US2444962A - Electrode structure for electrical gaseous discharge tubes - Google Patents

Description

July 13, 1948. P.W. STUTSMAN 2,

ELECTRODE STRUCTURE FOR ELECTRICAL GASEOUS DISCHARGE TUBES Filed March 27, 1946 2 Sheets-Sheet} C /NVENTOI? PAUL W STUTSMAN y 1948. P. w. STUTSMAN 2,444,962

ELECTRODE STRUCTURE FOR ELECTRICAL Filed March 27, 1946 GASEOUS DISCHARGE TUBES 2 Sheets-Sheet 2 INVENTQR PAUL, wsrursm/v m. 6 MM Patented July 13, 1948 ELECTRODE STRUCTURE FOR ELECTRICAL GASEOUS DISCHARGE TUBES Paul W. Stutsman, Needham, Mala, minor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application March 27, 1946, Serial No. 657,422

8 Claims. (Cl. 250-215) This invention relates to gaseous discharge devices particularly adaptable for use as half-wave rectifiers. It is among the objects of the present invention to provide a gaseous discharge device of relatively small dimensions and negligible power dissipation.

Further objects of the invention are to provide a device of the type described which is capable of operating directly from an 117 volt A. C. or; D. C. line, which will start readily and which requires no external heater voltage for the cathode.

The above and other objects and features of the invention will be made fully apparent to those skilled in the art from a consideration of the following detailed description taken in conjunction with the accompanying drawings in which:

Fig. 1 is a perspective view, with parts broken away for the sake of clearness, of a gaseous discharge tube constructed in accordance with the invention;

Fig. 2 is a front view showing on an enlarged scale the mount assembly of the tube shown in Fig. 1;

Fig. 3 shows a rear view of the mount assembly shown in Fig. 2;

Fig. 4 is a view of the mount assembly taken at right angles to that of Fig. 2;

Fig. 5 is a perspective view of a mount assembly illustrating a modified form of the invention; and

Fig, 6 represents a typical operating circuit of a tube embodying the invention.

In the drawings the tube illustrated consists of a glass envelope 6 adapted to contain a suitable gaseous atmosphere and having a reentrant stem 1 with a press 8 at the upper end thereof. The press 8 carries suitable lead-in conductors 9 and supporting elements for a cathode Ill, an anode H and an auxiliary anode or starting electrode indicated generally as It.

The cathode i0 is made in the form of a coiled coil, the ends of the coil being connected to standards IS. The coiled coil may be made by winding a line tungsten wire on a solid core. For example, the core may consist of molybdenum wire oi. approximately 0.005 inch on which the fine tungsten wire is spirally wound to provide a primary coil. The fine tungsten wire weighs about four milligrams per 200 millimeters and is wound closely with approximately 390 turns per inch. The reasons for this weight of wire will hereinafter appear. The primary coil may then be wound with approximately 62 .turns per inch on a wire rod of approximately 0.03 inch in diameter to provide the coiled coil. The wire rod may then be removed and the core dissolved. About eight turns of the resulting coiled coil are suflicient for one cathode. This coil is coated with an electron-emissive coating which may be prepared and applied in a manner known in the art. The interior of the primary coil from which the core has beendissolved will contain a substantial quantity of the emissive coating and functions as a reservoir of this material. The legs of the coil are left uncoated and are welded to the standards IS.

The anode H may be made of graphite and preferably is surrounded by a metallic shield i4 spaced therefrom at a distance not greater than the mean free path of the ions in the gaseous atmosphere of the tube in a manner described in the United States Patent No. 2,273,054. Preferably the anode is positioned with its active tip above the upper end of the shield i4 and also slightly above (for example, mm.) the level of the bottom of the coil of the cathode ill. The distance between the anode and the cathode is preferably of the order of mm. at the closest points.

A supporting rod ii. a portion of which is insulated, as indicated at i5, is inserted through the cathode coil in a position to support the top of the coil and has its uninsulated end welded to one of the standards IS. The other end of the supporting rod I4 is separated from the other standard by the insulation l5, but is rigidly attached thereto by the strap IS.

The auxiliary anode or starting electrode I2 is supported on standards I'I, one of which is connected to one of the lead-in conductors 9,

and the other of which terminates in the press I. The auxiliary anode I2 may be formed of sheet metal and is provided with fiat end tabs ll for welding to the standards l1 and a central portion ll of substantially greater width than the end tabs II. The central portion I9 is curved about the cathode coil ill, the upper portion thereof projecting outwardly so that it extends substantially beyond a vertical line cathode coil. The lower edge of the central portion is does not project inwardly to the same extent as the upper portion thereof for the sake of clearance. be apparent from Fig. 4, the auxiliary anode I2. is spaced closely to and extends parallel with the cathode coil ll. The auxiliary anode must clear the cathode at all points. For convem'ence in construction, the supporting stand- As will arcis l3 and I! may be positioned in alignment on the press 8 and the cathode standards l3 are bent forward at the upper portions thereof suiiiciently to support the cathode in properly spaced relation to the auxiliary anode. A getter assembly 20 may be attached to one of the standards H.

The gaseous atmosphere within the envelope 6 is preferably 90 per cent krypton and 10 per cent xenon at a pressure of the order of 10 to millimeters. The heavier of the monatomic gases are preferred to the lighter gases. such as neon and helium, due to the higher kinetic energy developed by the ions of the heavier gases. Since the heating of the cathode is inltiated and maintained by ionic bombardment. such heating is facilitated by the use of the heavier gases. Also the thermal conductivity of the gas decreases with increase in atomic weight. Accordingly in order to obtain a hot spot, the input may be less. A further advantage resides in the fact that the voltage drop through the tube decreases with increase in the atomic weight of the gas. It will be understood that with an anode of the type shown having an anode shield H, the spacing of the shield must be correlated to the gas pressure. With gases of the type described, the spacing between the anode shield and the anode may be 0.007 inch at a gas pressure of 16 millimeters.

Although the cathode I0 is intended to be ionically heated, each of the standards or supporting rods l3 for the cathode is connected to one of the lead-in conductors 9, and these conductors are brought out separately for greater flexibility of operation.

The operation and construction described in the foregoing may be best understood with reference to the circuit diagram shown in Fig. 6.

The tube 6 has its anode H connected through a resistor 23, which may be approximatelylOO ohms, to one of the terminals 24 of a 117 volt A. C. or D. C. line. The auxiliary anode I2 is connected to the main anode ll through a condenser 25 having a capacitance of the order of 0.002 microfarad and a resistor 28 of about 10 megohms in parallel therewith. The cathode III is connected to the other of the terminals 24 through a load resistor 21 of 1500 ohms having a condenser 28 of 40 microfarads in shunt thereacross.

During those portions of the cycle when no appreciable current flows between the cathode Ill and the auxiliary anode I2, the potentials on the auxiliary anode and the main anode are the same. However, when a glow discharge between the auxiliary anode and the cathode takes place, current flows through resistor 26, so that the resulting potential drop across this resistor lowers the potential on the auxiliary electrode relative to the potential on the main anode. The gases in the space between the cathode and the main anode become highly ionized resulting in an arc discharge. In a circuit having the parameters mentioned, a D. C. voltage of approximately 100 volts appears across the resistor 21.

The discharge initiated between the starting electrode I2 and the cathode I0 is ofthe nature of a glow discharge, which discharge results in a suflicient ionization of the gas in the region between the cathode Ill and the main anode II to initiate an arc discharge across the main electrodes. spot characteristic of such discharges at some The arc discharge results in a hot point upon the surface of the cathode. This point may move about the active surface of the cathode in a more or less haphazard manner. It has been found that the are spot, under certain conditions hereinafter described, tends to activate the surface of the cathode so that the surface at the point where the arc has previously operated is an active glow cathode, a characteristic it is capable of retaining for an appreciable period after the are itself has been extinguished. It is desirable that the distance between the active surface of the auxiliary anode and the activated spot on the cathode be not substantially different from the minimum potential breakdown distance. The minimum breakdown potential distance is defined herein as that distance at which a discharge will be initiated between two electrodes for a minimum applied potential. By providing the auxiliary aonde with a substantial surface area extending parallel to the axis of the cathode, a construction is provided which assures that some point upon this surface will be at the minimum potential breakdown distance from the point on the cathode where the arc has previously operated.

The conditions conducive to the formation of an active glow surface on the cathode will appear in conjunction with a consideration of the factors which enter into the determination of the weight of wire used in the cathode.

The weight of wire used in the cathode is determined by' the amount of average current the tube is required to deliver while maintaining a certain type of emission in the are state. There are two kinds of emission in the are state that a filament can furnish; one is an are spot similar to that in a mercury pool tube; the other is an are more nearly corresponding to that of a thermionic cathode mercury vapor tube. In each case the necessary energy for maintenance of the arc is furnished by positive ion bombardment. Almost any intermediate state between these two extremes can occur. If the wire weight is such that the end losses along the coil are too rapid, then an arc of the first kind is more likely to be present. If, however, the wire. is small enough, the coil is heated by ionic bombardment to a point where it is thermionically emissive. Operation in this latter condition tends to activate the surface of the coil so that, when the tube is next put in service, the surface of the coil at the place where the arc last operated is an active glow cathode and permits the formation of a glow discharge between the starter electrode and the cathode at a voltage within the range of the device. If the arc is more of the type which occurs, on a mercury pool, the surface of the coil is less likely to be an active glow cathode. Hence, for successive starts in service at low voltage, it is essential thatthe wire weight be correctly chosen.

A further reason for avoiding an arc of the I mercury pool type resides in the fact that such arcs are highly productive of R. F, noises when the tube is used in radio circuits; whereas when the arc originates in the copious thermionic emission of electrons, the noises produced due to the cathode function are at a minimum.

The lower limit of the wire size is determined by the fact that for A. C. operation the effective heating of the cathode is much greater than for the same average current on D. C. operation. It is necessary that the wire weight be great enough so that on the required A. C. operation the active coating is not evaporated by excessive heating. The heat retained by the cathode under the conditions described is sufficient to maintain thermionic emission but insumcient to result in excessive evaporation of the active coating.

The spacing of the cathode with respect to the anode has the effect of greatly reducing noise. Noise at radio frequencies is also produced by the steep wave front at the initiation of each cycle. The steepness of the wave front is determined by the voltage difference between the cathode and anode, and by the fact that the positive column has negative voltage characteristics,

depending upon the length of the column. Making the distance as short as possible eliminates the effect of the column. It is undesirable to have any element intervening in the region of the discharge space which may function as a grid.

While it is desirable from the standpoint of starting and from the standpoint of continuous wear and service on the cathode to have as high a gas pressure as possible, pressure is limited by the necessary close spacing requirements about the anode, the minimum potential breakdown spacing of the starter electrode from the cathode and the effect on R. F. noise. With increasing pressure the tendency of the plasma to oscillate and generate noises of various frequencies is increased. With the mixture mentioned, I have observed that working below pressure of 10 mm. was apt to produce tubes which ignite into the glow state, but do not pass from this state into the are or operating state when operating upon a 110 volt A. C. or D. C. line. Where the tube is designed for use at higher voltages, lower pressure would be permissible.

The circuit is suitable for either A. C. or D. C. operation. Rapid intermittent operation is not desirable because the wear on the cathode is greatest during starting. This is because the cathode is subjected to heavy ionic bombardment when unheated, and the load condenser 28 is not charged at that time. The initial starting current may attain a peak value of several ampei'eS while the steady state peak current is of the order of 0.5 ampere.

.The form of the invention shown in Fig. 5 corresponds generally to the form previously described except that the cathode is in the form of a pair of coils 30. This construction is desirable where a larger efiective cathode area is desired than is available in the form shown in Figs. 1-4, inclusive. This construction permits all portions of the effective cathode area to be spaced relatively close to the main anode 3i. If it is attempted to increase the effective cathode area by merely increasing the number of turns of the coiled coil iii of the previously described form, the turns adjacent the ends of the coil will be much farther removed from the main anode than the turn 3 in the central portion of the coil. Such large variations in the spacing are undesirable. By providing a pair of cathode coils and spacing the anode 3i so that its active tip is approximately midway between them, all parts of the cathode surface come within the permissible range of distances from the main anode. The auxiliary anode 32 is formed to provide a pair of hemicylindrical surfaces, each coacting with one of the coils 30, in the same manner in which the single hemi-cylindrical surface of the auxiliary anode i2 coacts with the single cathode coil ll of tain preferred embodiments of the invention, other embodiments within the scope of the appended claims will be obvious to those skilled in the art from a consideration of the forms shown and the teachings hereof.

What is claimed is:

1. A gaseous electrical space discharge device comprising an envelope, an ionizable gas in said envelope for supporting a discharge, a main anode and a cathode in said envelope between which an arc discharge is adapted to occur, an auxiliary anode partially surrounding said cathode and positioned on the remote side of said cathode from said main anode, said auxiliary anode being adapted to coact with said cathode to provide a glow discharge therebetween, said auxiliary anode being spaced from said cathode by a distance substantially equal to the minimum potential breakdown distance.

2., A gaseous electrical space'discharge device comprising an envelope, an ionizable gas in said envelope for supporting a discharge, a main anode and an ionically heated cathode in said envelope between which an arc discharge is adapted to occur, an auxiliary anode partially surrounding said cathode and positioned on the remote side of said cathode from said main anode, said auxiliary anode being adapted to coact with said cathode to provide a glow discharge therebetween, said auxiliary anode having an active area at least coextensive with said cathode and spaced therefrom by a distance substantially equal to the minimum potential breakdown dis-- tance.

3. A gaseous electrical space discharge device comprising an envelope, an ionizable gas in said envelope for supporting a discharge, a main anode and a cathode in said envelope between which an arc discharge is adapted to occur, and an auxiliary anode adapted to coact with said cathode to provide a glow discharge therebetween, said auxiliary anode partially surrounding said oath ode and having a surface conforming in shape to and extending parallel with a surface of said. cathode on the side thereof remote from said. main anode.

4. A gaseous electrical space discharge device comprising an envelope, an ionizable gas in said envelope for supporting a discharge, a main anode and a coiled coil cathode in said envelope between which an arc discharge is adaptedto occur, and an arcuate auxiliary anode exteriorly disposed with respect to said cathode and adapted to coact with said cathode to provide a glow discharge therebetween, said auxiliary anode having an interior surface conforming in shape to and extending parallel with said cathode over a substantial portion of the external area of said cathode.

5. A gaseous electrical space discharge device comprising an envelope, an ionizable gas in said envelope for supporting a discharge, a main anode and a cathode in said envelope between which an arc discharge is adapted to occur, said cathode having the form of a cylindrical coil, and an auxiliary anode adapted to coact with said cathode to provide a glow discharge therebetween. said auxiliary anode having a hemicylindrical surface conforming in shape to and extending parallel with the cylindrical surface of said cathode on the side thereof remote from said main anode.

6. A gaseous electrical space discharge device comprising an envelope, an ionizable gas in said envelope for supporting a discharge. a main anode and a cathode in said envelope between which an arc discharge is adapted to occur, said cathode "having the form or a cylindrical coil. and an auxiliary anode adapted to coact with said cathode to provide a glow discharge therebetween, said auxiliary anode having a hemicylindrical surface conforming in shape to and extending parallel with the cylindrical surface of said cathode, and spaced from said cathode by a distance substantially equal to the minimum potential breakdown distance;

7. A gaseous electrical space discharge device comprising an envelope containing an ionizable gas "and provided with a reentrant stem including a press, a first pair of spaced standards sealed in said press; a cathode supported between said standards, an auxiliary supporting member engaging said cathode and secured to said standards, a main anode supported by said press in spaced relationship with said cathode whereby an arc discharge is adapted to occur therebetween. a second pair of spaced standards sealed in said press, and an auxiliary anode supported between said second standards in spaced relationship with said cathode on the side thereof remote from said main anode whereby-a glow discharge is adapted to occur between said auxiliary anode and said cathode. g 8. A gaseous electrical space discharge device comprising an envelope containing an ionizable Number gas and provided with a reentrant stem including a press, a first pair of spaced standards sealed in said press, a coiled cathode supported between said standards. an auxiliary supporting member passing through said coiled cathode and secured to said standards, a solid, cylindrical main anode supported by said press in spaced relationship with said cathode'whereby an arc discharge is adapted to occur therebetween, a second pair of spaced standards sealed in said press, and an arcuate auxiliary anode supported between said second standards in spaced relationship with said cathode whereby a glow discharge is adapted to occur between said auxiliary anode and said cathode, said auxiliary anode partially surrounding said cathode on the side thereof remote from said main anode.

, PAUL W. STUTSMAN.

REFERENCES CITED The following references are of record in the file of this patent:

' UNITED STATES PATENTS Name Date 1,408,053 Wensley Feb. 28, 1922 2,106,847 Kniekamp Feb. 1, 1938 2,159,747 Mendenhall May 23, 1939 2,411,506 Chevigny Nov. 26, 1946

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