US3364374A - Compact source lamp having electrode construction providing arc stabilization - Google Patents

Description

Jan. 16, 1968 J. WILSON 3,364,374

COMPACT SOURCE LAMP HAVING ELECTRODE CONSTRUCTION PROVIDING ARC STABILIZATION Filed Sept. 28, 1964 Invervtov John WiLson b MW 8 His AFFL'OT'TWG United States Patent COMPACT SOURCE LAMP HAVING ELEC- TRODE CONSTRUCTION PROVEDING ARQ STABHLIZATEON John Wilson, Mayfield Heights, Ohio, assignor to General Electric Company, a corporation of New York Filed Sept. 28, 1964, Ser. No. 339,581 4 Claims. (Cl. 313-484) This invention relates to a direct current compact source high pressure gas are lamp and is more particularly concerned with a construction of the electrodes for stabilizing the arc.

A well-known form of compact source lamp is the high-pressure xenon arc lamp wherein the arc is electrode-stabilized between electrodes whose distance apart is less than the distance from the electrode axis to the envelope wall. Such lamps are frequently used in optical systems where the light source must be small and precisely placed and this requires that the electrodes be accurately positioned. A problem which remains is the tendency of the arc to bow or float upwards when the lamp is operated horizontally. The arc instead of being axially centered, moves up toward the upper side of the anode. This means that the arc is off axis and represents a form of instability. In the more extreme case, a portion of the are, sometimes referred to as a tail flame, projects beyond the upper side of the anode and may strike the bulb wall. Where the tail flame strikes the bulb wall, it may cause devitrification and as lamp life progresses, the wall becomes weakened and blows out, causing the lamp to fail. On account of the foregoing, high pressure compact source gas are lamps for DC. operation have generally been limited to vertical or almost vertical operation, for instance i30 off vertical.

The use of a magnetic field to stabilize the electric discharge arc and counter its tendency to bow upwards in horizontal operation is well known. A magnetic field may be produced by a permanent magnet or by an electromagnet placed outside the discharge lamp. One may also use a coil within the lamp envelope and surrounding one of the electrodes to produce an axial magnetic field. However magnetic stabilization arrangements are generally inconvenient and restrict freedom in moving and aiming the lamp. A magnetic stabilizing coil within the lamp envelope tends to obstruct light from the arc and this may be an unacceptable drawback where the lamp is intended for an optical projection system.

The object of the invention is to provide a new and improved lamp structure, and particularly anode configluration, which will reduce or eliminate bowing of the arc and generation of a tail flame, thereby improving stability of operation.

In accordance with my invention, I have found that in a compact source high pressure gas arc lamp for direct current operation, upward bowing or floating of the arc is greatly reduced and the generation of a tail flame sub stantially eliminated by changing the configuration of the anode from the conventional rounded or pointed tip to a flat tip or face normal or transverse to the interelectrode axis and having a diameter or transverse dimension at least 1.5 times the arc gap or interelectrode distance. In

a preferred embodiment, the face diameter of the flattipped anode is in the range of 1.75 to 2.5 times the arc gap under operating conditions.

For better appreciation of the invention and of its advantages, attention is now directed to the following description of a preferred embodiment to be read in conjunction with the accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

Patented Jan. 16, 1968 In the drawing:

FIG. 1 is a side elevation illustrating a compact source are tube embodying the invention.

FIG. 2 is a fragmentary view of. the arc gap region of a prior lamp showing the tail flame occurring with a rounded tip anode.

FIG. 3 is a view similar to FIG. 2 showing the effect of reducing the arc gap.

FIG. 4 is a side sectional view of a reflector lamp incorporating the arc tube of FIG. 1.

Referring to the drawing and more particularly to FIG. 1, the illustrated compact source are tube comprises a generally tubular envelope 1 suitably made of quartz or quartz-like glass. The mid-section of the tube is bulbous and the ends are sealed by full diameter pinch seals 2, 3 through which extend respectively composite inleads 4, 5. Each inlead includes a thin intermediate foil portion 4a, 5a preferably of molybdenum which makes the hermetic seal to the quartz, and an inner electrode supporting portion indicated 4b, 5b suitably of tungsten wire. The outer and inner wire portions of the inleads are welded to the intermediate portion and the quartz of the seal is thickened or relieved over the welds, as indicated at 6, 7 for greater mechanical strength.

The cathode 8 is a thoriated tungsten cylindrical body having a pointed tip or conical nose 9 and mounted on inlead portion 412. The anode 10 is a massive generally cylindrical tungsten body mounted on inner lead portion 522. Heretofore, conventional anode design called for a rounded tip on the anode as indicated at 1141 in FIGS. 2 and 3.

Alternatively a conical tip was provided which became rounded off when the lamp was put in operation. The anode in accordance with my invention is provided With a flat tip or face as shown at 11b in FIGS. 1 and 4. Where the flat tip does not extend across the entire transverse section or face of the anode, the shoulders are chamferred at 12. In the illustrated arc tube or lamp, the anode is heavy and additional support is provided by a springy wire coil 13 having several smaller turns wrapped tightly around the anode body and a single expanded turn hearing against the envelope wall. The coil is located towards the rear of the anode body in order to avoid obstructing light from the are occurring in the interelectrode gap. The are tube is exhausted and provided with a suitable ionizable filling, preferably xenon at a pressure of 1 or more atmospheres, through an exhaust tube which leaves a tip 14 in the envelope wall after tipping off.

In the lamp illustrated in FIG. 4, the arc tube is mounted within a seal zbeam type lamp housing formed of two pressed glass shells, at reflector 15 and a cover or lens 16, peripherally sealed together. The are tube is mounted with its longitudinal or electrode axis coincident with the optical axis of the reflector and the reflector is provided with a rearwardly projecting neck portion 17 in which the cathode pinch seal is accommodated. The cathode inlead 4 is connected to a metal thimble or ferrule 18 fusion-sealed into the neck portion, and the anode inlead 5 to a similar ferrule 19 in the center of lens 16, as per my copending application Serial No. 336,187,. filed January 7, 1964, entitled Reflector Arc Lamp, and assigned to the same assignee as this application. Said application has now been abandoned in favor of my continuation-impart application Serial No. 602,436 filed October 31, 1966, now Patent No. 3,341,731, issued September 12, 1967, similarly entitled and assigned. By way of example, the arc tube overall length was millimeters, cathode diameter 0.140", and anode diameter .250" (6.4 mm.). The reflector enclosure or outer jacket is filled with nitrogen at about /2 atmosphere through metal exhaust tube 21 which is shown tipped off.

The illustrated arrangement, by accommodating cathode pinch seal 2 in neck portion 17 permits placing the arc gap of the arc tube at the focus of the reflector in a compact structure. Over the surface of revolution formed by revolving the angle AOB about the optical axis ZZ', the reflector is elliptical with one focus of the ellipsoid located at the point 0. The surface of revolution ABAB is coated with a light-reflecting coating 22, either a metallic mirror film or a multiple layer optical interference film, which focuses the light to the other external or second focal point of the ellipse. The useful light from the arc is that which the reflector collects over the revolved angle AOB subtended at the focus. The conical pointed cathode blocks only the light which would proceed into the neck 17 of the reflector and be wasted anyway. An advantage fortuitously achieved by the invention is that the flat-tipped anode with beveled shoulders 12 also interferes very little with the useful light from the arc. Since the anode flap tip is oriented to face or look at the reflector, it blocks substantially only that light which would spread out through the cover or lens 16 and diverge uselessly. Thus the invention corrects upward bowing and instability by means of a flat-tipped anode without appreciably reducing the efliciency of the optical system.

FIG. 2 illustrates the kind of discharge which may occur with a compact source direct current lamp on horizontal operation when using a conventional rounded tip anode 11a. The are gap in this instance was 2.2 millimeters at the operating temperature. The plasma or luminous portion of the discharge indicated at 24 floats up towards the upper part of the anode and a tail flame 25 may extend obliquely beyond the anode and strike the bulb wall. The upward floating and the tail flame appear to be caused by convection effects. Where a tail flame strikes the bulb wall, devitrification may result and, as lamp life progresses, the wall nearest the tip of the tail flame is weakened and blows out causing the lamp to fail. With a floating or upwardly bowing arc, the tendency to flicker resulting from wandering of the are over the face of the anode is increased.

The tendency of the arc to bow upwardly may be re duced by shortening the interelectrode gap. This is illustrated in FIG. 3 showing a lamp utilizing a rounded tip anode 10 and an operating arc gap of 1.5 millimeters between anode 8 and cathode 9. The are or plasma 26 is more nearly centered in the interelectrode gap and there is no tail flame extending beyond the upper side of the anode. However it is undesirable to go to a shorter gap because lumen output and lamp voltage are reduced. In a compact source lamp, the voltage is low while the current is high and one of the limiting factors is the current carrying capacity of the electrode seals or inleads. Also when the current is increased, the reactive volt-ampere capacity of the ballast must likewise be increased, and this is in the direction of greater cost and reduced efficiency. Therefore it is advantageous to keep the lamp voitage as high as possible in order to avoid increasing lamp current to make up for reduced lumen output, and for this reason reduction of the arc gap is not desirable.

In accordance with my invention, are floating and upward bowing are substantially reduced and the tail flame eliminated by means of the flat tip 11b on the anode as illustrated in FIGS. 1 and 4. FIG. 1 shows at 27 the constrained or controlled are or luminous plasma obtained with the SOO-watt compact source xenon lamp illustrated when using an arc gap of 2.3 mm. under operating conditions, and an anode flat tip portion 5.2 mm. in diameter (dimension D). It is necessary to specify the gap under operating conditions rather than at room temperature because the expansion of the inleads is appreciable and operates to reduce the gap as the lamp warms up to operating temperature. In this case the gap would be about 2.6 mm. if measured with the lamp cold. The foregoing corresponds to a ratio of flat tip diameter to operating interelectrode gap of 2.25. As another example, with an interelectrode gap of 1.7 mm. under operating conditions, I have found a flat tip diameter of 3.6 mm. adequate; this corresponds to a flat tip diameter to interelectrode gap ratio of 2.1. In both lamps, the diameter of the flat tip (dimension D) of the anode is large enough that the upper part of the arc plasma does not extend beyond the fiat portion of the anode face and the arc does not show the tendency to overlap the anode and project beyond as a tail flame.

I have found that with compact source gas are lamps of the instant kind wherein the current is in the range of 10 to 50 amperes, the minimum diameter (transverse dimension D) of the fiat tip portion of the anode should be at least 1.5 times the arc gap or interelectrode distance, that is the distance along the axis of the lamp from the tip of the cathode to the face of the anode, under operating conditions. The longer the interelectrode gap, the greater must be the diameter of the flat anode face in order to avoid floating and bowing of the arc. However for reasons of economy and also to avoid unnecessary obstruction of light, it is desirable to hold the anode down to the minimum size which will meet the requirements of heat dissipation and arc stabilization by control of upward bowing. I have found that these conflicting requirements result in a preferred range for the diameter of the flat face portion of the anode from 1.75 to 2.5 times the operating arc gap of interelectrode distance. In the lamp illustrated in FIG. 4, when the ratio of the diameter of the flat anode tip to the arc gap is within the above preferred range, upward bowing or floating is substantially eliminated and the arc is stabilized without appreciable obstruction of useful light by the anode flat tip.

The specific embodiment of the invention which has been described in detail and illustrated is intended by way of example of the invention Whose scope is to be determined by the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A compact source high pressure gas are lamp comprising a vitreous envelope containing a filling of xenon at a pressure in excess of 1 atmosphere and a pair of electrodes sealed into said envelope and defining a short are gap whose length is less than the distance between the electrode axis and the envelope wall, one of said electrodes being an activated cathode having a pointed tip and the other of said electrodes being a more massive tungsten anode of generally cylindrical shape and having a flat tip whose minimum transverse dimension is at least 1.5 times the interelectrode arc gap.

2. A compact source high pressure gas are lamp comprising a quartz envelope containing a filling of Xenon at a pressure in excess of 1 atmosphere, a pair of electrodes sealed into said envelope and defining a short arc gap whose length is less than the distance between the electrode axis and the envelope wall, one of said electrodes being an activated pointed tip cathode and the other of said electrodes being a massive generally cylindrical tungsten body axially disposed within the envelope and having a flat tip at its forward face opposite the cathode, the diameter of the fiat portion of the anode face being in the range of 1.75 to 2.5 times the interelectrode arc gap.

3. A compact source high pressure gas arc lamp comprising a quartz envelope containing a filling of xenon at a pressure in excess of 1 atmosphere, a pair of electrodes sealed into said envelope and defining a short are gap whose length is less than the distance between the electrode axis and the envelope wall, one of said electrodes being an activated pointed tip cathode of thoriated tungsten capable of supporting a current in the range of 10 to 50 amperes, and the other of said electrodes being a massive generally cylindrical tungsten body axially disposed within the envelope and having a flat tip at its forward face opposite the cathode, the diameter of the flat portion of the anode face being in the range of 1.75 to 2.5 times the interelectrode arc gap, no part of said anode projecting beyond said flat portion toward the cathode.

4. A reflector lamp comprising a vitreous outer envelope including a concave reflector, a compact source are tube mounted Within said outer envelope with its longitudinal axis lying in the optical axis of the reflector, said are tube comprising a quartz envelope containing a filling of Xenon at a pressure in excess of 1 atmosphere, 3. pair of electrodes sealed therein and defining a short are gap Whose length is less than the distance from either electrode to the tube Wall, one of said electrodes being an activated pointed tip cathode of thoriated tungsten capable of supporting a current in the range of 10 to 50 amperes, and the other of said electrodes being a massive generally cylindrical tungsten body axially disposed Within the envelope and having a flat tip at its forward face With chamferred shoulders, the diameter of the flat portion of the anode face being in the range of 1.75 to 2.5 times the interelectrode gap, said anode face being oriented to look at the reflector in order to obstruct minimum light thereto and no part of said anode projecting beyond said flat portion toward the cathode.

References Cited UNITED STATES PATENTS 3,087,092 4/1963 Lafl'erty 313-188 3,113,234 12/1963 Schlegez 313l84 X 3,114,855 12/1963 Story 313217 X 3,138,731 6/1964 Beese 313-113 3,248,586 4/1966 Schlegel 313217 X 3,248,591 4/1966 Arndt 313217 JAMES \V. LAWRENCE, Primary Examiner. STANLEY D. SOHLOS'SER, Examiner. S. A. SCHNEEBERGER, Assistant Examiner.

Claims (1)

1. A COMPACT SOURCE HIGH PRESSURE GAS ARC LAMP COMPRISING A VITREOUS ENVELOPE CONTAINING A FILLING OF XENON AT A PRESSURE IN EXCESS OF 1 ATMOSPHERE AND A PAIR OF ELECTRODES SEALED INTO SAID ENVELOPE AND DEFINING A SHORT ARC GAP WHOSE LENGTH IS LESS THAN THE DISTANCE BETWEEN THE ELECTRODE AXIS AND THE ENVELOPE WALL, ONE OF SAID ELECTRODES BEING AN ACTIVATED CATHODE HAVING A POINTED TIP AND THE OTHER OF SAID ELECTRODES BEING A MORE MASSIVE TUNGSTEN ANODE OF GENERALLY CYLINDRICAL SHAPE AND HAVING A FLAT TIP WHOSE MINIMUM TRANSVERSE DIMENSION IS AT LEAST 1.5 TIMES THE INTERELECTRODE ARC GAP.

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