US2313646A - Gaseous discharge lamp - Google Patents

Description

March 9,1943. L. B. JOHNSON 2,313,646

GASEOUS DI SCHARGE LAMP Filed March 4, 1941 lnvenfor 5 H15 A'bkorneg.

Patented Mar. 9, 1943 2,313,646 GASEOUS DISCHARGE LAMP Lyman B. Johnson, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Application March 4, 1941, Serial No. 381,633

3 Claims.

My invention relates to electric gaseous discharge devices and especially to those designed for the production of electromagnetic radiation for lighting or for general irradiation purposes.

In the past, it has always been considered practically impossible to employ bare metallic cathodes in gaseous electric discharge lamps for the reason that the metal is quickly disintegrated and scattered as a black, finely divided deposit on the tube walls by the high positive ion bombardment which necessarily accompanies such cathodes. The prior art has persistently directed all its efiorts toward the improvement of the activating materials which have hitherto been the only means for controlling this disintegration and blackening of the walls. By relatively simple, yet radical, departure from the recognized procedure I have, however, made it possible to dispense with all such activatingmaterials and to operate with bare metallic cathodes. By the improved procedure, I have obtained even better suppression of these undesirable efiects.

It is accordingly an object of my invention to provide a new and improved gaseous discharge iamp, particularly one ofthe high pressure type, and a new and improved method of operating the same whereby cathode sputtering and consequent lamp blackening is matrially decreased.

It is a further object of the invention to provide a new and improved cathode for lamps of the aforesaid type which shall be simpler in construction and more economical in manufacture.

It is still a further object of the invention to eliminate the necessity for providing electron emissive coatings on cathodes used in gaseous discharge lamps.

Further objects and advantages of the invention will appear from the following detailed description and from the accompanying drawing, the single figure of which shows a lamp constructed in accordance with the invention along with a suitable circuit for operating the same.

Gaseous discharge lamps in which a discharge is passed between electrodes in a metallic vapor atmosphere have come into wide-spread use in recent years. Generally, these operate with mercury as the vaporizable and ionizable metallic vapor and include an ionizable inert auxiliary gas, such as one of the rare gases, at pressures up to several millimeters, to assist in start-- ing. The lamps are generally classified in one of two groups depending upon the pressure of the mercury vapor. In the so-called low pressure group, represented for example by the wellknown Cooper Hewitt lamps, the lamps operate at a relatively low temperature, and consequently at very low pressure. The discharge at such pressures characteristically spreads out into a uniform glow which fills the entire tube. In the high pressure field, of which the U. S. Patent No. 2,202,199 to Germer is representative, the lamp is allowed to heat considerably and, as a result, the pressures may gradually build up to very high values ranging anywhere from a fraction of an atmosphere to many atmospheres. At such pressures, the arc is characteristically constricted into a narrow thread. The dividing line between the two groups may be taken as the pressure at which the discharge begins to contract. While it is not well defined, it does occur somewhere between several hundred millimeters and about one atmosphere.

One of the chief problems in the design of any gaseous discharge lamps for a reasonably long, useful life has been the construction of cathodes which do not disintegrate under bombardment to which they are necessarily subjected by the current-carrying particles of the arc. This bombardment is caused principally by positive gas ions which are accelerated to high velocities by the so-called cathode fall. Unless preventative measures are taken, such ions attain sufiicient velocity to knock minute particles of metal or coating from the cathode. This phenomenon results in a gradual disintegration and final destruction of the cathode, as well as a blackening of the tube wall upon which the particles collect. The destructive efiect upon the cathode is particularly prevalent during the starting period of the lamp, 1. e., during the period of time before the cathodes have attained proper emission temperature. In the case of the high pressure lamp, the period will also include the time before the gaseous pressures have built up to their normal operating values. During this period a relatively high potential fall exists in the neighborhood of the cold cathode, and the positive ions which strike the cathode have, as a result, much more energy. Further, in the high 3 pressure lamp, because the gas pressure at this time is still low, there are relatively few gas particles with which the positive ions might collide and thereby lose velocity. 0n the other,

bythe pressure oi the emitted electrons, Further, in the high pressure case, when the gas pressure is atlnormal operating value,.the cathode is blanketed by gas particles which reduce by collision the velocity of the positive ions. As

a consequence of these conditions, the positive ions cannot acquire suillciently high energies to materially damage the cathode. The destructive effect of bombardment is added to by a tendency of the arc to jump around upon the surface of the cathode in an apparent attempt to'find' a.

highly active area. The latter phenomenon does not occur with unactivated electrodes.

The destructive and blackening effect of the bombardment has been so great that it has been hitherto impossible to construct a, commercially successful gaseous discharge lampwith bare electrodes. In the past, about the only commercially by any source of power, such as-the autotranssuccessful way to avoid or reduce the deleterious effects of bombardment has been to coat the .cathode with a layer of so-called activating ma-,

terial, that is, a material which when heated emits electrons much more copiously than does the cathode metal itself. Examples'of activators are certain metallic oxides,-or materials which break down to' form the oxides when heated, such as barium or strontium carbonates or hydroxides. As already stated, electrons emitted from such coatings when the electrode has been sufliciently heated reduce the cathode fall, which is largely responsible for the high velocity positive ions. This method has various disadvantages, however. In the first place, it is neces e ,sary to provide a tough binder to insure that thecoatings will adhere properly to the cathode. It is also generally necessary to subject the coatings'to some sort of an activation process before they are in proper condition for emission. All this adds to thetechnical difiiculties and cost ofmanufacture. Furthergfor various reasons, the coatings tend to flake oiT during the life of the cathode, which process in itself adds to th blackening of the envelope.

Now, I have discovered that it is possible to operate gaseous discharge lamps, particularly those of the high pressure type, with bare -metal electrodes for a substantial, useful life without undue sputtering or blackening of the tube walls, provided that the gas pressure of the auxiliary starting gas is raised to a value within the range hereinafter specified. Within that range there are a sufiicient number of auxiliary gas molecules with which the positive ions may collide and thereby lose their velocity. In. the past, the trend of the art .has been" in the opposite direction for it has been customary to choose very low auxiliary gas pressures in order that the discharge could be initiated by relatively simple means and at comparatively low voltages. The fact that a greater concentration of auxiliary gas particles in the vicinity of the cathode would aid in suppressing sputtering of electrode particles and thus allow the use of a cathode of simpler construction was apparently overlooked. While the starting voltage is raised by such increased concentration, I find that it need not be raised to a value which can not be readily provided by relatively simple and economical the prior art, which has persistently clung to the principle of lower auxiliary gas pressures and low voltagestarting. 1

Referring to the drawing, the lamp ilcomprises an envelope i! of vitreous material such as quartz. or glass, a pair of operating electrodes l3, -and a gaseous atmosphere comprising a metal vapor. Preferably mercury, and an auxiliary gas. The mercury vapor atmosphere during lamp operation may be in either a saturated or unsaturated state. If it is saturated, the pool I of mercury may be provided for the purpose of supplying an excess of mercury molecules in the atmosphere. If it is unsaturated, only a limited quantity of mercury lnsufilcient to produce a saturated condition at operating temperature is introduced. The electrodes II are preferably formed of bare and unactivated refractory metal such as tungsten. The lamp may be energized former II, having the secondary winding I6 and primary winding ll energized by a source ll of electrical energy. The source ll. may be the ordinary volt, 60 cycle supply. For the purpose of starting the lamp, the circuit is provided with the auxiliary relay is which, when held in the closed position by the spring 20, causes the lamp to be short-circuited by resistance 2|. As soon as the autotransformer is energized, the plunger 22 of the relay is attracted toward the transformer core by the leakage flux in gap 28 until its movement is arrested by the stops 23-23. The opening of the relay contacts 24-2 l when relay armature 25 is withdrawn causes a high voltage surge to appear across the lamp which initiates the breakdown of the arc discharge. The surge is carried to the proximity of the discharge space by starting coil 20. The

air gapl'l. is provided in order to insure that the voltage does not reach a destructive value. The aingap may, for example, be adjusted to break down at 4000 volts. Resistance 2| may be of such magnitude that it limits the curren through the switch to about 1 ampere.

The lamp in one particular case had a length of 15 centimeters between electrodes, and an internal diameter of 16 mm. The cathodes were formed of one or more turns of 30 mil tungsten wire wound closely around a core of 30 'mil tungsten wire. During operation the voltage across the t'erminals'of the lamp was about to volts R. M. S. at about 3 amperes, the maximum voltage delivered by the transformer being 250 volts R. M. S.. I have found that a lamp operating under these conditions, and with the auxiliary gas pressures hereinafter specified, will break down and initiate a discharge at voltage within the eppr'oximate range of 350 to 500 volts. The final operating pressure of the mercury vapor was about one-half of one atmosphere. The electrode temperature during operation was about 3000" Kelvin. f

It has been customary hitherto to employ auxiliary gas pressures up to about 20 mm? At such pressures a bare tungsten surface would sputter vto employ too high a pressure, otherwise an undesired spectral energy distribution and unduly high starting voltages in the tube may result.

" Several other advantages of my invention might be mentioned. In the first place, if the tungsten electrodes are used in preference to the activated type, the lamp may be kept considerably cleaner, since in that case there is only tungsten to sputter and evaporate, whereas in I the activated type the activating materials themselves are present in addition. Moreover, because of the much. lower vapor pressure of tungsten, the metal that does sputter tends to condense on that portion of the tube wall immediately adjacent the. cathode and therefore the tendency to become deposited on the central portions of the tube through which the light is emitted is less than when activators are used. Tungsten also has very much less tendency to initiate the. devitrification of a quartz envelope than do the usual activating materials. The alkaline earth compounds in particular show a marked tendency to activate or initiate devitrification of quartz or glass when deposited thereon.

Further, as already stated, the arc does not tend to wander around on the cathode surface if the bare electrode is used.

I have found that while the starting voltage will generally be increased by increase in the pressure of the auxiliary gas in the envelope, it is possible to minimize the starting voltage by suitable choice of the material of the starting gas. Thus, in one lamp having an auxiliary pressure of 65 mm. of argon, the starting voltage was measured as 265 volts. .By changing the starting gas to a mixture of 93% argon and 7% krypton-xenon mixed (96% krypton-4% xenon) at 65 mm. the starting voltage was decreased to 245 volts. The use of this particular mixture in 'a lamp having full wave starting electrodes (a starting electrode associated with each main electrode) made it possible to start an unactivated electrode high pressure lamp at 220 volts.

As has already been stated, with metallic vapor pressures of the orders here involved, the are will tend to be constricted and will center itself upon a very small area of the cathode. With electrodes of the type shown in the drawing it will tend to center on the tip of the electrode. The emission area will, therefore, be substantially constant for different values of arc current: i. e., it remains positionally unchanged, instead of wandering about on the electrode surface, as in the case of an activated electrode,regardless of the size of the emission area. In view of this, it becomes possible to choose an optimum size and shape of the electrode which will have a tip area of this size and which will at the same time cause that area to operate at the temperature at which the desired amount of electrons necessary to the maintenance of the are are furnished. For example, if, for a given value of arc current, the electrode area is too small and/or the rate of conduction of heat fromthe electrode is too small, the electrode will run at an unnecessarily high temperature during operation. The higher temperature will add to the rapid deterioration of the cathode by evaporation. On the other hand, if the electrode surface is too large and/or the rate of conduction too great, the cathode will operate at too low a temperature during normal operation. In this case, the cathode fall will be unnecessarily large and the cathode will be subject to more rapid disintegration by bomunnecessarily increased by the added losses which occur in the increased cathode drop and in the additional losses due to radiation and conduction from the unnecessarily large cathode area. By eflecting a compromise between these two limiting conditions, it is possible to design a cathode of optimum dimensions which will minimize the losses due to cathode evaporation on the one hand, and to high cathode drop, radiation and conduction, and sputtering on the other.

The operation of the aiore-described lamp will be somewhat similar to that of the hish pressure mercury lamps now common in the art. When the discharge is initiated, the heat of the arc will gradually build up the mercury pressure to that of normal operation, the current and voltages relations being substantially the same as those disclosed and claimed in the U. 8. Patent No. 2,202,199 to Germer. Under these conditions the mercury vapor will be sufficient in itself to protect the electrodes from sputtering. During the warming-up period when the mercury pressure is insuflicient to so protect the electrodes from bombardment, the auxiliary gas will perform that function. The auxiliary gas may thus be looked upon as a temporary means for protecting the electrodes from bombardment during the starting period. 5

By employing the principles of my invention. I have been able to construct high pressure lamps which have shown substantially no sputtering or blackening for a useful operating life of more than 1000 hours. The lamps have the further advantage that the starting voltage issubstantially constant throughout life. Such is not the case with the conventional type of activated electrode, since most activating materials are volatile and easily sputtered from the electrode.

While I have stressed the application of the principles of my invention in connection with high pressure lamps, it will nevertheless be understood that they may be applied to low pressure lamps, provided one is willing to forego the advantages of the lower starting voltage which is characteristic of those lamps, and which has hitherto been necessary it a low cost lamp is to be had. It may also be necessary to tolerate some impairment of the spectral quality of the low pressure discharge if the auxiliary gas pressure is raised. It should also be understood that while I have preferred the unactivated type of cathode for the reasons given, nevertheless, high auxiliary gas pressures may also be utilized to reduce deterioration of the activated type of cathode as well. Further, it is to be understood that many variations and modifications in the physical structure of the lamp, such as electrode shape, envelope shape, etc., as well as in the pressure ranges of both the auxiliary gas and the metallic vapor may be made without altering the general mode of operation. All such variations and modifications I aim to include within the scope of the appended claims.

1. In a high-pressure metal vapor electric discharge lamp of the cold-starting, solid, selfbardment. Further, the lamp wattage will be heating, widelyspaced electrode type having an elongated tubular envelope with the electrodes mounted near its opposite ends, and containing in said envelope vaporizable and ionizable metal which during normal running is vaporized and maintained at arc-constricting pressure suflicient to protect the electrode functioning as cathode from sputtering, though when unvaporized it is out of direct heat-conductins relation to said electrodes the combination with a compact. unactivated. tungsten cathode electrode which in running is locally heated by the discharge to emissive'temperature over a small area centered on the electrode tip that remains substantially unchanzinz for each value of the discharge current; oi. an atmosphere oi ionizable starting as in the lamp envelope at a pressure corresponding substantially to 50 to 150 mm. mercury pressure oi argon, whereby the cathode: electrode is protected irom sputtering, during startinz, until the heating of the lamp and the resultinavaporization oi. the aforesaid vaporizable metal produces sumcient metal .vapor pressure to protectv the a,s1a,e4e

electrode from sputtering, while the discharge starts readily under voltage moderately exceedin: the rated running voltage of the lamp.

2. The invention as set forth in claim 1 furture durinz normal running is oi'the order of LYMAN B; J OHN BON.

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