CA1165373A - Refractory helical overwound electrode for high pressure metal vapor lamp - Google Patents

Abstract

REFRACTORY HELICAL OVERWOUND ELECTRODE
FOR HIGH PRESSURE METAL VAPOR LAMP

Abstract of the Disclosure An electrode for a high pressure metal vapor lamp comprises a hollow helix of tungsten wire projecting from an inlead. An open-wound overwind of tungsten wire on the turns of the helix provides quasi-point-contact spacers between adjacent turns of the helix. This arrangement gives rigidity to the helix and provides a substantial increase in electrode surface area with low axial heat flow. The electrode has good spot transfer characteristics in moving the arc terminus to the tip and has low energy loss in operation. The electrode is particulary suitable as a cathode in a miniature d.c. metal halide lamp.

Description

S3'73 REFRACTORY HELICAL OVERWOUND ELECTRODE
FOR HIGH PRESSURE METAL VAPOR ~MP
This invention relates to a self-heating electrode for high pressure metal vapor lamps. It may be used in metal halide lamps wherein conventional alkaline earth oxides cannot be used, but it is also suited to carry emission materials when needed. It lends itself particularly well to cathode designs suitable for miniature metal halide lamps not using oxide emitters and operating on d.c. with a discharge current of 1 ampere or less.
B~CKGROUND''OF THE INVEN ION
Until recently, it has been the common view that the efficacy of high intensity discharge lamps inevitably goes into a rapid decline at lower wattage ratings starting at about 250 watts, and metal halide lamps in sizes below 175 watts were considered impractical for general lighting.
However, in Canadian patent 1,111,483 issued October 27, 1981 to Daniel M. Cap and William H. Lake, titled "High Pressure Metal Vapor Discharge Lamps of Improved Efficacy"
and assigned to the present assignee, design principles are set forth which permit high efficacy to be ' achieved in previously unheard-of small sizes of lamps. New miniature metal halide discharge lamps are disclosed having envelope volumes less than 1 cubic centimeter, ratings going down to less than 10 watts, and which operate with discharge currents of .

1 ampere or less. For good efficacy, a high ra-tio of arc w~$~s whi~h produ~e light~ to el~ctro~æ ~tts which do not, is necessary. In these new lamps, a high ratio approaching those found in larger sizes is achieved by increasing the mercury vapor pressure at the same time as the discharge volume is decreased. However it is necessary to attain the electrode temperature required for adequate electron emission even with the reduced energy input, and this is achieved primarily by reducing the physical size of the electrodes, inleads and end seals in order to reduce the heat loss from them. As physical size is reduced, ~ire commensurate in fineness must be used and this tends to make manu~acture more d f~icult.
In lamps where~n the electrodes do not carry elec-tron emission material in the conventional sense of alkaline earth metal or oxides, the criterla for elec-trode design and the permissible heat loss from the electrode are much more stringent than in lamps con-taining emission material. By ~ay of example, in a mercur~ ~apor lamp containing barium o~iae as emission material which has a wor]s function of 1.5 to 2 volts, the electrode temperature should not exceed 1500 K~
By contrast, in lamps ~ithout emission material or re-lying upon the presence o~ thorium or thorium iodide inthe fill for elec~rode activation with a work function of 3O5 to 4.5 electron volts, a temperature of 2500 to 3000 K is nece~sary ~or adequate electron emission.
On the other hand at temperatures above 3300 K, tung-sten vaporizes at such a rate that the small en~elopeso~ miniature lamps blacken rapidly and this imposes another design constraint.
When a lamp is operated on alternating current, each electrode sexves alternately as cathode and anode, and the heating ~rom the cathode half cycle i5 .~

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supplemented by that from the anode half cycle, resulting in an operating temperature which is less dependent on the cathode function~ Thus, when an electrode which was receiving adequate heat for i-ts cathode function in an a.c. circuit at a given current is used as the cathode in a d.c. circuit, it may no longer receive adequate heat at the same current. It is economically advantageous to operate miniature metal vapor lamps on ~ d.c. using transistors or solid state control devices ; 10 in the starting and ballasting circuit. Accordingly, the cathode must be designed to properly manage the energy balance in order to ensure that the cathode hot spot rapidly reaches its desired operating temperature during starting and does not exceed it during running.
If this is done, cathode damage and envelope blackening will be minimized. These requirements, in addition to those stemming from the small size and low operating current, must all be met if a miniature metal halide lamp is to operate satisfactorily on d.c.
SUMMARY OF THE INVENTION
The general object of the invention is to provide a new self-heating electrode design of general utility operable with or without alkaline earth emission material and adaptable to a wide range of operating currents.
: 25 A more specific object is to provide a cathode particularly suitable for miniature metal halide arc tubes operating on d.c. discharge currents of 1 ampere or less. Our design (1) permits the electrode surface area to be maximized, (2) allows the electrode heat conduction loss into the seal area to be minimized and (3) achieves the foregoing together with structural rigidity and ease of manufacture.
According to our invention, an electrode for a high pressure metal vapor lamp comprises a hollow helix of refractory metal wire which is provided with an open-wound overwind of refractory metal wire r~

on the turns of the helix. The open-wound overwind provides quasi-point-contact spacers between adjacent turns of the helix so -that conductive heat flow is compelled to follow a long helical path. The arrangement permits an increase in total electrode surface area as compared with the surface area of the helix without the overwind, and at the same time the arrangement assures a low axial heat conduction loss into the seal area.
In a preferred embodiment suitable for a miniature metal halide lamp, the open-wound overwind on the helix is wound to provide an optimum number of quasi-point-contact spacers between adjacent turns of the helix in order to give structural rigidity without appreciable increase in axial heat flow~ The electrode comprises a helix as described which is attached to a refractory metal inlead and projects distally from the inlead. The helix can be spudded onto a shank serving as an inlead and can be terminated in a solid cap by melting back a few turns at the distal end of the helix. The wire of the helix can be torsionally preloaded in order to provide a built-in force that biases the turns of the helix together against the spacers for extra rigidity.
DESCRIPTION OF DRAWINGS
In the drawings:
FIG. 1 illustrates, to the scale shown above the figure, a miniature discharge lamp for d.c. operation - provided with a cathode embodying the invention.
FIG. 2 is an enlarged view of a cathode embodying the invention.
FIG. 3 is an enlarged view of the primary over-wind wire open-wound on the primary mandrel.
FIG. ~ is a partly sectioned side view of cathode embodying the invention, enlarged to a greater extent than that of FIG. 2.
FIG. 5 is an end view of the cathode of FIG. 4.
DETAILED DESCRIPTION
An optimized electrode design must be capable of serving through the various modes encountered in lamp ~, J

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operation such as ~reakdown, glow-to-arc transition, normal operation ~whtch may be a.c. cr d.c.), -and ~ot restart after a temporary power loss. A given design will have a specific structure and surface condition including emission mix, a specific shape including sur-face curvature whicIl may influence the electric field, a specific mass distribution, and a particular ther-mal balance ~etween heat generation and heat loss in the structure. Our ~nventton provides an electrode de-sign which ofers a ~ide choice of independent param-eters which may be varied to achieve the desired opti-mization. Among these parameters are the refractory metal (e.g. tungsten) chosen for the structure, the emission material such as a coating if used, and parti-lS cularly the physical dimensions inherent in the struc-ture as will appear hereafter such as mandrel diameter, overwind diameter, over~ind pitch on the mandrel, tight-ness of the overwind on the mandrel, shank or inlead diameter and insertion length, overall length of the electrode and tip or end cap size. A11 of these may be varied to achieve t~e desired optimization.
When an electrode operates as a cathode on d.c., the forces that drive the point of arc attachment toward the tip are muc~ less than when the same electrode op-erates alternately as cathode and anode on a.c. This isbecause when an electrode operates as anode, electrons are collected at the point of least separation from the opposite electrode, namely at the tip, and the tip is hea~ed up thereby. Thus on a.c. operation the tip tem-perature is built up on successi~e anode half-cycles.
Such temperature build~up increases emission at the tip and facilitates transfer of the hot spot thereto on the ca-thode half-cycle. On d.c. operation, there is no such force driving the hot spot towards the tip. However our electrode design provides another driving force making it particularly suitable for d.c. operation.
The driving force arises through the resistance loss in the long spiral path which the current must follow coupled with the thermal insulation between electrode tip and the heat sink at the seal. With our design, the transfer characteristics in moving the arc terminus to the tip rapidly and without damage to the electrode are much superior to those of conventional electrodes using a winding around the shank with the shank tip protruding through it.
While our invention is useful in any size of lamp including high curren-t lamps, it is particularly valuable for miniature metal halide lamps such as those described in the previously-mentioned patent of Cap and Lake. An example of a miniature metal halide lamp is illustrated in FIG. 1 comprising a small arc tube 1 whose size may be judged from the centimeter scale shown above. By way of specific example, in a 35 watt lamp such as illustrated, the internal diameter of the arc chamber is from 6 to 7 millimeters. The envelope is made of quartz or fused silica and comprises a central bulb por-tion 2 which may be formed by -the expansion of quartz tubing, and neck portions 3,3' formed by collapsing or vacuum sealing -the tubing upon molyb-denum foil portions 4,4' of electrode inlead assemblies.Leads 5,5' welded to the foils project externally of the necks while electrode shanks 6,6' welded to the opposite sides of the foils extend through the necks into the bulb portion. The cathode shank 6 may be tungsten, or alternatively molybdenum which reduces tendency to back-arcing.
A suitable filling for the enevelope comprises ' argon or other inert gas at a pressure of several tens of torr to serve as starting gas~ and a charge com-prising mercury and one or more metal halides. A pre-:, 65~73 L~ 7759-- 7 --ferred filling comprises MaI, ScI3 and ThI4. The charge ma~ be introduced into ~he a-~c ch~amber through one o~
the necks before sealing in the second electrode; in such case the arc chamber portion is chilled during the heat S sealing of the neck to prevent vaporization of the charge.
Alternatively, the charge may be introduced through an exhaust tube extending from the side of the ~ulb which is then eliminated by tipping of. The arc tube is usually mounted within an outer pro~ective envelope or jacket ~not shown~ having a base to whose contact ter-minals the inleads 5~S' of the arc tube are connected.
In a direct current lamp, the anode is simply an electron collector and a conductor such as lead 6' pro-jecting into the envelope will suffice providing it has sufficient heat~dissipating capacity. The anode is made of refractory metal, suitably tungsten, and its tip may be eroded slowly during operation. In order to reduce such erosion and stabilize operation, an en-larged head or ball 7 may be provided on the tip of lead 6'. Such a ball is readily formed by directing a plasma -torch on the upper end of the wire while it is held upright. By way of example of dimensions, for the illustrated lamp the anode is tungsten and the shank may be 9 mil and the ball 7, 20 mil in dlameter.
Z5 The invention relates particularly to the cathode structure 10 formed upon or attached to the end or in-lead 6. As best seen in FIG~ 2 or in FIGS. 4 and 5, the cathode proper comprises a hollow helix 11 which may be described as consi~ting of a coiled primary mandrel 12 around whose coiled turns i5 wound a smaller primary wire forming an overwind 13. sath primary mandrel and primary wire are retained in the comple.ed cathode.
The wires are of tungsten or of other refractory me-tal suitable for electrodes. As shown ln FI~. 3, the over-3, wind wire 13' is wound around mandrel 12', such that io53~73 when the composite of 12' and 13' is tightly coiled a-round a secondary ~andrel to pr~duce t~e helica} struc~
ture 11, the turns of the over~ind or primary wire inier-digitate, producing a spacing of one primary wire di~
ameter 13 between tl~e turns 12 of the primary mandrel forming t~e main ~elix 11.
Our helical electrode with overr~ind meets the pre-viously stated criteria ~or good design, namely maxi-mized surface area for more rapid glow-to-arc transi-tion~ and controlled heat conduction loss into th~ seal.The surface area o~ the helix is large by comparison with that o~ a shank type electrode, even one with an overwind. The long helical path ~hich conduction heat from the electrode tip must follo~ greatly reduces the loss of heat ~y comparison with that in a shank type electrode. The overwind by providing points of support between turns of the helix assures structural rigidity which is particularly difficult to achieve in a small electrode.
The density of overwind turns 13' o~ the mandrel wire 12l, that is the pitch ratio, is determined by con-siderations o~ electrode sur~ace, thermal conductivity and structural rigidity with compromises or txade-off between these. It is desirable for stability to approach at least 3 evenly distributed spacers or xest points per turn of the primary mandrel (meaning that the angular interval between rest points cannot ~e much less than 120); less than 3 reduces rigidity and onl~ 2 rest points per turn corresponding to 180 between rest points) is of course inadequate. A density or pitch ratio of 1-1/2 turns of overwind ~ire 13' per turn of primary mandrel 12 generates 3 rest points per turn in the electrode structure. Elowever the distribution is uncertain, and unless the rest points are evenly spaced circumferentially when the overwirld turns interdigi-tate, the rigidity may be lessened. For this reason 3~3 _ g _ a minimum of 3 over~nd turns 13 ? per turn o~ the primary mandre~l 1i2' is preferred as illustrated in the drawings~
This generates 6 rest points which assures rigidity even under the worst condltion of contiguous pairing of rest points wh~ch would e~feckively reduce the 6 to 3.
The separation and resulting thermal insulation between primary mandrel turns ~hich the overwind assures is even more important later in the li~e of the cathode when sinter-ing, in the absence of spacers, woula tend to increase the thermal contact between mandrel turns. With the structure provided by our invention, sintering merely makes the helix into a mechanically stronger structure without significant change in heat flo~ characteristics~ This ls a great ad-vantage over other structures such as loop electrodes which tend to change shape during operation of the lamp, especial-1~ when the loops are made or ~ine wire as they must in miniature lamps.
The helix 11 is attached to inlead 6 in a mannex to project distally into the envelope. A convenient way to do so is to spud the helix onto the end o~ the wire inlead or shank 6. For such an attachment, the bore (diameter of the axial cavity) of the helix is made slightly less than that of the inlead; this causes the helix to expand slightly over the extent 15 as it is screwed onto the inlead and assures a tigh-t grip. -With our helix which caxries an overwind, the overwind pre-vents direct contact betwean wire 12 of the helix and the shank 6 and thus assures low-thermal conduction into ~ the shank. Another convenient manner of attachment is ; 30 by welding which may be used where greater thermal con-duction into the shank is desira~le. In ~IG. ~, the helix is shorter and ~ewer turns are spudded onto the shank or inlead than in FIG. 2; our electrode config-uration facilitates such variations in design to achieve the desired heat balance. The portion 15 of the elec-trode which grips the shank 6 may be embedded in the .5;3'73 LD 775g silica as shown in FIG. li embedding makes it easier to center the electrode in the bulb at the sealing step in lamp manu~acture.
- In FIGS. 1 and 2, the electrode 10 is ter~inated in a solid cap 14. Such an end provides a place where a hot spot may ~evelop where the arc attaches during normal operation and reduces the rate of erosion o~ the electrode. An end cap may be formed simply by heating the end O F the electrode~ suitably by a plasma torch, and melting back the last few ~urns of he helix. A1-ternatively, a small mass of suitable refractory metal may be welded or sintered to the distal end of the helix . portion 11.
In FIG. 4 the helix is not terminated by a solid end cap and FIG. 5 merely shows the electrode in end view. Some sintering together of the helix wire 12 and overwind wire 13 ~ill occur in operation, particularly at the distal end where the hot spot attaches in opera-tionn In a metal halide lamp wherein thorium.iodide is present, a bare electrode may be used as illustxated; in .~ other metal ~apor lamps a coating of electron-emissive material may be desirable and, in such case, the helical structure and the overwind are useful to retain the coat-ing~
~- 25 According to an optional feature of the in~ention .. even greater structural rlgidit~ may be achieved by pre-~ loading or over~inding the turns o~ the helix~ Greater : rigidity may become relatively more important as lamp and electrode are miniaturi~ed. When line or wire is coiled, there i5 an inherent or equi~alent twist of 360 per : loop put into it~ This is rea~ily seen when one pulls a line sideways off a spool instead of unrolling it from the spool; a 350 twist appears in the line for every loop pulled ofE the spool. If a twist greater than 360 per loop i9 put into the line, it may be said to be over-twisted or preloaded and the result is a built-in tor-5~3 sional stress that, in the ca~e of a close-wound heli~
o~ resilient wire, biases the turns laterally together and maintains them in tig!lt side-by-side contact. This may be observed in preloaded springs, such as those fre-quently used to close screen doors of houses; the springwill not stretch and the turns t~ill not open up at all until a certain minimllm force is excee~ed, and t~ere-alter the stretch is proportional to the excess of ~orce over the minimum. This minimum corresponds to the built-- 10 in torsional stress or pre-load that biases the turns together.
An overt~ist or preloaded condition may be achieved by putting a t~ist in the proper direction into t~e com-posite 12',13' prior to or during coiling around the sec-ondary mandrel. There are two coilings whic~ occur inthe electrode structure, that of overt~ind 13' around man-drel 12' ~FIG. 3~ and that of the composite 12,13 ~FIG.

2~ around a mandrel (not shown but corresponding general-ly to shank 6~. If both coilings are wound.in the sam~
di~ection (both left-hand or both right-hand), the ~inal structure t~ill have the overwind 13 tight on its mandrel wire 12. ~owever, i~ the coilings are wound in opposite directions, the ~inal coiling will cause overwind wire 13 to loosen on its mandrel 12. T~is generates clear-ance between the two exposing more o~ the electrode sur-face and creaiing fissures for emission mix. ~he fissure may also serve as an electrid field concentrator. This . . ef~ect.can be utiIized over a wide range to vary the physical characteristics of the electrode.
The ~ollowing is an example of a cathode in ac-cordance ~ith the invention suitable for a 35 watt metal halide lamp operating on d.c. current in the range of 200 to 500 milliamperes. The primary wire 13' is 5 mil tungsten and the primary mandrel 12' is 7 mil tung-sten wire. The primary coiling is relatively open and suitably provides an advance about equal to tt~ice mandrel ,~ j t~ '3 LD 775g diame-ter, that is 14 mil per turn, the objective being to hav~ a~pro~lmat~ly 3 tuxns of the o-~érwind around the primary mandrel per turn of the primary mandrel arouna the secQndary mandrel. Prior to coiling around the secondary mandrel, the composite wire resulking from the primary coil~ng is pretwisted approximately 1 turn per inch. If the primary coiling was conventional right-hand coliling, then the pretwisting is done with a rig~t-hand twist which has the ef~ect of loosening the overwind slightly. Finally the composite is coiled with a left-hand coiling around a 9 mil secondary mandrel (not shown). In this example the coiling sequence re-sults in a slight loosening of the overwlnd. The sec-ondary mandrel is molybdenum and is dissolved out by nitric and sulfuric acid which do not attack the tung-sten wires, and the helix is then spudded onto an inlead of more than 9 mil to assure a good grip.

Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrode for a high pressure metal vapor lamp comprising a hollow helix of refractory metal wire having an open-wound overwind of refractory metal wire on the turns of said helix, said open-wound overwind providing quasi-point-contact spacers between adjacent turns of said helix, whereby said open-wound overwind gives rigidity to said helix and provides a substantial increase in electrode surface area with low axial heat flow.

2. The electrode of claim 1, wherein the diameter of the overwind wire is smaller than the diameter of the wire of said helix.

3. The electrode of claim 2, wherein the number of said spacers per turn of said helix approaches at least three.

4. The electrode of claim 1, wherein said helix is attached to a refractory metal inlead and projects distally therefrom.

5. The electrode of claim 4, wherein said helix is spudded onto an end of said inlead.

6. The electrode of claim 5, wherein the distal end of said helix is terminated in a solid metal cap.

7. The electrode of claim 6, wherein said cap is integral with the distal end of said helix.

8. The electrode of claim 1, wherein the wire of said helix is under torsional stress forcing the turns of said helix together against said spacers.

9. A metal vapor arc tube comprising: a light-transmissive envelope containing an ionizable fill and a pair of electrodes connected respectively to a pair of refractory metal inleads sealed into said envelope, at least one electrode serving as a cathode, said one electrode comprising a hollow helix of refractory metal wire extending from the corresponding inlead and having an open-wound overwind of refractory metal wire on the turns of said helix, said open-wound overwind providing quasi-point-contact spacers between adjacent turns of said helix, whereby said open-wound overwind gives rigidity to said helix and provides a substantial increase in electrode surface area with low axial heat flow to the corresponding inlead.

10. The arc tube of claim 9, wherein said helix is spudded onto an end of the corresponding inlead and prjects distally therefrom.

11. The arc tube of claim 10, wherein the envelope is fused silica and at least part of the spudded end of the inlead is embedded in the fused silica of the envelope.

12. The arc tube of claim 10, wherein the distal end of said helix is terminated in a solid metal cap.

13. The arc tube of claim 12, wherein said cap is integral with the distal end of said helix.

14. The arc tube of claim 9, wherein the wire of said helix is under torsional stress forcing the turns of said helix together against said spacers.

150 The arc tube of claim 9 for d.c. operation, wherein the other electrode is a solid conductor of refractory metal.

16. The arc tube of claim 9 for a.c. operation, wherein both electrodes serve alternately as cathode and are constructed like said one electrode.

17. The arc tube of claim 9, wherein the wire of said helix and the wire of said overwind are of tungsten.

18. The arc tube of claim 17, wherein said ionizable fill comprises mercury and metal halides including thorium iodide.