High Current Thermionic Hollow Cathode Lamp

Abstract

A thermionically emitting hollow cathode for very high currents up to 400 amperes AC particularly suitable for xenon-filled wall-stabilized lamps. It comprises a hollow cylindrical body of porous tungsten, open in the direction of the arc and having a coil of tungsten wire lining the cavity walls except for a region at the rear deep within the cavity. The cavity wall but not the face nor the outside of the electrode are impregnated with emission material, suitably barium thorate. The cooler shank end of the cavity in which the electrode coil does not extend serves as a dead space into which gas can expand during the AC cycle, thereby reducing the rate of gas flow in and out of the open end.

Description

BACKGROUND OF THE INVENTION

The invention relates to thermionically emitting high current electrodes for use in very high power lamps, particularly xenon-filled wall-stabilized lamps.

In my U. S. Pat. No. 3,029,359 I have described and claimed a thermionic hollow cathode which operates well with a low cathode potential drop (1.2 volts) and with little envelope blackening at currents up to 75 amperes r.m.s. That cathode comprises a hollow cuplike body of tungsten open towards the front, that is in the direction of the arc, and has a tungsten coil lining the cavity walls. Emission material consisting of a barium containing compound is coated on the coil and lodged in the interstices between the turns of the coil and the cavity wall.

The object of my invention is to provide a yet more efficient cathode and one capable of operation at much higher currents, at least up to 400 amperes r.m.s.

SUMMARY OF THE INVENTION

A thermionic-emitting high current cathode according to my invention comprises a hollow body of porous tungsten, suitably a hollow cylinder open towards the front, that is in the direction of the arc. A coil of refractory metal wire, suitably tungsten, lines the cavity wall except for an inactive region or dead space at the rear, that is in the deepest portion of the cavity. Emission material, suitably barium thorate, is embedded in the pores of the tungsten of the inner surface of the electrode cavity. The material is filled into the pores to the depth required to hold the quantity necessary for the intended life of the lamp. The impregnated emission material causes the inner surface of the electrode cavity to show a whitish haze. The porous structure of the tungsten mechanically protects this reservoir of emission mix from peeling and also from disintegration by exposure to the arc. The structure allows the low-work function emission material to be transferred or dispensed to the coil in a controlled manner.

The application of emission material to the face and to the outer cylindrical surface of the electrode is carefully avoided in order to prevent undesirable burning of the arc in a spot-mode anywhere and to confine it to the cavity where it burns in a diffused mode.

A factor causing erosion of tungsten and envelope darkening in hollow electrodes is "breathing" during AC operation. Within the electrode cavity where the power density is extremely high, essentially all metal vapor, and particularly that of the easily ionized emission mix component, is ionized during burning of the arc. The ions can move only a millimeter or so during a half-cycle of the AC voltage. During the cathode half-cycle, the field tends to retain the ions within the cavity; during the anode half-cycle, the field may tend to force the ions out (although a negative anode field within the cavity is not excluded). The overall restraining effect of the cavity can be overpowered however by the heating and cooling of the gas during the AC cycle. The motion of an ion during a half-cycle under the action of expanding gas is likely to be much greater than under the action of the field. Various experiments have shown this " breathing" effect capable of eroding hot tungsten. The breathing effect carries metal vapors including tungsten out of the electrode cavity and into the lamp volume proper and deposition of the metal vapors on the envelope walls cause blackening.

My invention greatly reduces or substantially eliminates envelope darkening due to "breathing" by reason of two features:

1. The front end of the electrodes is much hotter than the rear or shank end, for instance 1690.degree. C. vs. 1040.degree. C.

2. The cathode emitter coil does not extend the entire depth of the cathode cavity.

As a result of the above factors, the arc burns towards the front portion of the cavity. As the arc current builds up during the AC cycle, heated gas tends to blow into and out of the inactive or dead space at the rear into which the arc does not extend. However the gas in the part of the lamp envelope where the positive column occurs is heating at the same time as the gas in the front portion of the cavity and a balancing effect takes place. The result is that there is much less gas transfer back and forth through the electrode entrance than there is between back and front portions of the cavity. To a first approximation, if the ratio of "live space" to "dead space" within the electrode is the same as the ratio of "live space" to "dead space" in the rest of the lamp, there will be no motion of gas through the cavity mouth during the AC cycle, and blackening from this cause will be minimized. Accordingly, in a preferred embodiment of my invention, this ratio is observed as between electrode and lamp volumes.

In order to assure the proper rate of dispensing of emission mix from the reservoir to the coil within cathode, an emission material appropriate to the operating temperature of the cathode must be used. A preferred emission material is barium thorate BaTh0.sub.3. Other materials which may be used as electron emitters are barium zirconate, barium aluminate, strontium thorate and thoria. In known manner, reducing agents may be added to the emission mix which are capable of reacting at the rate necessary to compensate for the evaporation of barium taking place. An advantage of using BaTh0.sub.3 is that as it decomposes, straight evaporation of Ba0 takes place and no reduction is necessary.

DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a high powered xenon lamp provided with cathodes embodying the invention. A central section of the lamp has been cut out in order to shorten the FIG. and one end of the lamp and its electrode have been sectioned.

FIG. 2 is a graph showing the volt ampere characteristic of the lamp.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1 of the drawing, illustrated lamp assembly 1 comprises a tubular envelope 2 made of quartz and containing xenon as the ionizable filling gas. At the ends of the envelope are mounted the thermionic electrodes 3 supported on internal rod conductors 4 which extend through reduced tubular vitreous extensions or necks 5. The necks 5 are made up of several vitreous sections having intermediate coefficients of expansion, the last being suitable for sealing to the thin thimblelike edge 6 of tubular external terminal member 7 in which rod 4 is engaged. The hermetic seal occurs between the neck 5 and the thin edge 6 of terminal member 7; elsewhere there must not be any bond between the vitreous and metal parts and the fine wire coil 8 wrapped around the rod next to the electrode assures clearance between the rod and neck at that point. The lamp is water-cooled by means of a glass jacket 9 which surrounds the envelope 2. A water-seal is made at each end by means of discs 10 and 11 drawn together by screws 12; one disc is flat and the other is chamfered internally and externally to accommodate rubber sealing rings 13, 14. The water inlet and outlet are indicated at 15, 16 respectively and a spiral glass rod 17 in the space between the envelope 2 and the jacket 9 assures sufficient velocity of water flow to prevent boiling at any place.

The electrodes 3 proper each comprise a generally cylindrical hollow body 18 of porous tungsten open towards the front, that is in the direction of the arc. Suitably the porous tungsten of the electrode body may be 70 percent of theoretical density. Within the body, a close wound emitter coil 19 of tungsten wire lines the cylindrical walls of the cavity from the front or open end to about three-fourths of the cavity depth. The portion of the cavity in which the wall is lined by the coil may be referred to as the "live space" within the electrode; the unlined portion at greater depth may be referred to as "dead space."

The interior wall of the electrode over the "live space " region has emission material embedded in the pores of the tungsten to the point where a whitish haze shows on the surface, indicated by the line of small x's in the drawing. The pores are filled to the depth required to hold sufficient emission material to last the intended life of the lamp. The porous inner wall of the electrode thus serves as a reservoir of emission mix which is mechanically protected from peeling. The emission mix is also protected from disintegration by exposure to the arc by reason of its dispersion through the porous tungsten structure. Transfer or dispensing of the low-work function material from the porous wall to the coil occurs in a controlled manner throughout the life of the lamp.

One way of embedding the emission material powder in the pores of the tungsten is to suspend it in a hydrophobic solvent such as toluene or n-heptane, and flow it onto the tungsten surface. Another way is to mount the electrode in a fixture, entrain the emission material powder into a flowing gas, and pump the gas radially out through the pores of the electrode. Application of emission material to the face and to the outer cylindrical surface is carefully avoided. This is necessary in order to maintain the arc in a diffused mode within the cavity and prevent it from burning elsewhere in a spot mode.

The great weight of the electrode requires that is be supported through the terminal member 7 and rod conductor 4 directly by means of some external fixture, and not through the vitreous envelope and the hermetic seal thereto. At the same time this arrangement permits dissipation of electrode heat through rod conductor 4 with the final result that the front end of the electrode is much hotter than the shank end, for instance 1690.degree. C. vs. 1040.degree. C. a ratio of cavity depth to diameter in the range of 1:1to 4:1is generally desirable. The difference in temperature between the front and shank end of the electrode, plus the fact that cathode emitter coil 19 extends only a certain fraction (e.g. three-fourths) of the cavity depth, assure that the arc burns toward the front portion of the cavity. As arc current builds up during the cycle, heated gas tends to blow in and out of the dead space at the rear. However the gas in the positive column of the lamp heats up simultaneously and there is less gas transfer back and forth through the electrode opening than there is between live and dead spaces of the cavity. Effectively the positive column within the lamp causes expansion to the region back of the electrodes and to the walls as well. The dead space within the lamp comprehends all such regions into which expansion occurs. The dead space within the electrode cavity is comparable to the dead space within the lamp. To a first approximation, when the ratio of dead space to live space within the electrode is the same as the ratio of dead space to live space within the lamp volume, there is no motion of gas across the cavity mouth during the AC cycle, and blackening from this cause is minimized.

By way of example of the invention, in a 100 kw. lamp design, electrodes embodying the invention were constructed capable of handling 400 amperes. The length of electrode body was 2 13/16 inch; outside diameter 1 5/16 inch, cavity diameter 7/8 inch. The shank or rod conductor 4 was 3/8 inch diameter tungsten rod. The cathode emitter coil 19 consisted of 50 mil tungsten wire initially coated with a light (5 milligram) coating of BaTh0.sub.3. The quantity of BaTh0.sub.3 embedded in the porous tungsten of the interior wall was about 165 milligrams per electrode. The electrodes were sealed into a quartz tube of 1.4 inch (3.5 cm.) internal diameter, as illustrated in FIG. 1. The tube or envelope was filled with sufficient xenon to provide an operating pressure from 178 to 11/2 atmospheres at a current intensity of 400 amperes. The lamp was provided with a water-cooling jacket as illustrated in the drawing.

Tests of such lamps at 400 amperes show a wall-stabilized discharge and a rising volt ampere characteristic is exhibited, as shown by curve 21 in FIG. 2. This means that such lamps can be operated without ballast or with minimum ballasting provided for safety reasons only. The r.m.s. voltage drop in these lamps was about 2.5 volts per centimeter. Under these conditions a 400 ampere lamp requires a length of about 1 meter for a 100 kilowatt input. The efficiency of such a lamp is about 36 lumens per watt.

Claims

I claim:

1. A high current thermionic self-heating electrode comprising an elongated hollow tubular body of porous tungsten open at the front end and closed at the rear end, a coil of tungsten wire located within said body and lining the inside wall thereof, and extending less than the full depth of said cavity, low work function emission material embedded in the pores of the inside wall only of said electrode at least coextensively with said coil, and a heavy rodlike conductor supporting said electrode from the rear end and assuring a substantial temperature difference between the front end and the rear end thereof.

2. An electrode as defined in claim 1 wherein the emission material is an alkaline earth metal compound.

3. A high current (wall-stabilized) discharge lamp comprising a tubular vitreous envelope containing an ionizable filling and having a pair of thermionic self-heating electrodes sealed into opposite ends, each electrode comprising an elongated hollow tubular body of porous tungsten open at the front end in the direction of the arc and closed at the rear end, a coil of tungsten wire located within said body and lining the inside walls only thereof and extending less than the full depth of said cavity, a low work function emission material embedded in the pores of the inside wall only of said electrode at least coextensively with said coil, and a heavy rodlike conductor supporting said electrode from the rear end and assuring a substantial temperature difference between the front and the rear end thereof.

4. A lamp as defined in claim 3 wherein said coil of tungsten wire extends only to a depth within said cavity corresponding substantially to the live space thereof, the ratio of live to dead space within said cavity corresponding to the ratio of live space within said lamp occupied by the discharge to dead space not so occupied.

5. A lamp as defined in claim 3 wherein the emission material is an alkaline earth metal compound and the ionizable filling is an inert rare gas.

6. A lamp as defined in claim 4 wherein the emission material is an alkaline earth metal compound and the ionizable filling is an inert rare gas.

7. A lamp as defined in claim 4 wherein the emission material is barium thorate and the ionizable filling is xenon.