needs a soft edge, which can be achieved by grinding. The relative diameters of the extractor bore  32  and the preform base  16  are also important, since there must be some gap space to prevent pinching off the wire. Although platinum wire is preferred, other metals such as gold could also be used. If the wire has excess length, it is trimmed off the bottom of the extractor cup after assembly of the extractor cup.  
         [0042]      FIGS. 3 and 4  show another arrangement for connecting high voltage to an extractor cup in the cathode of an x-ray tube. In  FIG. 3 a  pair of filament support posts  35  and  36  support a filament  38 , surrounded by an extractor cup  40 . The two legs of the filament  38  may be wound around the conductive support posts or pins  35  and  36 , as generally and schematically shown in  FIG. 3 , and firmly secured thereto. One post  35  is longer than the other post  36 , and may be placed wider from center, but in any event is placed wider than the opening  42  of the extractor cup, as shown. On assembly, the extractor cup is placed over the cathode filament such that the longer post  35  engages against the top inner surface of the extractor cup  40 , as shown, making electrical contact. Another advantage of this type of assembly and connection is that the filament position relative to the top of the extractor cup and the opening  42  is closely controlled by the length of the post  35 .  
         [0043]      FIG. 4  shows a variation of the above, wherein the one filament support post  35   a  need not be set widely, the post being curved outwardly at its upper end  35   b , where contact is made with the interior of the extractor cup  40 .  
         [0044]     In  FIG. 5  another embodiment uses another direct method of connection to connect high voltage from the cathode to the extractor cup. In this case the direct connection comprises a pigtail  44  extending from the filament beyond one of the filament support posts  46 . The support posts or pins  46  and  48  are preferably crimped over the filament  50  generally as shown, with an extending tail  44  directly in contact with the wall of the extractor cup  40 . The filament pigtail  44  may be connected to the wall by a braze alloy, with the connection made in the embodiment of  FIG. 1 , or the filament pigtail may simply act as springy wire which maintains contact with the extractor including during the high temperature operation of the tube.  
         [0045]      FIG. 6  shows another variation for direct connection with the extractor cup  40   a . The filament  52  in this arrangement is secured to only a single filament support post or pin  54 , and extends to the extractor cup  40   a , where it is permanently secured and where the filament is supported. The extractor cup may have a side slot or hole  40   b  for receiving the end or leg of the filament  52 . The side hole or slot  40   b  can be filled with a conductive material that cures upon firing. Alternatively, the end of the filament  52  could be brazed to the extractor wall (without a slot) or it could be coated with a braze alloy and permanently secured to the wall upon heating, as in the embodiment of  FIG. 1 . In this form of connection, the extractor cup serves as one lead of the filament power source, and it is connected to a lead  56  extending up from the base of the cathode assembly and from the catheter (not shown), then connected by a conductor  58  to the wall of the extractor cup  40   a . If the lead  56  reaches the surface of the base  60 , which may in some embodiments comprise a seal material, then the electrical connection  58  can comprise the material that seals the extractor cup to the base. The lead  56  may extend to a position to be bonded directly to the extractor cup, or it may be forced into contact with the side of the extractor when the extractor is assembled onto the base  60 . This arrangement is useful for smaller tube diameters, in that only a single power post is needed inside the extractor. It is also useful if coaxial conductors are used as leads to the filament, generally as shown in  FIGS. 11-14 , but with only the center conductor extending up into the extractor and a filament between the center conductor and the wall.  
         [0046]      FIG. 7  shows another arrangement for connecting an extractor cup to high voltage. In this assembly the seal  60 , which may comprise a glass preform as in previous embodiments, supports a pair of filament posts or pins  62  and  64 . The cathode filament is shown at  66 , crimped or otherwise retained to the top ends of the posts or pins  62 ,  64 . An extractor cup  68  surrounds the filament and posts, and the extractor is assembled against or over the edge of the glass preform base  60 . In this case the filament lead or post  62  is connected to the extractor by use of a vacuum stable conductive metallic paste or paint  70 .  FIG. 7  shows this conductive metal film  70  extending around and in contact with the bottom end of the post or pin  62  and also contacting the extractor cup  68 . The material  70  is a precursor cured by thermal processing to form the conductive metallic connector. For this purpose, reduced nickel oxide and organometallic gold inks were used successfully. This precursor material is applied by painting it in the area as shown, followed by thermal processing. Application can be with a brush, a paint preform (plastic tape with metallizing powder embedded), or with a needle applicator.  
         [0047]      FIG. 8  illustrates a connection method in which conductive metal is evaporated onto surfaces to connect one of the filament supporting posts or pins  72 ,  74  to the extractor cup  76 . The filament  78  of the cathode is coated with a conductive material that will evaporate off and be deposited onto adjacent surfaces when the filament is heated. Gold is one preferred material. In this case a shield  80  is connected to the filament post  74  which is not to be connected to the extractor.  
         [0048]     When the assembly has been made and the tube evacuated, the filament coating is evaporated off, as in a vacuum evaporation process. The filament is powered to raise it to a prescribed temperature, and this causes the gold to flash off the filament and to be deposited on the inside of the extractor cup and onto the base  82  and against the one filament support post or lead  72 . This forms a high-integrity connection between the base of the conductive post or pin lead  72  and the wall of the extractor cup. In addition, the inside of the extractor cup is coated with the conductive material, and if it is gold, this will reflect infrared radiation very well, thereby lowering the heat loss to the wall of the extractor cup and reducing power required to operate the filament  78  at a given temperature.  
         [0049]      FIG. 9  shows a variation of the above. This connection scheme is very similar to that of  FIG. 8 , but without the shield  80  to shadow an area of the base  82 . In this method the filament is coated with an evaporating semiconductor, so that the coating connects both the filament posts or pins  72 ,  74  to the extractor cup  76  via deposit on the base surface  82 . If the coating is in the thousands of ohms resistance, then the power loss in the coating will be very low, and the extractor cup will still remain at filament potential. The resistance can be about 200,000 to 300,000 ohms, up to about 1 megaohm. The resistive nature of the connection will also aid in reducing arcing and damage due to arcs, and will tend to drain off excess charge built up on the extractor. The excess charge builds up due to being struck by free electrons. This develops a voltage which will tend to flow to lower potential via available conductors. How fast the charge builds up, the maximum allowable voltage difference and the rate the charge is drained off determine if the connection is sufficient to do the job. Cutoff is a couple of volts above the filament voltage. The charge delivered will develop a voltage based on the capacitance of the extractor and the rate of drain.  
         [0050]      FIG. 10  shows in a plan view or flat view a connector  84  that may be placed in the cathode assembly to make the connection between a filament support pin and the extractor wall (pin and wall not shown). The connector element may be used above or below a glass preform base such as shown in previous embodiments. A braze preform wire can be placed around or against one of the filament pins or posts and, during the thermal cycle to flow the glass preform, the braze material will melt and create an electrical path between that filament post and the extractor. A braze alloy that melts below 900° C. preferably is selected, as glassing temperature typically is about 950° C. Instead of a wire, the preform can be shaped from braze foil as in the shape  84  shown in  FIG. 10 . Such a braze foil might be about 0.002 to 0.003 inch thick, and it can be chemically machined into a shape such as shown in  FIG. 10 , to match the geometry of a cathode assembly so as to conform closely to a filament post or pin at a small-radius end  86  and to conform to the wall of the extractor cup at a larger-radius end  88 .  
         [0051]      FIGS. 11-14  show further means of connecting a filament lead to the wall of an extractor cup, in an assembly using a coaxial pair of filament leads.  FIG. 11  shows a first example of such a construction. The coaxial pair of leads is shown with the outside conductor at  90  and the inside conductor at  92 , extending upwardly as a single post into an extractor cup  94 . In this embodiment the extractor cup includes a conductive bottom plate  96  with a central hole which slides down over the coaxial cable leads and will make electrical connection with the outside conductor  90  if the hole has the proper dimension. Brazing can be applied but is generally not necessary. The coaxial cable is shown extending up through a ceramic spool  98 .  FIG. 11A  shows a plan view cross-section of the  FIG. 11  assembly. Note that the inside conductor can extend up and loop over to make contact with one side of the outside conductor to serve as the cathode filament (detail not shown). In this case the filament will be somewhat off-center, and this can be compensated by eccentric positivity of the coaxial cable in the extractor.  
         [0052]     FIGS.  12   12 A and  12 B show variations wherein a conductive element is added to connect the coaxial leads  90 ,  92  with the extractor cup  94   a . Here, the extractor cup  94   a  has no bottom, but one or two conductive metal strips are inserted into the extractor to make contact between the external coaxial lead  90  and the extractor wall, providing the needed electrical connection. A single strip is shown at  100  in  FIGS. 12 and 12 A, and a pair of opposed such connector strips are shown at  100  and  102  in  FIG. 12B . Contact can be made by a tight fit or with brazing.  
         [0053]      FIG. 13  shows spring clips  104  extending radially from the coaxial cable  90 ,  92  into contact with the wall of the extractor  94   a . In addition to providing electrical connection between the outer conductor  90  and the extractor  94   a , the clips also hold the coaxial connector  90 ,  92  in place within the extractor.  FIG. 13A  shows this assembly in plan section.  
         [0054]      FIGS. 14 and 14 A show in plan section the use of a pair of wires to connect the outer coaxial lead  90  to the extractor  94   a . In  FIG. 14  the wires  106  are shown crossing over one another, whereas in  FIG. 14A  wires  107  are shown running parallel. In both cases the wires are both in contact with the outer coaxial conductor. The wires can be attached to the extractor cup by spot welding or other techniques. The distance between the wires, undeflected, is closer than the outside diameter of the coaxial cable. Electrical contact can be provided by twisting the wires ( FIG. 14 ), which are somewhat springy, and sliding the coaxial cable, i.e. the outer conductor  90 , between them. The distance between the two wires, in both  FIGS. 14 and 14 A, is smaller than the outer diameter of the coaxial cable to provide a tight fit and good contact.  
         [0055]      FIGS. 15 and 16  show an arrangement similar to  FIG. 14 , with a spring wire or spring strip  110  providing a conductive path between a filament support lead post  74  and an extractor  76 . In this case a single wire  110  is used, and the filament leads are not coaxial as in  FIG. 14 . The springy strip or sheet of foil or whisker  110  can be spot welded to the filament post  74 , and in constant spring compression against the wall of the extractor cup  76 . The spring material can be one of the nickel alloys such as Hastalloy or Kovar that can be welded and remains springy at 300° to 400° C. Tungsten, Molybdenum stainless steel can also used. The strip can take the form of a foil or wire as well as the flat strip  110  shown in  FIG. 16 .  
         [0056]      FIGS. 17-19  show a further embodiment of a connection scheme. In this arrangement a plate  112  is included on the bottom of an extractor cup  76  as shown schematically in  FIG. 19 . The plate has an oblong hole  114  through which the filament leads  72 ,  74  are extended, these leads supporting a filament  78 .  FIG. 17  shows that the opening  114  can be generally D-shaped, with the long edge of the D lying parallel to the two posts  72  and  74  upon initial assembly. The opening  114  could be oval, elliptical, other oblong shapes or even circular, as long as it is non-symmetrically positioned about the two leads  72 ,  74 . Once the filament and posts have been inserted into the extractor cup through the hole  114 , the extractor cup and bottom plate  112  are rotated, about  900  or sufficiently to firmly place a wall of the plate opening  114  into engagement with one lead  72  of the cathode assembly. The extractor cup is glassed or brazed into position after proper assembly. The extractor could be already in place, glassed to the frame, and the filament assembly rotated to make contact. In this case the filament assembly would be heated to seal it into the frame and fix the relationship with the extractor cup.  
         [0057]      FIG. 20  and  FIG. 21  show a simple mechanical connection for placing high voltage potential at the extractor cup. In the schematic view of  FIG. 20 , the inner wall  120  of an extractor cup is indicated, along with two filament support posts or pins  122  and  124 . As discussed above, the cathode filament  126  is crimped to the top ends of these two conductive metal posts or pins in several embodiments, to secure and electrically connect the filament to the posts. During the attachment of the filament to the posts, which may be Kovar, a crimping tool is used. The crimp plastically deforms the Kovar around the filament wire. In this arrangement shown in  FIG. 20 , a non-symmetric crimp is used on the pin  126 , in order to form an oblong shape that will contact the inner wall  120  of the extractor cup. The shape of this deformation can be set by the geometry of the crimping tool  128  as shown in  FIG. 21 . The crimping tool jaws can be machined non-symmetrically at  130 , to form the elongated, oblong crimp. A standard crimp forming cavity  132  can also be included on the tool, to form the crimp at  122  in  FIG. 20 . As an alternative to this method, an upset can be put in one of the posts to cause contact between the post and the cup.  
         [0058]      FIG. 22  shows a variation wherein the extractor cup  24  is connected not to the cathode filament  22  or either end of the filament, but to a third conductor  140 . This third conductor  140 , also at high voltage and electrically isolated from the two HV filament leads  18  and  20 , allows the extractor to be electrically biased with respect to either of the HV leads  18 ,  20  independently. This permits a level of electronic control of the availability of electrons to the anode (electronic gain control). As seen in  FIG. 22 , one arrangement for connecting this third HV conductor  140  to the extractor  24  is similar to what is shown in  FIG. 2 ; the conductor wire  140  is positioned over the edge of the insulative base  16  such that the metal extractor cup  24  will crimp or deform the wire  140  as the cup is assembled onto the base  16 , thus making a good electrical contact.  
         [0059]     The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.