GB2043331A - Electrode for high pressure metal- vapour lamp - Google Patents

Abstract

An electrode for a high pressure metal vapour lamp comprises a hollow helix 12 of refractory metal, e.g tungsten wire, projecting from an inlead, an open-wound overwind 13 on the helix providing quasi-point contact spacers between turns of the helix which gives rigidity with only minor increase in axial heat flow. In an embodiment wire 13 is 2-5 mils in diameter and wire 12 is 7 mils in diameter. Ball end 14 reduces tip erosion. The electrode has good spot transfer characteristics in moving the arc terminus to the tip and low energy loss in operation, and is suitable as cathode in a miniature d.c. metal halide lamp with or without alkaline earth oxide electron emitters. The turns of the helix may be torsionally preloaded to bias them together. <IMAGE>

Description

SPECIFICATION Electrode for high pressure metal vapor lamp The 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 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.

Until recently, it has been the common view that the efficacy of high intensity discharge lamps inevitably goes into a rapid decline starting at about 250 watts, and metal halide lamps in sizes below 175 watts were considered impractical for general lighting. However, in UK Patent Application No.28756/78 (Serial No. 2000637) 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 ratio of arc watts which produce light, to electrode watts 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, wire commensurate in fineness must be used and this tends to make manufacture more difficult.

In lamps wherein the electrodes do not carry electron emission material in the conventional sense of alkaline earth metal or oxides, the criteria for electrode design and the permissible heat loss from the electrode are much more stringent than in lamps containing emission material. By way of example, in a mercury vapor lamp containing barium oxide as emission material which has a work function of 1.5 to 2 volts, the electrode temperature should not exceed 1500" K. By contrast, in lamps without emission material or relying upon the presence of thorium or thorium iodide in the fill for electrode activation with a work function of 3.5 to 4.5 electron volts, a temperature of 2500 to 3000 K is necessary for adequate electron emission.On the other hand at temperatures above 3300 K, tungsten vaporizes at such a rate that the small envelopes of miniature lamps blacken rapidly and this imposes another design constraint.

When a lamp is operated on alternating current, each electrode serves alternately as cathode and anode, and the heating from the cathode half cycle is 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 its cathode function in an a.c. circuit at a given current (root-mean-square) 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 in the starting and ballasting circuit. Accordingly the cathode must be designed to properly manage the energy balance in order to insure that the cathode hot spot rapidly reach its desired operating temperature during starting and 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.

The general object of the invention is to 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.

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 conduction loss into the seal area to be minimized, and (3) achieves the foregoing together with structural rigidity and ease of manufacture.

An electrode according to our invention comprises a hollow helix of refractory wire which is provided with an overwind which keeps the turns of the helix separated from each other so that conductive heat flow is compelled to follow a long helical path. The overwind permits an increase in total electrode surface area and at the same time the arrangement assures a low conduction loss into the seal area.

In a preferred embodiment suitable for a miniature metal halide lamp, the overwind on the helix is wound to provide an optimum number of quasipoint-contact spacers between 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 spudded onto a shank and which may be terminated in a solid cap by melting back a few turns at the distal end. The wire of the helix may be torsionally preloaded in order to provide a built-in force that biases the turns together against the spacers for extra rigidity.

The present invention will be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 illustrates, to the scale shown above the figure, a miniature discharge lamp for d.c. operation provided with a cathode embodying the invention.

Figure 2 is an enlarged view of a cathode embodying the invention.

Figure 3 is an enlarged view of the primary overwind wire open-wound on the primary mandrel.

Figure 4 is a partly sectioned side view of cathode embodying the invention, enlarged to a greater extent than that of Figure 2.

Figure 5 is an end view of the cathode of Figure 4.

An optimized electrode design must be capable of serving through the various modes encountered in lamp operation such as breakdown, glow-to-arc transition, normal operation (which may be a.c. or d.c.), and hot restart after a temporary power loss. A given design will have a specific structure and surface condition including emission mix, a specific shape including surface curvature which may influence the electric field, a specific mass distribution, and a particular thermal balance between heat generation and heat loss in the structure. Our invention provides an electrode design which offers a wide choice of independent parameters which may be varied to achieve the desired optimization.

Among thesn parameters are the refractory metal (e.g. tungsten) chosen for the structure, the emission material such as a coating if used, and particularly the physical dimensions inherent in the structure as will appear hereafter such as mandrel diameter, overwind diameter, overwind pitch on the mandrel, tightness of the overwind on the mandrel, shank of in lead diameter and insertion length, overall length of the electrode and tip or end cap size. All of these may be varied to achieve the desired optimization.

When an electrode operates as a cathode on d.c., the forces that drives the point of arc attachment toward the tip are much less than when the same electrode operates alternately as cathode and anode on a.c. This is because 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 heated up thereby. Thus on a.c.

operation the tip temperature is built up on successive anode half-cycles. Such temperature build-up increases emission at the tip and facilitates transfer of the hot spot thereto on the cathode 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 current lamps, it is particularly valuable for miniature metal halide lamps such as those described in the previously mentioned copending application. An example of a miniature metal halide lamp is illustrated in Figure 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 portion 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 molybdenum foil portion 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 envelope comprises argon or other inert gas at a pressure of several tens of torr to serve as starting gas, and a charge comprising mercury and one or more metal halides. A preferred filling comprises Nal, Sic13 and The4. The charge may be introduced into the are chamber through one of the necks before sealing in the second electrode; in such case the arc chamber portion is chilled during the heat 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 bulk which is then eliminated by tipping off. The arc tube is usually mounted within an outer protective envelope or jacket (not shown) having a base to whose contact terminals the inleads 5,5' 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' projecting 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 enlarged 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 diameter.

The invention relates particularly in the cathode structure 10 formed upon or attached to the end of in-lead 6. As best seen in Figure 2 or in Figures 4 and 5, the cathode proper comprises a hollow helix 11 which may be described as consisting of a coiled primary mandrel 12 around whose coiled turns is wound a smaller primary wire forming an overwind 13. Both primary mandrel and primary wire are retained in the completed cathode. The wires are of tungsten or of other refractory metal suitable for electrodes. As shown in Figure 3, the overwind wire 13' is wound around mandrel 12', such that when the composite of 12' and 13' is tightly coiled around a secondary mandrel to produce the helical structure 11, the turns of the overwind or primary wire inter-digitate, producing a spacing of one primary wire diameter 13 between the turns 12 of the primary mandrel forming the main helix 11.

Our helical electrode with overwind meets the previously stated criteria for-good design, namely maximized surface area for more rapid glow-to-arc transition, and controlled heat conduction loss into the seal. The surface area of the helix is large by comparison with that of a shank type electrode, even one with an overwind. The long helical path which conduction heat from the electrode tip must follow greatly reduces the loss of heat by 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' on the mandrel wire 12', that is the pitch ratio, is determined by considerations of electrode surface, thermal conductivity and structural rigidity with compromises or trade-off between these. It is desirable for stability to approach at least 3 evenly distributed spacers or rest points per turn of the primary mandrel (meaning that the angular interval between rest points cannot be much less than 120"); less than 3 reduces rigidity and only 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 wire 13' per turn of primary mandrel 12 generates 3 rest points per turn in the electrode structure. However the distribution is uncertain, and unless the rest points are evenly spaced circumferentially when the overwind turns interdigitate, the rigidity may be lessened.For this reason a minimum of 3 overwind turns 13' per turn of the primary mandrel 12' is preferred as illustrated in the drawings. This generates 6 rest points which assures rigidity even under the worst condition of contiguous pairing of rest points which would effectively reduce the 6 to 3.

The separation and resulting thermal insulation between primary mandrel turns which the overwind assures is even more important later in the life of the cathode when sintering, in the absence of spacers, would 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 flow characteristics. This is a great advantage over other structures such as loop electrodes which tend to change shape during operation of the lamp, especially when the loops are made of fine wire as they must in miniature lamps.

The helix 11 is attached to inlead 6 in a manner to project distally into the envelope. A convenient way to do so is to spud the helix onto the end of 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 tight grip.

With our helix which carries an overwind, the overwind prevents direct contact between wire 12 of the helix and the shank 6 and thus assures low thermal conduction into the shank. Another convenient manner of attachment is by welding which may be used where greater thermal conduction into the shank is desirable. In Figure 4, the helix is shorter and fewer turns are spudded onto the shank or inlead than in Figure 2; our electrode configuration facilitates such variations in design to achieve the desired heat balance. The portion 15 of the electrode which grips the shank 6 may be embedded in the silica as shown in Figure 1; embedding makes it easier to center the electode in the.bulb at the sealing step in lamp manufacture.

In Figure 1 and 2, the electrode 10 is terminated in a solid cap 14. Such an end provides a place where a hot spot may develop where the arc attaches during normal operation and reduces the rate of erosion of the electrode. An end cap may be formed simply by heating the end of the electrode, suitably by a plasma torch, and melting back and last few turns of he helix. Alternatively, a small mass of suitable refractory metal may be welded or sintered to the distal end of the helix portion 11.

In Figure 4the helix is not terminated by a solid end cap and Figure 5 merely shows the electrode in end view. Some sintering together of the helix wire 12 and overwind wire 13 will occur in operation, particularly at the distal end where the hot spot attaches in operation. In a metal halide lamp wherein thorium iodide is present, a bare electrode may be used as illustrated; in other metal vapor 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 coating.

According to an optional feature of the invention even greater structural rigidity may be achieved by preloading or overwinding the turns of the helix.

Greater rigidity may become relatively more important as lamp and electrode are miniaturized. When line or wire is coiled, there is an inherent or equivalent twist of 360" per loop put into it. This is readily seen when one pulls a line sideways off a spool instead of unrolling it from the spool; a 360" twist appears in the line for every loop pulled off the spool. If a twistgreaterthan 360 per loop is put into the line, it may be said to be overtwisted or preloaded and the result is a built-in torsional stress that, in the case of close-wound helix of resilient wire, biases the turns laterally together and maintains them in tight side-by-side contact.This may be observed in preloaded springs, such as those frequently used to close screen doors of houses; the spring will not stretch and the turns will not open up at ali until a certain minimum force is exceeded, and thereafter the stretch is proportional to the excess of force over the minimum. This minimum corresponds to the built-in torsional stress or pre-load that biases the turns together.

An overtwist or preloaded condition may be achieved by putting a twist in the proper direction into the composite 12', 13' prior to or during coiling around the secondary mandrel. There are two coilings which occur in the electrode structure, that of overwind 13' around mandrel 12' (Figure 3), and that of the composite 12,13 (Figure 2) around a mandrel (not shown but corresponding generally to shank 6). If both coilings are wound in the same direction (both left-hand or both right-hand), the final structure will have the overwind 13 tight on its mandrel wire 12. However, if the coiling are wound in opposite directions, the final coilings will cause overwind wire 13 to loosen on its mandrel 12. This generates clearance between the two exposing more of the electrode surface and creating fissures for emission mix. The fissure may also serve as an electrid field concentrator. This effect can be utilized over a wide range to vary the physical characteristics of the electrode.

The following is an example of a cathode in accordance with 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 2.5 mil tungsten and the primary mandrel 12' is 7 mil tungsten wire. The primary coiling is relatively open and suitably provides an advance about equal to twice mandrel diameter, that is 14 mil per turn, the objective being to have approximately 3 turns of the overwind around the primary mandrel per turn of the primary mandrel around the secondary mandrel.

Prior to coiling around the secondary mandrel, the composite wire resulting from the primary coiling is pretwisted approximately 1 turn per inch. If the primary coiling was conventional right-hand coiling, then the pretwisting is done with a right-hand twist which has the effect 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 results in a slight loosening of the overwind. The secondary mandrel is molybdenum and is dissolved out by nitric and sulfuric acid which do not attack the tungsten wires, and the helix is then spudded onto an inlead of more than 9 mil to assure a good grip.

Claims (22)

1. An electrode for a high pressure metal vapor lamp including a hollow helix of refractory metal wire and an open-wound overwind on the turns of the helix providing spacers between turns giving rigidity to the helix.

2. An electrode as claimed in claim 1 wherein said overwind provides a substantial increase in emitting surface area without major increase in axial heat flow to achieve heat balance.

3. An electrode as claimed in claim 1 or claim 2 wherein the size of the overwind wire is less than that of the wire forming the helix.

4. An electrode as claimed in any one of the preceding claims wherein the number of supports per turn of the helix approaches at least 3 in order to achieve structural rigidity.

5. An electrode as claimed in any one of the preceding claims wherein a length of the hollow helix is attached to a refractory metal inlead and projects distally therefrom.

6. An electrode as claimed in claim 5 wherein the length of hollow helix is spudded onto the end of the refractory metal inlead.

7. An electrode as claimed in any one of the preceding claims having a distal end terminated in a metal cap.

8. An electrode as claimed in claim 7 wherein the cap is the result of melting back a few distal end turns of the helix

9. An electrode as claimed in any one of the preceding claims said helix was torsionally preloaded in order to bias the turns together against the spacers.

10. A metal vapor are tube comprising a tight transmitting envelope containing an ionizable fill, electrodes connected to inleads sealed into said envelope, at least one electrode serving as cathode, the cathode comprising a hollow helix of refractory metal extending from the inlead and an open-wound overwind on the turns of the helix providing spacers between turns giving rigidity without major increase in heat loss by conduction to the inlead.

11. An arc tube as claimed in claim 10 wherein the length of hollow helix is spudded onto the end of the refractory metal inlead.

12. An are tube as claimed in claim 11 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 wall.

13. An arctube as claimed in any one of claims 10 to 12 wherein distal end of said one electrode is terminated in a metal cap.

14. An arc tube as claimed in claim 13 wherein the cap is the result of melting back a few distal end turns ofthe helix.

15. An arc tube as claimed in any one of claims 10 to 14 in claim 10 wherein the wire of said helix was torsionally preloaded in order to bias the turns together against the spacers.

16. An arc tube as claimed in any one of claims 10 to 15 for d.c. operation wherein the other electrode is a solid conductor of refractory metal.

17. An arc tube as claimed in any one of claims 10 to 15 for a.c. operation wherein both electrodes serve alternately as cathode and are constructed like said one electrode.

18. An arc tube as claimed in any one of claims 10 to 17 wherein the wires of the helix and of the overwind are of tungsten.

19. An arc tube as claimed in claim 18 wherein the ionizable fill comprises mercury and metal halides including thorium iodide.

20. An electrode as claimed in claim 1 substantially as hereinbefore described with particular reference to the accompanying drawings.

21. A metal vapour arc tube as claimed in claim 10 substantially as hereinbefore described with particular reference to the accompanying drawings.

22. An electrode as claimed in claim 1 substantially as hereinbefore described in the Example.