Abstract:
An arc discharge metal halide lamp having a discharge chamber with visible light permeable walls bounding a discharge region through which walls a pair of electrode assemblies are supported with interior ends thereof positioned in the discharge region spaced apart from one another. These electrode assemblies each also extend through a corresponding capillary tube affixed to the walls to have exterior ends thereof positioned outside the arc discharge chamber. At least one of these electrode assemblies comprises an electrode discharge structure with a discharge region shaft extending into the capillary tube corresponding thereto. A discharge region shaft extends outwardly in that corresponding capillary tube to be in direct contact with an interconnection shaft extending outside of that corresponding capillary tube to provide an exterior end of this electrode assembly, and which is in direct contact with a sealing cap over the end of the tube.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to high intensity arc discharge lamps and more particularly to high intensity arc discharge metal halide lamps having high efficacy.  
         [0002]     Due to the ever-increasing need for energy conserving lighting systems that are used for interior and exterior lighting, lamps with increasing lamp efficacy are being developed for general lighting applications. Thus, for instance, arc discharge metal halide lamps are being more and more widely used for interior and exterior lighting. Such lamps are well known and include a light transmissive arc discharge chamber sealed about an enclosed a pair of spaced apart electrodes, and typically further contain suitable active materials such as an inert starting gas and one or more ionizable metals or metal halides in specified molar ratios, or both. They can be relatively low power lamps operated in standard alternating current light sockets at the usual 120 Volts rms potential with a ballast circuit, either magnetic or electronic, to provide a starting voltage and current limiting during subsequent operation.  
         [0003]     These lamps typically have a ceramic material arc discharge chamber bounding a discharge region that usually contains quantities of metal halides such as CeI 3  and NaI, (or PrI 3  and NaI) and T1I, as well as mercury to provide an adequate voltage drop or loading between the electrodes, and also an inert low ionization potential starting gas. A pair of electrodes is arranged on opposite ends of the discharge tube extending from outside the tube into the discharge region to allow electrical energization to occur in that region. Such lamps can have an efficacy as high as 145 LPW at 250 W with a Color Rendering Index (CRI) higher than 60, and with a Correlated Color Temperature (CCT) between 3000K and 6000K at 250 W.  
         [0004]     Referring to  FIG. 1  in describing such a lamp in more detail, a typical arc discharge metal halide lamp,  10 , known in the prior art is shown in a side view having a bulbous, transparent borosilicate glass envelope,  11 , fitted into a conventional Edison-type metal base,  12 . Lead-in, or electrical access, electrode wires,  14  and  15 , of nickel or soft steel, each extend from a corresponding one of the two electrically isolated electrode metal portions in base  12  parallely through and past a borosilicate glass flare,  16 , positioned at the location of base  12  and extending into the interior of envelope  11  along the axis of the major length extent of that envelope. Electrical access wires  14  and  15  extend initially on either side of, and in a direction parallel to, the envelope length axis past flare  16  to have portions thereof located further into the interior of envelope  11  with access wire  15  extending after some bending into a borosilicate glass dimple,  16 ′, at the opposite end of envelope  11 . Electrical access wire  14  is provided with a second section in the interior of envelope  11 , extending at an angle to the first section that parallels the envelope length axis, by having this second section welded at such an angle to the first section so that it ends after more or less crossing the envelope length axis.  
         [0005]     Some remaining portion of access wire  15  in the interior of envelope  11  is bent at an obtuse angle away from the initial direction thereof parallel to the envelope length axis. Access wire  15  with this first bend therein past flare  16  directing it away from the envelope length axis, is bent again to have the next portion thereof extend substantially parallel that axis, and further along bent again at a right angle to have the succeeding portion thereof extend substantially perpendicular to, and more or less cross that axis near the other end of envelope  11  opposite that end thereof fitted into base  12 . The succeeding portion of wire  15  parallel to the envelope length axis supports a conventional getter,  19 , to capture gaseous impurities. Three additional right angle bends are provided further along in wire  15  to thereby place a short remaining end portion of that wire below and parallel to the portion thereof originally described as crossing the envelope length axis which short end portion is finally anchored at this far end of envelope  11  from base  12  in glass dimple  16 ′.  
         [0006]     A ceramic arc discharge chamber,  20 , configured about a bounded or contained region as a shell structure having polycrystalline alumina walls that are translucent to visible light, is shown in one of various possible geometric configurations in  FIG. 1 . Alternatively, the walls of arc discharge chamber  20  could be formed of aluminum nitride, yttria (Y 2 O 3 ), sapphire (Al 2 O 3 ), or some combinations thereof. Discharge chamber  20  is provided in the interior of envelope  11  which interior can otherwise either be evacuated, to thereby reduce the heat transmitted to the envelope from the chamber, or can instead be provided with an inert gaseous atmosphere such as nitrogen at a pressure greater than 300 Torr to thereby increase that heat transmission if operating the chamber at a lower temperature is desired. The region enclosed in arc discharge chamber  20  contains various ionizable materials, including metal halides and mercury, which emit light during lamp operation and a starting gas such as the noble gases argon (Ar), xenon (Xe) or neon (Ne) or some mixture thereof.  
         [0007]     In this structure for arc discharge chamber  20 , as better seen in the cross section view thereof in  FIG. 2 , a pair of polycrystalline alumina, relatively small inner and outer diameter truncated cylindrical shell portions, or capillary tubes,  21   a  and  21   b , are each concentrically joined to a corresponding one of a pair of polycrystalline alumina end closing disks,  22   a  and  22   b , about a centered hole therethrough so that an open passageway extends through each capillary tube and through the hole in the disk to which it is joined. These end closing disks are each joined to a corresponding end of a polycrystalline alumina tube,  25 , formed as a relatively large diameter truncated cylindrical shell with the inner diameter thereof designated as D, so as together to be about the enclosed region in providing the primary arc discharge chamber. The total length of the enclosed space in chamber  20  extends between the junctures of tubes  21   a  and  21   b  with the corresponding one of closing end disks  22   a  and  22   b . The length of primary central portion chamber structure  25  of chamber  20  extends between the junctures therewith and each of closing end disks  22   a  and  22   b . These various portions of arc discharge tube  20  are formed by compacting alumina powder into the desired shape followed by an initial sintering of the resulting compact to thereby provide the preformed portions, and the various preformed portions are joined together by a final sintering to result in a preformed single body of the desired dimensions having walls impervious to the flow of gases.  
         [0008]     Chamber electrode interconnection wires,  26   a  and  26   b , of niobium each extend out of a corresponding one of tubes  21   a  and  21   b  to reach and be attached by welding to, respectively, access wire  14  at its end portion crossing the envelope length axis and to access wire  15  at its portion first described as crossing the envelope length axis. This arrangement results in chamber  20  being positioned and supported between these portions of access wires  14  and  15  so that its long dimension axis approximately coincides with the envelope length axis, and further allows electrical power to be provided through access wires  14  and  15  to chamber  20 .  
         [0009]      FIG. 2  shows the discharge region contained within the bounding walls of arc discharge chamber  20  that are provided by structure  25 , disks  22   a  and  22   b , and tubes  21   a  and  21   b  of  FIGS. 1 and 2 , and shows in cross section view the electrode arrangements having capillary tubes  21   a  and  21   b  and the corresponding electrodes extending therethrough into the discharge region in greater detail. Chamber electrode interconnection wire  26   a , being of niobium, has a thermal expansion characteristic that relatively closely matches that of tube  21   a  and that of a glass frit,  27   a , affixing wire  26   a  to the inner surface of tube  21   a  (and hermetically sealing that interconnection wire opening with wire  26   a  passing therethrough) but cannot withstand the chemical attack resulting from the forming of a plasma in the main volume of chamber  20  during operation. Thus, a tube or wrapped foil of niobium,  28   a , is used to connect a cermet lead-through rod,  29   a , which can withstand operation in the plasma, to one end of interconnection wire  26   a  by welding where this end is also surrounded by a portion of frit  27   a  in a hermetic seal. The other end of lead-through rod  29   a  has one end of a tungsten main electrode shaft,  31   a , positioned thereagainst and connected thereto by laser welding.  
         [0010]     In addition, a tungsten electrode coil,  32   a , is integrated and mounted to the tip portion of the other end of first main electrode shaft  31   a  by press fitting, so that an electrode,  33   a , is configured by main electrode shaft  31   a  and electrode coil  32   a . Electrode  33   a  is formed of tungsten for good thermionic emission of electrons while withstanding relatively well the chemical attack of the metal halide plasma. Lead-through rod  29   a  serves to dispose electrode  33   a  at a predetermined position in the region contained in the main volume of arc discharge chamber  20 . This configuration results in lower temperatures in the sealing regions in capillary tube  21   a  during lamp operation since electrode  33   a , in extending through this capillary tube into the chamber discharge region a significant distance, is thereby spaced further from the seal region in capillary tube  21   a  as is then the discharge arc established between this and the opposite end electrode during operation.  
         [0011]     A portion of first main electrode shaft  31   a  is spaced from tube  21   a  by a molybdenum coil,  34   a , having one end thereof welded to the interior end of cermet rod  29   a  that is positioned in frit  27   a . Since tungsten rod  31   a  with electrode coil  32   a  mounted thereon to form electrode  33   a  must be placed in the corresponding end of capillary tube  21   a  and then positioned to extend into the discharge region in arc discharge chamber  20   a  selected distance after the fabrication of that chamber has been completed, the inner diameter of capillary tube  21   a  must have inner diameters exceeding the outer diameter of the electrode coil  32   a . As a result, there is a substantial annular space between the outer surface of tungsten rod  31   a  and the inner surfaces of capillary tube  21   a  which must be taken up in part by the provision of molybdenum coil  34   a  around and against the corresponding portion of tungsten rod  31   a  to complete the interconnections thereof and reduce the condensation in these regions of the metal halide salts occurring in chamber  20  during lamp operation. A typical diameter for both interconnection wire  26   a  and cermet rod  29   a  is 0.9 mm, and a typical diameter of electrode shaft  31   a  is 0.5 mm.  
         [0012]     Similarly, in  FIG. 2 , chamber electrode interconnection wire  26   b  is affixed by a glass frit,  27   b , to the inner surface of tube  21   b  (and hermetically sealing that interconnection wire opening with wire  26   b  passing therethrough). A niobium material tube or wrapped foil,  28   b , is used to connect a cermet lead-through rod,  29   b , to one end of interconnection wire  26   b  by welding where this end is also surrounded by a portion of frit  27   b  in a hermetic seal, and the other end of lead-through rod  29   b  has one end of a tungsten main electrode shaft,  31   b , laser welded to it. A tungsten electrode coil,  32   b , is integrated and mounted to the tip portion of the other end of the first main electrode shaft  31   b  by press fitting, so that an electrode,  33   b , is configured by main electrode shaft  31   b  and electrode coil  32   b  which is disposed at a predetermined position in the discharge region of chamber  20  to thereby provide sufficiently lower temperatures in the corresponding seal region. A portion of second main electrode shaft  31   b  is spaced from tube  21   b  by a molybdenum coil,  34   b , connected by welding to the interior end of cermet rod  29   b  and fills in part the resulting annular space therebetween needed to allow electrode  33   b  to pass, the outer end of that coil also being in frit  27   b . A typical diameter for both interconnection wire  26   b  and cermet rod  29   b  is also 0.9 mm, and a typical diameter of electrode shaft  31   b  is again 0.5 mm.  
         [0013]     These electrode arrangements have “compromise” properties components in the seal regions within capillary tubes  21   a  and  21   b , these being outer electrode parts of cermet rods  29   a  and  29   b  which provide good thermal expansion matching to the polycrystalline alumina but which are expensive to manufacture. The exposure length of each of outer electrode portions  26   a  and  26   b  must be limited thus requiring the presence of the bridging middle part of the electrode arrangement, typically a cermet rod as above or possibly a molybdenum wire or rod, between such outer electrode portion and the corresponding tungsten electrode portion. Special welding techniques such as laser welding are necessary to join the ends of tungsten electrode rods  31   a  and  31   b  to the ends of cermet rods  29   a  and  29   b , respectively. Furthermore, as a brittle materials cermet rods  29   a  and  29   b  cannot be resistance welded to outer lamp parts and so they are affixed to the corresponding ones of interconnection wires  26   a  and  26   b  with corresponding ones of niobium sleeves  28   a  and  28   b  by use of laser welding.  
         [0014]     Care must also taken to ensure that the melted sealing frits  27   a  and  27   b  flow completely around and beyond the corresponding niobium rods to thereby form a protective surface over the niobium against the chemical reactions due to the halides preventing condensation of salts. The frit flow length inside the corresponding capillary tube needs to be controlled very precisely. If the frit length is short, the niobium rod portion of the electrode is exposed to chemical attack by the halides. If this length is excessive, the large thermal mismatch between the frit and the solid middle electrode portion molybdenum, tungsten or cermet rod following inward from the niobium rod leads to cracks in the sealing frit or polycrystalline alumina, or both, in that location.  
         [0015]     In these circumstances, of course, other ceramic arc discharge chamber constructions for ceramic metal halide lamps that make use of different sealing methods or structural arrangements have been resorted to. These include methods such as direct sintering of polycrystalline alumina to the electrode arrangement, the use of cermets in and about electrode arrangements or substituting other alternative materials in such electrode arrangements, frit position limiters and graded temperature coefficient of expansion seals, or even the use of new arc tube materials that enable straight sealing of the tube body to a single material electrode such as molybdenum or tungsten.  
         [0016]     However, these alternative methods have not yet been able to demonstrate an overall advantage with respect to improved lamp performance, lower cost, or compatibility with simpler lamp factory processes. Thus, a further alternative structural arrangement has been used in which a metal lid is welded to the electrode arrangement in an air-tight joint and a metal pipe or sleeve over the outside of the chamber capillary tube in which the electrode arrangement is positioned is sealed against this lid with a first melted and then resolidified frit seal. Such a configuration, however, prevents the escape of gases during formation of this frit seal leading to voids therein and increasing pressures that result in repositioning parts of the molten frit perhaps even violently. Thus, there is a desire to provide another sealed electrode structure for the arc discharge chamber that avoids cracks in some portion thereof due to thermal mismatches between materials and voids in sealing materials to thereby provide an more reliable structure at lower costs.  
       BRIEF SUMMARY OF THE INVENTION  
       [0017]     The present invention provides an arc discharge metal halide lamp for use in selected lighting fixtures having a discharge chamber with visible light permeable walls bounding a discharge region through which walls a pair of electrode assemblies are supported with interior ends thereof positioned in the discharge region spaced apart from one another. These electrode assemblies each also extend through a corresponding capillary tube affixed to the walls to have exterior ends thereof positioned outside the arc discharge chamber. At least one of these electrode assemblies comprises an electrode discharge structure located at the interior end thereof, the electrode discharge structure having a discharge region shaft extending into the capillary tube corresponding thereto to be in electrical contact with an interconnection shaft either directly or through an intermediate connection with the interconnection shaft having a portion extending outside of that corresponding capillary tube to provide the exterior end of this electrode assembly which is in direct contact with a sealing cap provided over the end of the tube. Such an arrangement can also be provided for the other electrode assembly.  
         [0018]     The interconnection shaft is sealed in the corresponding capillary tube with a sealing frit with this shaft either having the other end of a helical coil wound there about or being provided by an extended end of the helical coil. A spatial volume occupying structure can be used to reduce the amount needed of such frit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a side view, partially in cross section, of an arc discharge metal halide lamp of the present invention having a ceramic arc discharge chamber of a selected configuration therein,  
         [0020]      FIG. 2  shows a known arc discharge chamber for the arc discharge chamber of  FIG. 1  in cross section in an expanded view,  
         [0021]      FIG. 3  shows a portion of an arc discharge chamber in cross section with an embodiment of the present invention,  
         [0022]      FIG. 4  shows a portion of an arc discharge chamber in cross section with an alternative embodiment of the present invention,  
         [0023]      FIG. 5  shows a portion of an arc discharge chamber in cross section with an alternative embodiment of the present invention,  
         [0024]      FIG. 6  shows a portion of an arc discharge chamber in cross section with an alternative embodiment of the present invention,  
         [0025]      FIG. 7  shows a portion of an arc discharge chamber in cross section with an alternative embodiment of the present invention, and  
         [0026]      FIG. 8  shows a portion of an arc discharge chamber in cross section with an alternative embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0027]     In a typical arc discharge tube structure sufficient to form a reliable sealing of the electrode in each of the polycrystalline alumina material capillary tubes extending from the remainder of the polycrystalline alumina material arc discharge tube, each of the electrical conducting leads, the sealing frit and the polycrystalline alumina need to have similar thermal expansion coefficients to thereby reduce thermal stresses in the sealing regions of the arc discharge tube resulting from the large temperature increases occurring during lamp operation. The use of niobium metal cap assemblies in connection with each of the electrodes in these sealing regions will result in significantly lower thermal stresses therein over temperature changes as its thermal expansion coefficient is similar to that of polycrystalline alumina. Placing the niobium metal cap assembly outside the arc tube capillary can eliminate the possibility of chemical reaction between the niobium and metal halide fill materials.  
         [0028]     One such cap assembly electrode arrangement is shown in  FIG. 3  in a fragmentary view of a portion of arc discharge chamber  20  that includes capillary tube  21   a  with the associated electrode extending therethrough into the chamber discharge region to form an expanded partial cross section side view thereof. There, a molybdenum coil,  34   a ′, is wound around an extended length tungsten rod,  31   a ′, that extends from the discharge region of arc discharge chamber  20  through the full length of capillary tube  21   a , and continues outside beyond the end tube of that tube with this outer portion serving as a chamber electrode interconnection wire,  26   a ′. Molybdenum coil  34   a ′ also extends a few turns outside past the end of capillary tube  21   a  and the outside end of this coil is attached by crimping or spot welding to a niobium metal cap,  40   a , so that cap  40   a  will form an external seal about the electrode provided by the coil and extended tungsten metal rod  31   a ′ in sealing off the discharge region in arc discharge chamber  20 . Affixing cap  40   a  to the end of molybdenum coil  34   a ′ by crimping or spot welding serves to control the insertion length of the electrode into the discharge chamber. The use of a crimp or just a spot weld for this joining assures that an unsealed passageway is formed at this point in the sealing process elsewhere between cap  40   a  and extended tungsten metal rod  31   a ′ to thereby allow gases to escape therethrough that are formed in the melting and resolidifying of frit  27   a . Thus, in  FIG. 3 , a spot weld is shown with a concave curve representing the meniscus of the weld material on the lower side of chamber electrode interconnection wire  26   a ′ at cap  40   a.    
         [0029]     Sealing frit  27   a  with a thermal expansion coefficient chosen to match that of polycrystalline alumina and niobium, at least at the operating temperature of arc discharge chamber  20 , is used to complete this electrode seal by sealing the gap between polycrystalline alumina capillary tube  21   a  and cap  40   a . Some excess frit resolidifies outside of cap  40   a  in the gas passageway space between it and chamber electrode interconnection wire  26   a ′ at which the spot weld is absent as shown by the convex curve on the upper side of chamber electrode interconnection wire  26   a ′ at cap  40   a . Preventing reactions between the metal halide salts and cap  40   a  of niobium metal requires having sealing frit  27   a  distributed such that it conformably covers the inner surface of that cap. This glass frit also seals the gap or passageway between cap  40   a  and molybdenum coil  34   a ′ of the electrode formed by this coil and tungsten metal rod  31   a ′. During the arc discharge chamber sealing process, melted frit  27   a  should flow inwardly in the interior channel of polycrystalline alumina capillary tube  21   a  from its outer end sufficiently to cover 2 to 4 turns of molybdenum coil  34   a ′ as wrapped about extended tungsten rod  31   a ′. The coverage of the end of molybdenum coil  34   a ′ will prevent metal halide salts from accumulating on the inner surface of cap  40   a  over the duration of lamp operation such that lamp performance will not change over time. The same electrode sealing arrangement can be provided at the other end of arc discharge chamber  20  in connection with capillary tube  21   b.    
         [0030]      FIG. 4  shows, in a fragmentary partial cross section side view that includes capillary tube  21   a , a further alternative embodiment of the present invention having a different electrode being used with the cap assembly. An extended length molybdenum coil,  34   a   41  , is wound around tungsten rod  31   a  and also stretched in the portion thereof near the outer end of capillary tube  21   a  and permanently deformed into an extended helical coil in that region. This helical coil portion of molybdenum coil  34   a ″ is continued outside past the end of tube  21   a  a couple of turns after which it is straightened into an extended linear portion to form a chamber electrode interconnection wire,  26   a ″. Approximately at the point the helical coil portion of molybdenum coil  34   a ″ straightens into an extended linear portion, this coil, or wire  26   a ″, is attached to niobium metal cap  40   a  by crimping or spot welding. Again, cap  40   a  will form an external seal about the electrode provided by the coil in sealing off the discharge region in arc discharge chamber  20 , and again the use of a crimp or a spot weld avoids a seal all about wire  26   a ″ at this point in the sealing process so that a passageway is formed this time between the cap  40   a  and this linear portion of molybdenum coil  34   a″.    
         [0031]     Sealing frit  27   a  with a thermal expansion coefficient chosen to match that of polycrystalline alumina and niobium, at least at the operating temperature of arc discharge chamber  20 , is again used to complete this electrode seal by sealing the gap between polycrystalline alumina capillary tube  21  a and cap  40   a . As before, preventing reactions between the metal halide salts and the cap  40   a  of niobium metal requires having sealing frit  27   a  distributed such that it conformably covers the inner surface of that cap. This glass frit also seals the gap or passageway between cap  40   a  and linear wire  26   a ″ of the electrode formed by this coil and its extended linear portion. During the arc discharge chamber sealing process, frit  27   a  should flow in the interior polycrystalline alumina capillary tube  21   a  inwardly from its outer end sufficiently to cover 2 to 4 turns of molybdenum coil  34   a ″ as wrapped about extended tungsten rod  31   a  so as to also cover the end of that rod to again prevent metal halide salts from accumulating on the inner surface of cap  40   a  over the duration of lamp operation. Here, too, this same electrode sealing arrangement can be provided at the other end of arc discharge chamber  20  in connection with capillary tube  21   b.    
         [0032]      FIG. 5 , in another fragmentary partial cross section side view that includes capillary tube  21   a , shows another embodiment with an electrode arrangement similar to that shown in  FIG. 4  but with the omission of a stretched helical coil portion which is replaced by a longer linear extension. A molybdenum coil,  34   a ′″, again has an interior end portion thereof wound about an outer end portion of tungsten rod  31   a  but this coil has its outer end portion straightened into a linearly extending portion that begins well within the interior of capillary tube  21   a  and continues outside that tube past the end thereof as a chamber electrode interconnection wire,  26   a ′″. Interconnection wire  26   a ′″ is affixed to niobium metal cap  40   a  again by crimping or spot welding to thereby leave a passageway between them preparatory to cap  40   a  forming an external seal about the electrode provided by the coil linear extension in sealing off the discharge region in arc discharge chamber  20 . Sealing frit  27   a  completes the seal as before just as for the seal shown in  FIG. 4 , and the opposite end of the chamber with capillary tube  21   b  can be configured with the same electrode arrangement as shown in  FIG. 5 .  
         [0033]      FIG. 6  shows yet a further fragmentary partial cross section side view that includes capillary tube  21   a  with an electrode embodiment substituting a wrapped foil for the longer linear extension coil portion in the electrode shown in  FIG. 5 . Molybdenum coil  34   a  of  FIG. 1  is essentially again used with its interior end portion wound about an outer end portion of tungsten rod  31   a , and with the outer end portion thereof welded to niobium tube or wrapped foil  28   a . Tube or foil  28   a  begins well within the interior of capillary tube  21   a  and continues outside that tube past the end thereof where it is spot welded to niobium metal chamber electrode interconnection wire  26   a  and to niobium metal cap  40   a  to thereby leave a passageway between them preparatory to cap  40   a  forming an external seal about the electrode provided by foil  28   a  and wire  26   a  in sealing off the discharge region in arc discharge chamber  20 . Sealing frit  27   a  completes the seal as before just as for the seal shown in  FIG. 5 , and the opposite end of the chamber with capillary tube  21   b  can be configured with the same electrode arrangement as shown in  FIG. 6 .  
         [0034]      FIG. 7  shows the fragmentary partial cross section side view of the embodiment of  FIG. 4  with a solid polycrystalline alumina rod,  41   a , inserted within the helical coil portion stretched from molybdenum coil  34   a ″. Rod  41   a  thus has a diameter smaller than the inner diameter of this helical coil portion which from the coil of  FIG. 1  will typically be between 0.4 and 0.5 mm. Since the helical coil portion occurs in the sealing region provided by resolidified frit  27   a , the addition of polycrystalline alumina rod  41   a  reduces the volume of this sealing frit If a relatively large volume of sealing frit is provided in the sealing region, some voids in the form of spherical cavities can occur during arc discharge chamber capillary tube sealing processes which is thus alleviated by the presence of rod  41   a . Rod  41   a  should not be tightly fitted into the interior region of the helical coil portion of molybdenum coil  34   a ″ so that frit  27   a  can bond to this molybdenum helical coil over all of its surface areas including in the gap between the helical coil and rod  41   a.    
         [0035]      FIG. 8  shows the fragmentary partial cross section side view of the embodiment of  FIG. 5  with a polycrystalline alumina sleeve,  41   a ″, positioned about the linear extension portion of molybdenum coil  34   a ′″. In keeping with molybdenum coil  34   a  of  FIG. 1 , sleeve  41   a ′ has an outer diameter of 1.0 mm, an inner diameter of 0.5 mm, and, for typical choices of length for the linear extension portion of molybdenum coil  34   a ′″, a length of 3.5 mm. Here, too, as with rod  41   a  above, the presence of sleeve  41   a ′ will reduce the volume of frit glass  27   a  that is provided in the sealing region provided by resolidified frit  27   a . The presence of sleeve  41   a ′ also makes easier the wetting by frit glass  27   a  of the surfaces of structures about the gaps that are to be filled by frit glass  27   a  in the sealing region.  
         [0036]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.