US2084751A - Gaseous electric discharge device - Google Patents

Description

June 22, 1937. c. J. R. H. VO N WEDEL GASEOUS ELECTRIC DISCHARGE DEVICE Filed Feb. 15, 1935 IN VE N TOR l Y d M w m n T O T UIMA P J. fv. a c m Patented June 22, 1937 UNITED STATES PATENT OFFICE assignor to Thomas A. Edison, Incorporated,

West Orange, N. J., a corporation of New Jersey Application February 15, 1935, Serial No. 6,612

12 Claim.

This invention relates to gaseous electric discharge devices; and while various features of the invention have utility in broader connections, the invention has especial reference to devices adapted to operate with high vapor pressures. Throughout the specification the term gaseous has been employed as an adjective relating to either a gas or a vapor or a combination thereof, while the terms gas and vapor are each used in a more specific sense.

A general object of my invention is to provide an improved high pressure discharge device.

Another object is to provide a high pressure discharge device adapted to operate over a satisfactory period of life without the use of external cathode heating means.

Another object is to provide improved means for starting the high pressure discharge in such devices.

Another object is to provide an improved electrode structure for use in such devices.

Still another object is to provide for use therein a cathode element characterized by great resistance to disintegration.

It is another obiect to provide improved methods and preparations for coating the cathode elements.

It is another object to provide improved and convenient means for introducing into the device 30 small quantities of auxiliary vapors wherewith to influence. the color of the discharge.

It is still another object to reduce the cooling of the envelope of such a device whereby to permit higher vapor pressures.

Other and allied objects will more fully appear from the following description and the appended claims.

In the description of-my invention, hereinafter set forth, reference is had to the accompanying 40 drawing, of whlch:--

Figure 1 is a longitudinal sectional view, with parts shown in elevation, of a discharge device according to my invention, with a schematic illustration of appropriate circuit components for 45 use therewith;

Figure 2 is an enlarged view of a portion of the device of Figure 1, showing one of the electrodes with an outer portion thereof partially broken away;

Figure 3 is a perspective view of the components of the electrode with the exception of the cathode element, the components being shown separated by slight distances one from another in this figure for purposes of better illustration;

Figure 3a is a cross-sectional view of an alternative construction of one of the components appearing in Figure 3;

Figure 4 is an elevational view of the helical cathode element of the electrode; 1

Figure 5 is a view of a section of the cathode element before its helical formation; and

Figure 6 is a view illustrating the formation of the outwardly extending envelope seal which is a feature of my invention.

The various features of my invention, with their functions and advantages, are best described following a brief description of the structure and general operation of a typical discharge device in which they are co-operatively employed. Such a device is illustrated in Figure 1. Herein I represents a sealed glass vessel or tubular envelope I evacuated of air and containing a filling of a monatomic gas at a pressure of a few mm. Hg. as well as some source 5 of metal vapor. This source for example may be mercury or alkaline metal or a mixture of the two, may cling to the wall of the envelope I when cold, and is adapted upon heating of the envelope to vaporize and to develop a vapor pressure within the envelope.

Closely adjacent the respective ends of the envelope are the main electrodes 2 and 3, respectively supported on the lead-in wires 2b-2c and 4b-4c sealed in the ends of the envelope. Die main electrodes 2 and 3 are mutually similar; their components and assembly are illustrated in Figures 3 through 5 and hereinafter described in detail. while there appears in Figure 2 an en-- larged view of one of them in assembled form (electrode 3) with a portion of the outermost member or cylindrical shield can broken away to show the inner member or cathode element 28. An extremity of the cathode element in each electrode is electrically connected to the surrounding shield can, while the other extremity is connected to the corresponding lead-in wire 2b or to, as the case may be. In each main electrode, as hereinafter more particularly set forth, the shield can is closed by suitable end members so that the cathode element is quite enclosed excepting for a hole 24 in the center of the end member facing the central portion of the envelope. Close to one of the main electrodes-e. g., electrode 3is shown a starting electrode 6 in the form of a metal ring supported on and electrically connected to a lead-in wire 4a sealed in the adjacent end of the envelope i alongside the lead-in wires 4b and 4c.

The envelope l is placed within an outer glass I bulb 'l of tubular form, which may be evacuated or filled with gas, and which serves to insulate the envelope I from outside atmospheric convection currents and thus to reduce cooling oi the envelope i. The end of the envelope wherethrough pass the lead-in wires 4a--4b4c may be held in place within the bulb 1 by the sealing of lead-in wires 41) and 4c in the stem 40 of the bulb; the opposite end of the envelope i may be centered in the bulb l as by means of a disc 8 of heat insulating material such as mica. Thus the lead-in wires 2b-2c may be passed through this disc, and the wire 2b folded over and re-passed through the disc to be connected as later specifled, this serving to secure the disc 8 to the envelope i; the periphery of the disc is made to fit the interior cross-section of bulb '5, allowing only for longitudinal sliding to permit diiierent longitudinal expansions and contractions of emveiope i and bulb i.

The lead-in wire 2b, after its re-passage through the disc 8, is extended along the outside of en'- velope itoward the other extremity thereof by means of wire it, this wire being held in position near each end of the envelope as by the metal rings iila encircling the envelope. The wire iii is then sealedly led through the stem ii! to one terminal of the Edison base ii secured on the bulb l, the lead-in wire db being extended through the stem ill to connect to the other terminal of base ii. A resistance d is connected within the bulb d from the starting electrode lead in wire do to the wire it. The base it is connected to the power supply terminals id through a choke coil ii, the terminals it being shunted by a capacity it for power factor correction, as will be understood.

While in the operation of initiating a discharge in the illustrated device the cathode element in one or those in both of the electrodes 2 and 3 might be pro-heated to some emissivity by appropriate external heating means, I have preferred to avoid the lead-in and other complications which this would entail, and first to start anauxiliary discharge from the starting electrode E to the cathode element 28 in electrode 3 with this element cold. This element being connected through the lead -in wire db to one of the supply terminals M, and the starting electrode 8 being connected to the other supply terminal through the resistance 9 and choke coil 312, this auxiliary discharge is started as a glow discharge during alternate half cycles as soon as the supply voltage is applied to the terminals. This discharge of course occurs in the simple low pressure gas atmosphere, the envelope i being cold and the metal vapor having therefore not yet formed. The glow discharge current is made of suflicient magnitude, by proper choice of the value of resistance 9, so that it presently breaks down the normal cathode resistance to glow discharge, heating the cathode element to the extent that arc spots are formed thereon. The auxiliary discharge is thus increased to a value limited principally by the value of resistance 9; and the ions formed by this discharge largely diffuse to the walls of the envelope 8, clearing them of electronic wall charges and permitting an arc to strike from the one main electrode to the other. So the main arc discharge is initiated.

This main arc discharge will be understood to to take place from electrode 2 to electrode 3 in one half cycle, from 3 to 2 in the next, then from 2 to 3, and so on, each of the electrodes 2 and 3 acting alternately as cathode and anode. It will be appreciated that the arc discharge when first initiated has widely different characteristics from those final ones which obtain during its later and normal continuance; and the period through which its characteristics change to the final ones I have herein termed the starting period.

At the beginning of the starting period the discharge spreads in the low gas pressure throughout the cross-section of the envelope I,

passing through the hole- 24 of the electrode momentarily acting as cathode to reach the cathode element proper therein. The device is already warming up and vaporizing some of the metal 5. The vapor thus formed at first largely condenses on the relatively coldest part of the envelope, which may be the top-illustrated portion thereof near the seal of electrode 2. This section gradually warms, however, and the vapor pressure and density continuously increase, increasing the total gaseous density within the envelope i. The vapor, having a lower ionization potential than the gas filling, will be preferredly ionized and soon will be the sole source of ions in the discharge path for carrying the discharge and neutralizing space charges. At first the potential gradient and voltage drop in the device do not change materially because, while the vapor has a lower ionization potential than the gas, the total gaseous density is increasing.

As the starting period progresses, however,

more and more vapor is created and the total gaseous density increases sufiiciently to increase the potential gradient and voltage drop within the device, decreasing the mobility of the gaseous molecules and ions by increased collisions. This leads in turn to further increase of temperature; but this increase now exhibits a selectivity between the cros's-sectional portions of the envelope, occurring to the greatest extent in the central such portion. Accordingly the total gaseous density becomes selectively distributed, the central cross-sectional portion being one of relatively low density and being surrounded by a gaseous shell of relatively high density. The mean free path of the electrons and ions is then of course much greater in the central portion than in the surrounding gaseous shell, and the discharge tends to concentrate in the central portion. As the vaporization of metal 5 continues the gaseous shell surrounding the central cross-sectional portion becomes so dense that it acts'as a discharge path wall, intercepting ions straying from the now central discharge path and causing recombination thereof with slow electrons diffusing in the gaseous shell. This shell then performs the same function as the walls of the envelope I performed before at low total gaseous densities, and the effect of the shell is similar to that which would be produced by constricting the walls of the envelope to a very small radius.

The starting period may be considered as continuing, with steadily increasing temperature of the walls of envelope 8 and increasing total gaseous density, until the radiation losses equal the heat generation of the device; only then has the discharge acquired its final or normal characteristics. Throughout the starting period the potential gradient and voltage drop within the device steadily increase. Meanwhile of course the voltage drop across the choke coil I2 is seadily decreasing, as is likewise the current through the choke coil and discharge path-- 1. e., the discharge current. The excess of the initial discharge current-i. e., that flowing on initiation of the main arc discharge-over the final discharge current may thus be, and indeed normally is, very appreciable.

The voltage drop herelnabove referred to is an average drop across the discharge path. The 5 instantaneous drop in the final arc discharge path and the instantaneous discharge current therethrough are interrelated with the instantaneous temperature of this discharge path, which temperature is capable of rapid and mam terial fluctuation. This path, finally in the form of a narrow light pencil designated as I! in Figure 1, has as above noted a low gaseous density relative to the surrounding gaseous shell, and because of its low density and small size has 5 small heat storage capacity or inertia. This contrasts with the relatively high heat inertia of the balance of the device-i. e., the gaseous shell, the envelope I, etc. The temperature of the pencil I5 is very high at instants of high 20 current, but reduces materially as the current stops flowing each half cycle. This reduction increases the instantaneous potential gradient, and thus increases the restarting voltage of the devicei. e., the voltage required for starting 25 the discharge in the next half cycle. The restarting voltage is therefore materially higher than the average voltage drop in the device, and this average drop can never become comparable with the supply voltage. Dependable restarting is, of course, obtained by the employment of a 30 proper choke coil l2 for the particular discharge device in use.

This general description of a typical discharge device may be concluded with mention of certain structural and operating parameters of devices with which I have actually employed my invention. Thus I have employed a device with a discharge path length (e. g., from electrode 2 to electrode 3) of 15 centimeters and with an envelope diameter of 30 millimeters; the device 40 had a filling of rare gas (e. g. krypton or argon) at about 4 mm. Mg. pressure (as well as a vapor source 5 of mercury), and the voltage drop therein on initiation of the arc discharge (i. e., at the beginning of the starting period) was of 45 the order of 22 volts. The choke coil l2 employed was of such reactance that the initial discharge current was approximately 4.5 amperes. The choke coil and the device cooling facilities co-operated to produce a final or nor- 5 mal voltage drop within the device of about 80 volts, with a final discharge current value of about 2.5 amperes. These current and voltage values are given on the basis of an alternating supply voltage of approximately 120 volts.

55 The attainment of a satisfactory cathode life in a high pressure device of the general class described, without the use of some means other than the discharge itself to heat the cathodes, is a serious problem. It has been shown that in v(:0 normal operation the gaseous density excepting in the high temperature portions-e. g., the pencil Iis relatively high, and consequently the potential gradient excepting in high temperature portions would be very high. It is also to be 65 appreciated that a cathode, because of its relatively higher thermal capacity, has a significant influence on the temperature of the gaseous filling immediately in front of it. The discharge, approaching a simple cathode which is wholly 7 or principally heated through ionic bombardment by the discharge itself, may be considered as confronted with two alternatives: first, to distribute itself evenly over the entire cathode surface, rendering that entire surface at medium 75 temperature with corresponding thermal effect on'the gaseous filling immediately in front of and influenced by the surface; or secondly, to concentrate itself on a very small surface portion of the cathode, heating that portion to a very high temperature and maintaining a high 5 temperature in that small amount of gaseous filling which is in front of and thermally infiuenced by that portion. In seeking the path of least potential gradient, the discharge, of course chooses the second of these alternatives, 1o constricting its pencil as it approaches the cathode to concentrate itself on the small surface portion. This very small portion, or arcing spot,

is thereby called upon to emit sumcient electrons to support the entire discharge, or in other words to operate at an extremely high current density per unit area.

The emissivity of a cathode and its temperature of course rise together; and the practical limit of the emissivity-i. e., of the emission per unit area-is the temperature at which there occurs serious dissociation and disintegration of the cathode material. (It should be noted that this temperature is not uniquely determined by the characteristics of the cathode alone, but is also dependent to some extent on the density and molecular weight of the gaseous molecules in front of thecathode surface-in other words, on the mean free path of the molecules in that space; the permissible cathode temperature being. higher thehigher be that density and molecular weight, or the smaller be that mean free path.) In a device of the character described a simple type of cathode, heated solely by the discharge, is found to be rapidly ruined by the trem/endous temperatures to which the arcing spot rises. The temperature of the arcing spot is not only raised by the localized ionic bombardment above discussed, which occurs in alternate half cycles", but also by the heat produced by the elec- 40 tronic currents during the other half cycles when the electrode in question is acting as an anode. This heat is that liberated when electrons from space adjacent the cathode surface enter that surface with a velocity determined by the anode drop. This heat in watts is roughly equal to the discharge current in amperes times the anode drop in volts, and its effect on the temperature of the arcing spot may be very appreciable.

According to my invention I employ electrodes in which at least the principal part of the electronic currents just mentioned enters an electrode portion other than the cathode element proper, whereby these currents are prevented from significantly further heating any arcing spot; in which there is provided high heat conductivity between portions of the cathode surface-in directions normal to the path of the discharge entering the surface, whereby the heat of any initial arcing spot is induced to spread consider- 50 ably and to divide among other surface portions which will in turn divide the discharge with the initial arcing spot; in which heat conductivity away from active cathode surface portions in the line of the discharge entering the surface is 5 minimized, whereby the heat conduction to useful surface portions is rendered most efiective; in.which an improved cathode surface is employed capable of withstanding very high temperatures without serious disintegration; and in which other desirable characteristics cooperate with those already mentioned to make possible the satisfactory use of the electrodes to support the normal discharge in the device without heating means other than the discharge itself. At 7 the same time the electrodes according to my invention are peculiarly adapted for efficient operation throughout the preliminary discharge and throughout the starting period, so that a desirable overall operation of the device is assured. as will hereinafter become apparent.

Reference is now particularly invited to Figures 3, 4, and 5 for the details of construction of an electrode. The cathode element 28 is made 01 a length of rather heavy core wire 4|. such as may be heated by a few volts and several amperes. The surface of the cathode is desirably indentured or corrugated to provide large surface in small space; and accordingly I may wind around the core wire 4| a fine wire spiral 42 (see Figure 5), plating or welding the wire 42 to the wire 4| .to insure good current and heat conductivity therebetween throughout the entire cathode length. The surface metal of the cathode element should be preferably mainly nickel or other neutral metal or alloys of the iron group the oxyacids of which are not more electronegative than that of molybdenum. The core wire 4| may consist of any refractory resistance wire, plated if necessary to provide the desirable surface characteristics. The cathode element 28 is formed from the so-prepared wire 4| by winding the latter into the form of a helix having an inside diameter of 4.5 to 5 millimeters and with the windings thereof of small pitch, for example, such that adjacent convolutions are spaced apart a fraction of 1 mm., for the typical device whose parameters have been mentioned above, excepting that in the lower portion of the helix the diameter may desirably be progressively reduced to approximate an inverted cone. This is shown in Figure 4. The spacing of adjacent turns of the helix should be made sufiiciently close, that the helix shields electrically the inside of the adjacent shield can surface from the arc discharge when said surface becomes activated by disintegrating cathode material. After this formation of the cathode element 28, the element may be sprayed or otherwise treated with its coating, the desirable nature and characteristics of which are hereinafter set forth. The electrode may optionally further include a disc 29 of fine wire mesh or netting, welded to the external surface of disc 26, for purposes hereinafter mentioned.

While the electrode 3 with the cathode element in place therein is illustrated in Figure 2, the portions of the electrode other than the cathode element are best seen in Figure 3, wherein they have been longitudinally separated by slight distances one from another. The main such portion is the cylindrical shield can 20 of nickel or iron sheet,

of diameter but slightly larger than the outside helix diameter. This can is conveniently provided with the outwardly extending flange 2| parallel to its axis, and is closed by top and bottom metal end members or thimbles 22 and 23, respectively. The end portion of the top thimble 22 forms a disc 26, is provided with the central hole 24 hereinafter more fully considered, and is desirably of a slightly larger diameter than the annular wall of this thimble so as to form a slight overhang or flange. It is important that the disc 26 be of highly refractory material such as molybdenum or other still more refractory metal or alloy; and therefore if the thimble 22 be a single integral part, it should be in its entirety of such material. Alternatively, however, the thimble 22 may be formed, as illustrated in cross-section in Figure 3a, ofa nickel thimble 22' having an end portion 26' without overhang but with the central hole 24, this end portion 26' having thoroughly welded thereto the molybdenum or other highly refractory disc 26'-also of courseprovided with hole 24 and of diameter slightly greater than the thimble 22' diameter. The lower thimble 23, which may be simply of nickel or iron, is provided with a central hole 25, into which fits the centrally pierced ceramic insulating bushing 21.

The wall of top thimble 22 is pierced with a small hole 28', and is slipped over the upper end of cylinder 20 so that this hole 28 is brought into juxtaposition with a small longitudinal slot 28" in the cylinder at the end of flange 2 I. The thimble 22 is then firmly welded all around to the cylinder 20. The cathode element 28 is placed within the cylinder 2|), its upper or large diameter extremity 28a passing outwardly through the slot 28" and hole 28' to be folded over and welded to the flange 2|. ment 28 is fed through the bushing 21 in bottom thimble 23, and that thimble slipped in place over the bottom end of the cylinder 20 and secured thereto as by welding of rod 39a (already welded to thimble 23) to flange 2| and welding of rod 3% (already welded to cylinder 20) to the thimble 23.- The appropriate lead-in wire-e. g., 4cis welded to the flange 2|, and the appropriate leadin wire-e. g. 4bto the cathode element extremity 28b.

These lead-in wires, together with the starting electrode lead-in wire in the case of the particularly illustrated electrode 3, are formed to relatively close adjacency as near below the electrode as practicable; there each is headed with a. glass bead Hi. This is illustrated in Figure 6. The lead-in wires are now clamped just below the seal as by a. temporary clamp I9, and the crossbeads l8 formed between the beads I 8 to result in a rigid structure and the electrode welded thereon as explained. The envelope l, at this stage having an open, reduced-diameter end portion la'large enough to pass the electrode, is slipped thereover until the end portion la is in juxtaposition with the beading |8-|8; the end portion la is then heated, and clamped fiat against and fused with the also heated beading |8-|8' therebyiorming a common press or seal, this operation being conveniently performed in a sealing machine. 1

The desirability of the structure lies in the fact that it reduces to a minimum the area of envelope wall beyond the main discharge path, which, being subjected to less heating effect from the discharge column than the rest of the envelope wall, is the coolest wall area and by virtue of vapor condensation a limiting factor on the vapor density and pressure which can exist in the tube throughout the normal discharge. A further reduction of undesired cooling by this envelope wall portion may be effected by plating, spraying or otherwise coating this portion or at least the extremity thereof on the outside of the envelope with an inwardly reflecting metal mirror, as indicated by the shading II in Figure 1. Aside from its effect on the cathode position and vapor pressure, the outwardly extending envelope seal l6 remains relatively cool and co-operates with the inwardly extending bulb stem 40 to provide a minimum length of lead-in wires therebetween; consequently these wires may be and preferably are made relatively thin, thus facilitating the making of a dependable seal l6, while still re- The other extremity 28b of the cathode ele-' maining well able by reason of their combined 7 strength to support the envelope l within the bulb I.

It will be understood that the electrode 2 may in all respects similar to the electrode 3, the lead-in wires 2b and- 2c and the seal 38 bein respectively analogous to the lead-in wires 4b and 4c and the seal Hi. If desired, however, advantage may be taken of the fact that no lead-in wire analogous to M1 (the starting electrode leadin wire) need be employed adjacent the wires 2b and 2c, and a third lead-in wire 2d may instead be sealed in the seal 38 and utilized as a second support for the cylindrical shield can of electrode 2; this may be welded for example to a second flange 31 provided on this can.

The cathode element 28 is most desirably treated or covered with a mixture of alkaline earth metal oxides with metals or oxides of tial heating of the cathode element and further in the intense heat of the arc discharge. In practice, after incorporation of the electrode structures in the envelope I, each treated cathode element may be heated by passing a heavy current therethrough (for example, in the case of electrode 3, by connecting an appropriate low voltage source across the lead-in wires lb and so that it may be properly de-gassed in vacuum by the exhausting pumps; the oxide covering then in part dissociates, and, instead of remaining loose and of light color, melts or sinters together in the form of a thin, black, strongly metallized, dense, hard crust, like iron forging scale, providing a coating strongly adherent to the element so that it is difflcult to scrape off.

The formation of the metallized coating abovementioned may also be effective by spraying or otherwise applying the covering mixture on nickel and heating the same to a white-yellow temperature a little below the melting point of nickel, as by a reducing-flame in the open air. A cathode treated in this way may be heated in vacuum and activated with a gas discharge. It is also possible to take a cathode having this metallized coating out of a gaseous discharge device in which it has been operating, expose it to the atmosphere, and place it in a new envelope which is thereupon evacuated and filled with gas, and the cathode will perform its duty as formerly, little the worse because of this experience.

To obtain this improved metallized .cathode coating I in general avoid the ordinarily preferred use of carbonates of the alkaline earth metals, using to a. large extent instead hydroxides which melt first on heating in their crystal water. I abandoned the idea that the coating should be a very porous mass of oxides as used in Wehnelt cathodes in devices wherein the current density per unit cathode surface remains low and wherein excessive sintering causes poor emissivity by raising the coating resistance. Instead I strongly metallize the cathode coating and, although sintering occurs to a high degree upon first heating, my improved coating retains high conductivity by virtue of this strong metallization.

As an example of a covering mixture which I have employed in producing my improved. cath- I ode coating I may, cite the following:

This mixture was milled in normal amyl acetate solution containing a few percent of cotton to make the mixture stick to the surface on sprayns.

This completes the structural description of my improved electrodes, excepting for a few desirable relations between certain parameters which are most conveniently brought out in the following description of operation.

The operation of these electrodes throughout the normal arc discharge in devices of the character described has been found quite satisfactory, most of the electronic current flowing to the electrode when that acts as an anode enters and heats the disc 26 rather than penetrate the hole 24 to the cathode element 28, and therefore does not contribute to any overheating of an arcing portion of the cathode element surface; it exerts a small and uniform heating effeet on the entire cathode element by virtue of its flow therethrough, the cathode element being serially connected between the shield can 20 (and hence the disc 26) and the current supply source. The discharge approaching the electrode when that acts as a cathode is deterred, however, by the high work function of the refractory disc 28 from playing thereon, and enters the hole 24 to approach the cathode element 28 generally perpendicular to the axis of that element. High heat conductivity of the cathode element, as well as of the metallized coating thereon, rapidly conducts the heat of any initial arcing spot to adjacent sections of the cathode element surface (i. e., in directions normal to the path of the discharge entering the surface), dividing the total heat with those other cathode element sections and thus inducing the discharge to occur to a greater surface area of the cathode element at lower maximum surface temperature. This desirable heat conduction is aided by the minimization of heat dissipation in other and useless directions, the cathode element being of small dimension in'a direction in the line of the discharge (1. e., of the dimension of the wire diameter), and the shield can 20, closely adjacent the cathode element and itself heated by conduction from disc 26, preventing convection away from the cathode element. It is true that the normal arc discharge will occur most strongly to the portion of the cathode element nearest the hole 24 (e. g., the upper portion of the cathode element in electrode 3), but this portion will be of materially greater area and lower maximum. temperature than the spot which would be selected by substantially the entire discharge in the case of a simple unenclosed cathode; and the improved coating, especially in view of the now relatively high vapor density, easily withstands the existing very high current density per unit surface area.

Throughout the normal arc discharge there exists the tendency of the arc to shorten its path length and to take place to the disc 26 whenever it can obtain sufficient emission from this surface. This tendency is enhanced by the fact that some of the small amount of active cathode material which is bound to disintegrate and leave the cathode enclosure will settle on this disc and thereby reduce its normally high work function: the discharge may shift thereto vfor long enough to consume or disintegrate this material before returning to the cathode element proper. Under these circumstances the disc 26 may become very highly heated, and is likely to melt unless made of refractory material as above specified. (Its thorough welding to the portions of the electrode which support it, abovementioned, isalso important for the rapid dissipation of heat acquired by the disc.) The more electronegative the material of the disc 26, the greater is its tendency to form with disintegrating cathode material oxide compounds which need higher temperatures to yield a given emission, and hence the less will be the tendency of the discharge to leave the cathode element proper; When the arc discharge occurs to-the disc 26, it wanders thereover; and in wandering near the edge of the disc might in the absence of preventive measures heat the adjacent non-refractory metal sufiiciently to render it emissilve. then melting it and destroying the shield can 20. This possibility is guarded against by, the slight overhang of the disc abovementioned.

The temperature to which the disc 26 will rise, and hence again the tendency of the discharge to leave the cathode element, is also increased by reduction of the diameter of hole 24, and the consequent restriction of the high temperature discharge column and of its potential gradient in the restricted part. Independently of the tendency of the discharge to leave the cathode element, too small a diameter of hole 24 will invite the discharge to enlarge this hole by disintegrating the metal therearound, with consequent deposit on and blackening of the envelope walls. But hole 24 is desirably small enough, and the current density therethrough consequently high enough, so that active cathode material disintegrating from the cathode element surface will be strongly ionized and therefore principally transported back to the cathode surface; this reducesthe net loss of cathode material and blackening of the envelope walls thereby. And in any event the opening must be as small as the maximum inside diameter of the helix formed by the cathode element 28, so that the top extremity of the helix will be protected by the disc 26 from ionic and electronic current impact otherwise occurring in the direction of the envelope axis.

The operation of the electrodes during the preliminary or auxiliary discharge and during the starting period may now be considered. The initial glow discharge takes place from the starting electrode 6 through the hole 24 to the cathode element proper; and I have found that with the described arrangement of the cathode element and adjacent electrode portions it is possible to pass in the glow discharge, with normal cathode drop, a relatively high current-i. e., one many times exceeding that capable of being passed with normal cathode drop to a plane cathode surface of equivalent area. The value of the resistance 9, in series with this glow discharge path, should be made low enough to permit sufficient glow current so that the normal cathode drop of the active surface of cathode element 28 is exceeded; and because this element can as above noted pass a relatively high current with normal cathode drop, the resistance 9 cannot be of very high value. For the typical discharge device with parameters abovementioned I have found satisfactory a resistance value of the order of 500 ohms; the resistance should then be capable of dissipating around 11 watts. This handling capacity is indicated by the fact that the current is limited almost solely by the resistance 9 in the latter part of the preliminary or auxiliary discharge; during the normal arc discharge, however, the resistance 9, still remaining in circuit, will dissipate not more than 8 watts-i. e., the loss power" will not exceed 4% of the total power (approxmately200 watts) consumed by the device. While of course a second starting electrode similar to 6 might be employed adjacent the electrode 2, connected through a resistance similar to 9 to the lead-in wire 4b, to aid the electrode 6 in its function, this arrangement would double the loss power just mentioned; and consequently were the double arrangement used it would be desirableto provide a thermostatic switch in series with the second starting electrode and adapted to open in response to the heat of the envelope l as the vapor pressure is in the process of rising. With the devices having parameters as above, however, I have found unnecessaary this double arrangement, and consequently have illustrated only the single one.

The relatively high glow current which is thus an incident of my improved electrodes facilitates the preheating of the cathode element and decreases the duration of the glow discharge- -i. e., it increases the rapidity with which hotspots and cathode emissivity are created. Moreover the shield can 20 prevents convection from the relatively low-mass cathode element; and the net result is a very rapid break-down of the normal cathode resistance to glow discharge, and development of arcing spots on that element; these spots are promptly spread in the same manner as described for normal discharge arcing spots above, and with the same beneficial result of avoiding too high a maximum temperature.

In connection with the auxiliary discharge it may be noted that the starting electrode or ring 6 must be properly apportioned and spaced from the disc 26 in order to obtain proper field distribution and generally favorable results. imaginary truncated cone having the hole 24 as its bottom and smaller base and the ring as its upper and larger base must not have too great an angle of upward divergence, otherwise the field of the ring with respect to the cathode element is strongly intercepted by the disc 26 and the starting voltage may prove insuflicient. On the other hand the ring must be of sumcient diameter to permit the pencil 15 of the normal arc discharge to pass therethrough with a margin, or gaseous cushion, therebetween. For if the pencil l5 impinge upon or even come into too close adjacency with the ring, the latter will become unduly heated and disintegrated or even melted by that discharge. A proper apportionment and spacing ,for ordinary purposes is obtained when the internal diameter of the ring is about twice the diameter of the hole 24 and the ring is spaced away from the disc 26 by about the diameter of hole 24.

When the ionization produced by the final portion of the auxiliary discharge has, by diffusion of ions, cleared away the electronic wall charges and the main arc discharge strikes through the device, the starting period has begun; the opera- The.

tion of my electrode structure during the early portion of this period may now be considered. At the very beginning of this period the cathode elements, having been heated only by the auxthe small rare gas density. Therefore'until the cathode elements can respond with a greatlyand uniformly increased temperature in their various portions, overheating and disintegration by spot arcing will tend to occur. While my improved metallized coating greatly helps the cathode elements to withstand these tendencies, it still remains of the utmost importance that the cathode elements respond with the mentioned temperature increase with great rapidity, so that the time during which these unfavorable tendencies obtain may be reduced to a very low value. The cathode elements in the electrodes according to my invention do so respond; and several reasons therefor may be noted, over and above the low cathode element mass, its high heat conductivity in useful directions only, and the elimination of convection currents by shield can 20, these having been already discussed in connection with the continuing normal arc discharge.

The electronic current approaching one of the electrodes when that acts as an anode largely enters and heats the disc 26, and this heat is quickly spread to the entire shield can 20, by virtue of the high heat conductivity therebetween, and aids in quick response of the enclosed and closely adjacent cathode element. Also as abovementioned this current flows serially through the cathode element and helps to heat it. These considerations are now of especial significance because of the especially high current value. That portion of the electronic current which does pass through the hole 24 encounters the gas within the helix formed by the cathode element and ionizes it; but the electrons are here not further accelerated and are slowed down by collision and spread laterally to the cathode element, contributing somewhat to heating of its lower or reduced diameter portions.

When the current reverses in the next half cycle and the same electrode acts as a cathode, a positive ion cloud is established along the axis of the cathode element by virtue of ionization of the gas by electrons leaving the cathode element surface perpendicularly toward the element axis.

surface cause ionization in the vicinity of the ele-' ment axis. In reverse terms, the mean free path of electrons and gas ions in the gaseous filling as it exists within the shield can must be materially exceeded by the internal radius of the cathode element (in its upper or full diameter section), or in other words the internal element diameter must be several times such mean free path. If the cathode element diameter is appreciably less than so specified, the positive ion cloud will extend outside the shield can, and the cathode element will be unevenly heated by fewer ions of higher field acceleration along a longer path. Because the even heating and current distribution are particularly desired during the early portion of the starting period, the specification just laid down must be referred to the gas filling alone (i. e., without vapor) at the temperature then prevailing within the shield can.

It is also necessary for a relatively even current distribution on the different portions of the cathode element that the voltage drop in the element be small--i. e., that the element be generally of a low voltage, high current type, as has been elsewhere herein specified. The length of the helix formed by the element may be as great as twice the full internal helix diameter without itself causing appreciable inequality of the current distribution over the various portions. The diameter contracton in the lower end of the helix, producing an inverted conical or domelike internal surface, I have found helpful in compensating for the stronger tendencies toward cooling of this section obtaining under discharge conditions.

The cathode elements accordingly respond to the initiation of the starting period with a rapid and even increase of temperature to a point where they sustain the particularly heavy early current without any undue spot heating or tendencies toward disruption. As the starting period progresses the gaseous density and the potential gradient within the device become higher and the current reduces: and gradually the action of the electrodes-in particular the cathode elementsshifts from that which has just been described to that which was'above described as obtaining throughout the normal arc discharge. The cathode elements, for example, shift gradually from a condition of practically uniform heating and total emissivity in low pressure gas to support a large discharge current, to a condition of more localized heating and total emissivity in high pressure vapor (and gas) to support the somewhat smaller normal discharge current. i

I have described above a special coating for the cathode elements which will not develop too much oxygen in the very high current density per unit surface produced on the cathode elements by the concentrated high pressure are discharge. This contrasts with the normal type of Wehnelt coating adapted for operation in low gaseous densities and pressures and consisting of some 10,000 molecular layers of alkaline earth metal oxides, which type of coating in the case of very high current densities per unit surface rapidly dissociates the oxides and liberates oxygen into the discharge path, causing the formation of undesired blackening compounds of OXY'. gen on the envelope walls beside disrupting the cathode. Because the entire cathode element length is active in the early part of the starting period of devices of the character herein described. the current density per unit surface (in spite of the then somewhat higher total current) is then much less than the very high densities above-mentioned, and the Wehnelt type of coating in an electrode structure otherwiseas above described would be satisfactory in this limited part of the operation of the device. It follows that the electrode structure as described, but with normal Wehnelt type coating, may be employed in a low pressure device, particularly if the cathode element or elements therein be preheated to moderate emissivity before any are is permitted to strike. It will be obvious that most of the merits of the electrode structure other than those peculiar to the cathode element coating will remain beneficial; and it is to be noted that these merits include the ability to maintain the cathode element at the 'emissive heat required for the normal discharge in such device solely by that discharge, so that any pre-heating 10 current supply used may be cut off after the discharge has stabilized. It is, however, very important to observe the specified relationship between helix diameter and mean free path of electrons in the gaseous filling as existing within the shield can; and in the cases of some gaseous fillings of small atomic weight, at low pressure and density, the mean free path length may become so great and the cathode element and shield can therefore necessarily so large that the advantages of the structure as to heat conservation are somewhat ofi'set.

A special feature of my invention which may be optionally employed in the electrode structure is the welding of a disc 29, of fine wire mesh or netting, on top of the disc 26, thedisc 29 of course being provided with the hole 24 similarly to the disc 26. The disc 29 should be of neutral metal or compound such as nickel, and may be alloyed or coated with one or more of those met- 30 als of the I and II groups of Mendeleeffs Periodic System which have a lower ionization potential than mercury, such as zinc and/or cadmium or compounds thereof, so that the arc discharge when playing on the disc 26 and hence on the disc 29 will vaporize such alloying or coating metal, the small and limited amount of vapor thus produced mixing with the vapor from the source 5 to influence the color of the light emitted by the arc discharge. Of course this disc 29 may, by the play of the arc discharge thereon, be melted on the refractory material of disc 26, but the alloying or coating metal e. g.,

the zinc or cadmium) will have first been vaporized as desired.

.On the other hand the mesh disc 29 may be coated with a similar compound to that specified for the cathode element, so that upon suflicient heating of the disc 29 its surface will become metallized and will offer an active surface on which the arc discharge may play, and which will fairly well support the discharge, as soon as sufficient vapor pressure is developed. This arrangement will relieve the associated cathode element from operating throughout the normal arc discharge, but the cathode element will still perform its outlined functions during the auxiliary discharge and during the starting period. Some disintegration of the disc 29 and 'its coating will of course occur, and the disintegrating material will not be subject to the above described restricting action of the shield can, so that envelope wall blackening near the electrodes may be expected; accordingly this arrangement may be most useful for devices with relatively long discharge columns and envelopes. The disintegration, however, is not violent; and should the activity of the disc 29 at any time be too greatly exhausted, the arc discharge will simply return to the cathode element. It is to be noted that in the envelope exhausting process the disc 29 cannot be sufllciently heated for proper degassing by merely heating the cathode element, so that other heating means for disc 29, such as a high frequency bombardment, must be then employed.

-A special feature of my invention which may be employed in connection with the cathode element coating compound is the addition to this compound of a few per cent-of zinc oxide and/or cadmium oxide as as'ource of small and limited amount of zinc and/on cadmium vapor to mix with vapor from the source 5 as above mentioned in connection with-the disc 29. When the cathode is heated in vacuum apparently these oxides dissociate, theoxygen is bound or removed by the exhausting pumps, and the metals vaporize.-

Whether these; vapors (e. g., the zinc and/or cadmium vapors) are produced from disc 29 as above described or from oxides in the cathode coating as just outlined, they do not of course'return to their source when condensing, but are deposited on the envelope walls in a dark amalgam with condensate ofthe vapor from the source 5 (e. g., with mercury). With the heating of the walls attendant upon the arc discharge, however, the dark deposit will vaporize and disappear while the temperature of the walls is yet below 250 degrees C.

I do not intend that the scope of my invention be limited by the details of the particular structures shown and described, as it will be obvious that these may be modified without departure from the spirit of the invention. And it is my intention herein to claim, as broadly as the state of the art will permit, all the various novel combinations, sub-combinations and features-hereinabove disclosed. Y

I claim:

1. A gaseous discharge device adapted to attain a high temperature in operation and comprising a filling of low pressure gas; atemperatureresponsive source of vapor pressure; a pair of relatively spaced electrodes and means for initiating and means for maintaining therebetween a gaseous discharge, each said electrode comprisingv a cylindrical conductive shield, end members for said shield, one of said members being provided with an aperture, and a cathode element in the form of a helix within and closely adjacent said shield and having a first extremity connected to said shield, the internal diameter of said helix being several times the mean free path of electrons in said low pressure gas at temperatures existing within said shield before said vapor pressure rises; said discharge maintaining means being connected between the second extremities of the respective two said cathode elements.

2. In a. gaseous discharge device, a cathode element in the form of a closely wound helix, a cylindrical shield closely surrounding said helix, and an end member for said shield provided with a central aperture at least as small as the internal diameter of said helix.

3. In a gaseous discharge device, a cathode element in the form of a helix, a cylindrical shield surrounding said helix in close spaced relationship to the main portion thereof, and an end member for said shield provided with a central aperture, the diameter of said helix in its end portion further from said aperture being progressively reduced.

4. In a gaseous discharge device, a cathode ele-' end member forming a part of said enclosure, said member being provided with a discharge-conducting aperture and having at least an outer surface of refractory material, and said material at least slightly overhanging other portions of said enclosure.

6. An electrode for a gaseous discharge device, comprising a cathode element, a shielding enclosure thereabout provided with an aperture, and a mesh member secured on the surface of said enclosure about said aperture and comprising neutral metal together with some form of a metal falling in group I or II of Mendeleefis Periodic System and having a lower ionization potential than mercury.

7. An electrode for a gaseous discharge device, comprising a cathode element, a shielding enclosure thereabout provided with a discharge-conducting aperture, and a mesh member secured on the surface of said enclosure about said aperture and carrying on its surface a coating comprising an alkaline earth metal oxide mixture and oxides of metals of the iron group, adapted to dissociate at temperatures below the melting point of nickel.

8. In a discharge device having a gaseous filling, a cathode element in the form of a closely wound helix, means for maintaining thereto a gaseous discharge, a cylindrical shield around and closely adjacent said helix, and an end member for said shield provided with a discharge-conducting aperture, the internal diameter of said helix being several times the mean free path length of electrons in said gaseous filling at operating temperatures within said shield.

9. In a gaseous discharge device, an electrode and means including a current source unilaterally connected to said electrode for alternately maintaining a gaseous discharge thereto and therefrom, said electrode comprising a conductive shield adapted to support the discharge leaving said electrode and provided with an aperture, and a filamentary cathode element within said shield adapted to receive the approaching discharge through said aperture, said cathode element being serially connected between said shield and said current source whereby to be heated by the current attendant upon the discharge leaving said electrode.

10. A gaseous discharge device adapted to attain a high temperature in operation and comprising a filling of low pressure gas; a temperature-responsive source of vapor pressure; a pair of relatively spaced electrodes and means for initiating and means for maintaining therebetween a gaseous discharge, at least one of said electrodes comprising a cathode element of hollow formation, and a shield closely surrounding the exterior surface of said cathode and provided with a discharge-conducting aper ture communicating freely with the space bounded by the interior surface of said cathode element; the transverse internal dimensions of said cathode element being several times the mean free path length of electrons in said low pressure gas at temperatures existing within said shield before said vapor pressure rises.

11. A gaseous discharge device adapted to attain a high temperature in operation and comprising a filling of low pressure gas; a temperature-responsive source of vapor pressure; a pair of relatively spaced electrodes and means for initiating and means for maintaining therebetween a gaseous discharge, at least one of said electrodes comprising a cathode element of hollow formation, and a shield closely surrounding the exterior surface of said cathode and provided with a discharge-conducting aperture communicating freely with the space bounded by the interior surface of said cathode element;

the transverse internal dimensions of said cathode element being several times the mean free path length of electrons in said low pressure as at temperatures existing within said shield before said vapor pressure rises, and the interior surface of said cathode carrying an emissive coating strongly metallized whereby to resist disintegration when said discharge concentrates at higher temperatures and vapor pressures.

12. In a gaseous discharge device, an electrode comprising a cathode element, a shielding enclosure thereabout provided with a dischargeconducting aperture, and an annular starting electrode in front of and spaced from said enclosure by approximately the diameter of said aperture and having a diameter of the order of twice-said aperture diameter.

CARL J. R. H. vow WEDEL.

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