US3013175A - High output discharge lamp - Google Patents

Description

12, 1961 J. F. WAYMOUTH ETAL 3,013,175

' HIGH OUTPUT DISCHARGE LAMP Filed May 1, 1957 IN VENTORS: JOHN E war/wary WARREN a cu/vsm y an BER/VIE)? ATTORNE Y I rit;rates .datent @iiice 3,913,175 Patented Dec. 12, 1861 3,lll3,175 HIGH @UTPUT DISCHARGE LAMP John F. Waymenth, Marblehcad, Warren C. Gungie, Salem, and (Carl .i. Bernier, Beverly, Mensa, assignors to ylvania Electric Products inn, alern, Mass, a corporation of Massaehnsetts Filed May 1, 1957, Ser. No. 656,356 3 E'Jlaims. (Cl. I l3--l85) This invention relates to electric discharge lamps. It relates especially to those of the type utilizing an electric discharge in a gas at low pressure, and particularly to fluorescent lamps of that type.

It is generally desirable in such devices to have at least one thermionic electrode, in which a metal piece or Wire carries a quantity or" electron-emissive materials, such as alkaline earth oxides. Such an electrode, used as cathode, is susceptible to disintegration by bombardment with high energy positive ions produced in the gas. If the discharge occurs in the presence of a low pressure of mercury, say at a pressure of about microns of mercury, then the addition of an inert gas at a considerably greater pressure, say or a pressure of the order of a millimeter of mercury, will generally give considerable protection against positive ion bombardment, because of the mass of the gas atoms. Argon and krypton are especially eifective in giving such protection to the cathode.

However, when it is desired to operate such a discharge lamp at very high current densities, the efficiency of production of desired radiation from the discharge is found to be greater with neon or helium than with argon or krypton. But neon and helium are lighter gases, with less protective action on the cathode.

Moreover, neon has about the same atomic mass as the oxygen present in the cathode coating, so neon ions will be more likely to dislodge oxygen atoms than would argon or krypton ions.

Since electric discharge lamps are usually operated commercially on alternating current, the same type of electrode is generally used at each end of the discharge. Each electrode will act as cathode during one-half of the cycle and as anode during the other half. To avoid overheating the electrode during the anode portion of the cycle, auxiliary electrodes to take part of the anode current are generally necessary. These are often in the form of straight wires placed near the thermionic electrode and connected to it. For convenience, the thermionic electrode will be hereinafter called the cathode to distinguish it from the auxiliary electrode, although during one-half the cycle it will function as part of the anode.

The disintegration of the cathode coating and of the auxiliary electrodes is much greater in neon and helium than in argon or krypton. The sputtering of the auxiliary electrodes is quite rapid, and the products of the disintegration tend to deposit or plate over the cathode coating, destroying the effectiveness of the latter, which in turn further accelerates the disintegration process. This is especially true if the lamp is operated with cool spots behind the cathode to control the vapor pressure for high efficiency.

The disintegration takes place throughout the operation of the lamp, but is most severe during the starting of the lamp. in a discharge taking a total electron and ion current of about one and one quarter amperes, the auxiliary electrodes can draw as much as 0.3 ampere of ion current, composed of ions having energies which may be as high as 100 volts for a brief period during starting.

We have discovered, however, that the disintegration of the auxiliary electrodes can be greatly reduced, and the plating of the cathode reduced, by positioning the auxiliary electrodes at such a distance ahead of the cathode that when the cathode is negative they will be in the so-called Faraday dark space. In that region, the ion density is considerably lower than in the negative glow; thus the sputtering or" the auxiliary electrode will be substantially reduced.

We have found that by so placing the auxiliary electrode, We have been able to greatly extend the lamp life and to improve the lumen output of the lamp during life because of the consequent reduction in blackening of the lamp envelope.

It might be thought that by moving the auxiliary electrodes ahead of the cathode, the Faraday dark space would be moved ahead also, but that is not the case, because the main current flow still emanates from the thermionic portion of the cathode when the electrode is negative.

Although the disintegration of the auxiliary electrodes is reduced by placing them in the Faraday dark space, positive ions still reach the cathode, and a further protection is desirable for best life.

We have discovered that the addition to the neon of argon in amounts of about 3% or more, and preferably from 10% to 25%, by volume, will further protect the cathode. Neon has a metastable energy level at about 16.6 electron-volts, and in the absence of argon, or with too small an amount of argon, the voltage drop at the cathode may be of about that value, or somewhat higher. Neon ions may therefore be produced by two-stage ionization in the cathode sheath, and neon ions of 16.6 electronvolts energy will thus strike the cathode and disintegrate it. We have discovered, however, that if suflicient argon is added to the neon, the voltage drop at the cathode will be reduced to a much lower value, approaching the 12.5 electron-volt value of the metastable level of argon, and hence no neon ions can be produced, and only argon and mercury ions strike the cathode. Under those conditions, the cathode life will be greatly increased.

Such an amount of argon is far greater than the small fraction of 1% sometimes used in neon to reduce the starting voltage, and in fact there is no need to reduce the starting voltage in the neon-mercury fluorescent lamp, because it is already so low that the lamp has a tendency to start as soon as the voltage is applied, before the cathodes have time to warm up. For the present purpose, that of protecting the cathode, the preferred amount is between 10% and 25%, because that is the amount which will give a low voltage drop at the cathode without decreasing the light output more than a few percent, and without decreasing the total voltage drop across the lamp very much.

Although the auxiliary electrodes themselves are pro tected by being placed in the Faraday dark space, they must still be supported from the envelope by leads which are connected to the cathode, directly or indirectly, and which must ordinarily pass out of the dark space and through the negative glow region where ion densities are high.

The supporting wires, therefore, will be subject to disintegration by ion bombardment in the vicinity of the cathode, and unlike the auxiliary electrodes, they cannot be moved entirely into the dark space. They must extend out of the latter and into the disintegrating region, because they have to reach a supporting member. We have discovered, however, that all metal parts which are not required by their function to be exposed to the discharge should be protected from it by solid insulating material, such as a ceramic. This includes particularly the cathodecoil supports, which by their nature must be-located in the negative glow region, and the anode supports.

The insulation must be resistive enough to maintain its outer surface at the floating potential in the discharge at the point considered, so that the energy of ions reaching it will be very small.

In low pressure mercury vapor fluorescent lamps of very high output, a shield is sometimes used between the cathode and the end of the lamp to cool the end enough to keep the mercury vapor pressure low, say at about 10 microns. These shields can be of metal, and connected to one or the other of the lead-in wires to the electrode. If a shield so connected is placed close to the cathode, it also may be disintegrated by ion bombardment, although it operates at much lower current density. It is accordingly desirable to place the shield far enough hehind the cathode to be out of the negative glow surrounding the cathode. If the shield is out of the negative glow, the positive ion current density reaching it will be low.

Other objects, features and advantages of the invention will be apparent from the following specification, taken in connection with the accompanying drawing in which:

FIG. 1 is a view, partly in section, of a discharge lamp according to an embodiment of the invention;

FIG. 2 is a schematic view of the discharge in such a lamp; and

FIG. 3 is a cut-away view showing the mount at an end of the same lamp.

In FIGURE 1, the lamp 1 has the elongated tubular envelope 2, the interior surface of which carries the phosphor coating 3. A stem 4 is sealed across each end of the envelope 2, and has a stem press 5, through which the lead-in and support wires 6, 7 are sealed. The auxiliary electrodes 8, 9 are in the form of wires bent into a plane transverse to the envelope 2, each wire being welded to an end of one of said lead-in wires.

The lamp is filled with an inert gas mixture at a pressure of about 1.3 millimeters of mercury, the gas containing about 20% argon and 80% neon. The lamp also contains a drop of mercury, sufficient in amount to produce a pressure of about 10 microns during operation of the device. A metal disc 10, concentric with the axis of said tube, is attached to and electrically connected to leadin wire 6 near the stem press 5, to control the mercury vapor pressure by acting as a radiation shield for the end of the lamp. The other lead-in wire 7 passes through the disc 10, from which it is insulated by the glass collar 11 which extends upward from stem press and around said lead-in wire through a hole in said disc.

The filamentary cathode 12 is supported between the lead-in wires 6, 7 in a position intermediate the disc and the auxiliary electrodes 8, 9. One end of the filament wire is electrically connected to filament lead-in wire 6 and the other end to lead-in wire 7. The cathode can be a triply-coiled wire carrying a filling of alkaline earth oxides in the manner known in the art. A similar electrode mount structure is placed at the other end of envelope 2. At each end of the envelope 2, an opening 13 in a stem 4 is in communication with a sealed exhaust tube 14. The mount structure is shown in more detail in FIGURE 3.

FIGURE 2 shows schematically the various regions of the discharge between the electrodes. The cathode glow and cathode dark space would be very narrow in extent and invisible inside the negative glow around the cathode, so they are omitted from the diagram.

The negative glow N surrounds the left-hand cathode 12, extending ahead of it to the beginning of the dark region F called the Faraday Dark Space. In the latter, the electron energies are too small to produce much ionization or excitation, so the region remains dark.

The dark space F extends from the end of the negative glow N to the beginning of the region P called the positive column, which in turn extends to the electrodes at the right-hand end of the discharge in FIGURE 2. In a long discharge lamp, the positive column occupies most of the length of the envelope 2.

The auxiliary electrodes 8, 9 are placed in the region which will be the Faraday Dark Space when the cathode 12 to which they are connected is negative. For neon pressures of from about 1 to 3 millimeters, the dark space 15-. is centered at about three-quarters of an inch in front of the cathode 12, and may be about one-quarter to one-half inch in length. (By in front of or ahead of the cathode, we mean nearer to the electrode at the other end of the envelope.)

In order to insure that the whole of each auxiliary electrode 3, 9 will be within the dark space F, the electrodes are best shaped to lie entirely in a single plane, as shown in the figures. To further insure that ordinary variations in manufacturing will not shift the auxiliary electrodes to a position in the negative glow, it is generally desirable to position the electrodes 8, 9 slightly ahead of the middle of the dark space F. In practice, for neon pressures of about one to three millimeters, it is best to place the electrodes 8, 9 about seven-eighths of an inch in front of the cathode 12, so that the cathode, if accidentally placed oif center during manufacturing, will be more likely to be nearer the positive column than to the motor deleterious negative glow.

The disc 10 is placed far enough behind the cathode to be out of the negative glow, which extends around the cathode in all directions. In one embodiment of the invention, the envelope 2 was about 45 inches long and about 1 /2 inches in diameter. The distance between the filaments 12 at opposite ends of the envelope was about 40% inches, with the auxiliary electrodes 8, 9 set Va inch in front of the center line of the cathode 12, and the disc 10 set about inch behind said center line. The distance from either disc 10 to the nearest end of the envelope 2 was about 1% inches. The tube was filled with an inert gas mixture of 20% argon and neon, by volume, at a pressure of about 1.3 mm. of mercury, and a drop of mercury sufiicient to provide a vapor pressure of about 10 microns during normal operation of the lamp.

The cathode 12 was made of 2.9 mg. wire wound at 280 t.p.i. (turns per inch) on a double mandrel of 60 mg. tungsten and 8 mg. molybdenum wires, the molybdenum afterward being removed by acid in the usual manner, to give a coil fitting loosely on the tungsten mandrel. (The wire size is given in milligrams per meter, indicated merely by mg..) This coil was then coiled again, this time at 36 t.p.i. on a 12 mg. molybdenum mandrel, which is afterward removed by acid. This double-coil is then coiled once more, this time at 24.5 t.p.i. on a 50 mil steel pin, which is afterward mechanically removed. The coil is then shaped to have 5 turns extending over a distance of 6 to 7 millimeters with straight legs at the ends extending in a direction axial to the coil, so that the total outside length is about 16 to 18 millimeters.

The coil is then dipped into a coating suspension of alkaline earth oxides and zirconium dioxide, as shown in US. Patent 2,530,394, issued November 21, 1950, to E. F. Lowry et al. About 18 milligrams of the mixture when dried is carried by the coil.

The auxiliary electrodes were of 0.070 inch diameter nickel wire, 18 mm. long along their main portion parallel to the cathode, and 6 mm. along their bent portion.

The disc 10 was 27 mm. in diameter, of 0.005 inch thick nickel sheet.

A base (not shown) such as described in copending patent application Serial No. 637,196, filed January 30, 1957, by Stanley C. Shappell is fixed to the end of each lamp.

The lead-in wires 6, 7 are encased in the aluminum oxide tubes 17, 18 to protect them from ion bombardment. This is desirable in order to protect them from high energy ions because they must extend out of the Faraday dark space and through the negative glow before reaching the filament 12 and the stem press 5. Instead of being encased in the aluminum oxides tubes 17, 18 the lead-in wires may be protected by a heavy coating of suitable glass or may be coated with insulating material in some other suitable manner.

The protecting tubes 17, 18, 19, 20 can be of Alsimag #656, sold by the American Lava Company. We have used tubes of that material of 0.125 inch outside diameter and about 0.056 inside diameter, over nickel leadin wires of about 0.05 inch diameter. (The lead-in wires are of 0.025 dumet wire where they go through the glass.) It is convenient to apply the tubes 17, 18 in two sections, as shown in FIGURE 3, one on the lead-in wire above the cathode, and one on the lead-in wire below the cathode. The region between the two sections, that is, the regions where the cathode 12 is welded, clamped or otherwise attached to lead-in wires 6, 7, should be coated with an insulating cement, such as that known as sauereisen, suitable for vacuum use, unless the space between the sections, for example, the distance between tubes 17 and 18, is small enough so that no appreciable current is taken by the metal exposed at the junction.

In operation the lead-in wires 6, 7 which extend outside the envelope 2 to the base (not shown) are connected to a source of heating voltage of about 3.6 volts, and a source of discharge voltage of about 320 volts open-circuit is connected between one of the lead-in wires 6, 7 at one end of the lamp and one of the lead-in wires 6, 7 at the other end, in series with another identical lamp and an impedance or ballast in the usual socalled series-sequence type of rapid-start circuit. The lamp current is about 1.3 amperes.

When the various voltages are applied to the lamp the filaments 12 will begin to heat up and an arc will strike between them. If the fill gas is predominantly neon the discharge will generally start before the filaments 12 have reached their full operating temperature. The auxiliary electrodes 8 and the metal disc 10 will therefore participate in the discharge, starting as socalled cold electrodes. Such action produces considerable sputtering of these parts; but the sputtering will be less when the electrodes are placed as mentioned above and the neon filling-gas contains enough argon to give a low voltage drop at the filament 12.

The metal discs 10 placed at each end of the envelope 2, reflect radiation from the discharge away from the ends of the tube to provide a cool end chamber. Although the main tubular portion of the envelope 2 may operate at 85 C., the end will be sufliciently cool to fix the mercury pressure at the value corresponding to about 40 C. namely about 10 microns. In that way a lamp may be operated at very high power inputs without increasing mercury vapor pressure above the 'value for maximum efiiciency. In this manner a high-efiiciency lamp having a light output several times as great as is obtainable from lamps that do not use such shield is quite practicable.

The lamp of the specific example gave a light output of 6200 lumens after 100 hours of operation, with a current of about 1.3 amperes and a voltage of about 87, corresponding to an input of about 100 watts.

Although for convenience the invention has been described with respect to a specific embodiment, that was by way of example and not by way of limitation, and various modifications therein will be apparent to a person skilled in the art, without departing from the spirit and scope of the invention.

What we claim is:

1. An electric discharge lamp having a Faraday dark space, said lamp comprising an elongated sealed envelope, mercury therein, a filling of inert gas at low pressure therein, and an electrode at each end of said envelope, one at least of said electrodes having an electron-emitting cathode to fix the position of the Faraday dark space, and an auxiliary electrode, connected to said cathode and positioned in the region of said Faraday dark space.

2. The lamp of claim 1, in which neon is a major component of the inert gas.

3. An electric discharge lamp having a Faraday dark space, said lamp comprising an elongated sealed envelope, mercury therein, a filling of inert gas at low pressure therein, and an electrode at each end of said envelope, one at least of said electrodes having an electron-emitting cathode to fix the position of the Faraday dark space, an auxiliary electrode positioned in the region of said Faraday dark space, electrical connections in said envelopes from said auxiliary electrode to said cathode, an insulating covering for said connections, and a metal shield between said cathode and the corresponding end of said envelope, said shield being outside the negative glow region around the cathode.

4. An electric discharge lamp having a Faraday dark space in its discharge region, said lamp comprising an elongated sealed envelope, a vaporizable metal therein, a filling of insert gas at low pressure therein, and an electrode at each end of said envelope, at least one such electrode including a metal coil carrying alkaline earth oxides as electron-emissive materials, an auxiliary electrode between said cathode and the electrode at the other end of said envelope, said electrode being positioned in the region of said Faraday dark space, electrical connections in said envelope, from said auxiliary electrode to said cathode, an insulating covering for said connections, and a metal shield between said cathode and the corresponding end of said envelope, said shield being outside the negative glow region around the cathode.

5. An electric discharge lamp having a negative glow region, said lamp comprising an elongated sealed envelope, a vaporizable material therein, a filling of inert gas at low pressure therein, and an electrode at each end of said envelope, one at least of said electrodes having an electron-emitting cathode to fix the position of the negative glow region therearound, and a shield between said cathode and the nearest end of said envelope, said shield being positioned outside said negative glow region.

6. The lamp of claim 5, in which neon is a major component of the inert gas.

7. The lamp of claim 5 in which the shield is electrically-conductive and is electrically-connected to said cathode.

8. An electric discharge lamp having a negative glow region and a Faraday dark space, said lamp comprising an elongated sealed envelope, mercury therein, a filling of inert gas at low pressure therein, and an electrode at each end of said envelope, one at least of said electrodes having an electron-emitting cathode to fix the position of the Faraday dark space, an auxiliary electrode, connected to said cathode and positioned in the region of said Faraday dark space, and a shield between said cathode and the nearest end of said envelope, said shield being positioned outside the negative glow region around said cathode.

References Cited in the file of this patent UNITED STATES PATENTS

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