US3304456A

US3304456A - Slot cathode

Info

Publication number: US3304456A
Application number: US262446A
Authority: US
Inventors: Lany Beatrice Pearson De; Gertrude P Copeland
Original assignee: Individual
Current assignee: Individual
Priority date: 1963-03-04
Filing date: 1963-03-04
Publication date: 1967-02-14
Anticipated expiration: 1984-02-14

Description

1967 B. P. DE LANY ETAL 6 SLOT CATHODE 2 Sheets-Sheet 1 Filed March 4, 1963 .M r my a W W? M g m a FZJA/ wa w ZY United States Patent Ofihce 3,304,456 Patented Feb. 14, 1967 3,304,456 SLUT CATHODE Beatrice Pearson De Larry, Miami Beach, Fla, and Paul L. Copeland, Bensenville, TIL; Gertrude I. (Iopeland, executor of the estate of said Paul L. Copeland, deceased, assignor of one-half interest to Gertrude P. Copeland, individuality Filled Mar. 4, 1963, Ser. No. 262,446 19 Claims. (til. 313-482) This invention relates to an electrical discharge tube. It has for one object to provide in an electrical discharge tube, a cathode of increased efficiency, whereby large currents may be established at low voltages.

Another object is to provide a construction by which a moderate potential difference will produce a current in the glow discharge from a small cathode which is of the order of those usually obtained in an arc.

A still further object is to provide a hollow cathode and to shape that cathode to make more rapid and certain the initiation of the discharge Within the cathode and the transition from the exterior surface of the cathode.

Another object is to form a cold cathode lighting tube operable at conventional voltages.

Other objects will appear from time to time throughout the specification and claims.

The invention is illustrated more or less diagrammatically in the accompanying drawings wherein:

FIGURE 1 is a sectional view taken through one form of the device,

FIGURE 2 is a transverse sectional detail of the cathode of FIGURE 1, on an enlarged scale,

FIGURE 3 is a version similar to FIGURE 2, illustrating a modified construction of the cathode slot edge,

FIGURE 4 is a side elevation of a further modified form of electrode,

FIGURE 5 is a side elevation of a still further modification of the cathode,

FIGURE 6 is an enlarged side elevation, in part section, of a further form of cathode, 4

FIGURE 7 is a graph of voltage vs. current for a cold cathode tube of a conventional type, and

FIGURE 8 is a graph of voltage vs. current comparing a conventional tube and a tube formed in accordance with the present invention.

Like parts are designated by like characters throughout the specification and drawings.

It should be understood that the present invention finds utility as a cold cathode lighting tube of the fluorescent type. In such case there would be two similarly formed electrodes, the polarity of which would reverse each half cycle with the applied alternating current. However, for purposes of illustration, one electrode will be called the anode and the other the cathode.

As shown in the form of FIGURE 1, the device includes an envelope 1, which is preferably formed of glass but may be formed of any air-impervious electrically insulating material. 2 is an anode positioned within the envelope and connected to a lead-in wire 3, by mean of which the anode may be put into electrical connection within a suitable system. The anode 2 may be formed of a variety of metals. Tungsten is one suitable metal.

Within the envelope is positioned a cathode 4, which is secured to a lead-in wire 5 which passes through and extends outside of the envelope wall, as shown at 6. The lead-in wire 6 is connected in circuit to a suitable electrical system. Thus, the wires 3 and 6, when the device is to be used, are connected to a source of electrical current.

The cathode 4 may be formed of a wide variety of metals. Refractory metals may be used to permit the temperature of the cathode to rise. Among suitable metals for the formation of the cathode are tungsten, tantalum, molybdenum, nickel and stainless steel. The cathode is not limited to any particular metal. The cathode may also be made of cerium or other rare earths.

As shown in FIGURES 1, 2 and 3, the cathode 4 is generally tubular. Its one end may be closed, for example, by a member 7, to which the lead-in wire 5 is secured. The end of the cathode may be closed by means other than the added plate or plug member 7. That member serves only as one convenient closure for the tube 4- and is a convenient attachment means for the leadin Wire 5.

The cathode 4 is open at it inner end, as at 8, and is provided along its side with a slot 9. This slot may run substantially from end to end of the tube 4. It need not do so, however, but it must be of substantial length. It may be formed merely by slotting all or a part of the length of the tube. As shown in FIGURES 1 and 2, the slot 9 is formed merely by a saw or comparable tool which leaves relatively

thick edges

10, 10 along the slot R.

The slot in the tubular electrode 4 may be formed to produce relatively sharp edges 11, 11 along the slot 9, as shown in the modified form of FIGURE 3.

The modified form of the cathode shown in FIGURE 4 is generally tubular and closed at both ends. Thus the tubular member 12 is closed at its outer end by member 13, to which is secured the lead-in wire 5. At its inner end, the cathode tube 12 may be closed by a member 14. Adjacent to the inner end of the tubular cathode 12 is an opening 15. This is of relatively large diameter and is formed in the side wall of the cathode, adjacent its inner end. Although the hole or opening is shown as being round, in fact it may be of almost any shape.

Formed in the tubular cathode I2 is a slot 16, generally similar to the slot 9 shown in FIGURES 1, 2 and 3. It communicates with the opening 15. Actually, it need not communicate with that opening as long as one end of the slot 16 approaches closely to the opening 15. The slot Id need not extend the full length of the tubular cathode 12, but it should be of substantial length and should be in communication with, or closely approach, the opening or perforation 15.

A still further modification of the cathode is shown in FIGURE 5. As shown there, the cathode comprises a generally tubular member 17, closed at its outer end by the parts 18, to which is secured the lead-in wire 5. The tubular cathode 17 is open at its inner end, as at 19, and is provided with a generally helical slot 20, which at its upper end communicates with the open end 159 of the cathode 17.

When the expression outer end of the cathode has been used, it is used to indicate the end of the cathode which is remote from the anode. correspondingly, when the expression inner end of the cathode has been used, it refers to the end of the cathode which is more nearly adjacent the anode.

FIGURE 6 illustrates still another form of cathode. A hollow body 21 is closed at its outer end 22 and includes openings or perforations 23 which may be random or arranged and which extend generally the length of the cathode. The body 21 may be formed of perforated or expanded metal or formed in any other way to permit positive ions to enter the inside of the cathode through substantially its entire length to cause secondary emission of electrons from within the cathode.

Several forms of the tubular cathode have been shown. In each, there is at, or adjacent, the inner end of the cathode, a relatively large opening. In three forms of the cathode, there is an extended and narrow slot, or slit, or opening in the wall of the cathode. This relatively narrow opening communicates at one end with the larger opening, or closely approaches that opening. Thus, the slots 9 and are in communication with the open end of the tubular cathodes 4 and 17 respectively. In the case of the cathode 12, when both ends are closed, the large opening adjacent the inner end is in communication with, or closely approached by, the extended slot-like opening 16. Although the curved and generally helical slot 20, shown in FIGURE 5, is a suitable form for the extended narrow opening, it is shown also as an indication that the slot or narrow opening need not be straight, as the slot 9 is straight, and need not be axially aligned with the tubular cathode, as the slots 9 and 16 are. The relatively narrow slot-like opening in the cathode wall may be disposed in a variety of positions as long as it is of sufiicient length and closely approaches or communicates with the major opening, such as the open end 8 or the opening 15. In the fourth form the cathode is formed with many openings or perforations. What is important in all forms is to permit positive ions from the exterior to enter the cathode and cause secondary emission of electrons from the interior cathode surface.

The use, operation and function of the invention are as follows:

FIGURES 7 and 8 are graphs illustrating the effect of providing a means for allowing positive ions to enter the cathode cavity generally throughout its length. In FIG- URE 7, a tube with a -inch diameter cathode and containing argon gas was kept at a temperature of -78.6 centigrade and at a gas pressure of 280 microns. The curve at the left side of FIGURE 7 shows an increase in current with an increase in voltage. The increase in current is fairly linear. After the voltage reached a level of roughly 800 volts the curve was discontinuous and there was a substantial increase in current. In fact the voltage was reduced considerably at this point, but the current still remained at a very high level.

In a tube of this type which does not have any means, except the main orifice, for permitting positive ions to reach the inside of the cathode hole, the glow is on the external surface of the cathode and does not penetrate the cavity or hole. Conditions are such that a plasma is not established within the cavity. A continuing increase in voltage will ultimately establish a plasma within the cavity at which point there will be a sudden and substantial increase in current. This is shown in FIGURE 7. Once the plasma is established within the cavity, the tube provides excellent voltage regulation and the voltage will stay generally almost constant for increases in current. The action of the hollow cathode is so effective that the parts of the cathode outside of the cavity may not have any discharge. It is necessary and essential to establish a plasma within the cavity so that positive ions will strike the inner wall of the cavity and provide secondary emission of electrons. It is in this way that there can be a rapid increase in the current provided by the tube.

The curves of FIGURE 8 show the effect of a slotted cathode. The values for the curve at the left side of FIGURE 8 were taken from a tube with a cathode having a -inch outer diameter and a -inch inner diameter. The argon gas was at a pressure of 340 microns. The curve at the right or along the bottom of FIGURE 8 was with an identical cathode except that a ;-inch wide slot was milled along one side of the cathode. This slot opened into the cathode cavity. Note that in the left-hand curve of FIGURE 8 there were small increases in current for rather substantial increases in voltage. No plasma was formed within the cavity. In the slotted cathode however there was an immediate plasma formed within the cavity and hence very large currents were obtained at 250 volts. Note that there was excellent voltage regulation. By providing a slot in the side of the cathode or some other means permitting the positive ions to move into the cathode cavity and strike its wall it is possible to have substantial secondary electron emission from the wall. The secondary emission electrons stabilize the glow within the cathode and permit substantially higher currents at conventional voltages. Very large changes of current can be produced by rather small changes of voltage. This characteristic is accounted for by the rapid variation of secondary emission with primary energy.

The action of a hollow cathode depends on trapping electrons in the plasma and providing positive ion bombardment of the interior cathode surface to produce secondary electrons. The slot provides means for positive ions to enter the central plasma and provides means for controlling the rate of loss of positive ions from the central plasma to the space outside of the hollow cathode.

While it has been stated that the cathode may be formed of a variety of different metals, the metal of which the cathode is made should be one having a high melting point and a low work function. Also, alkali earths and rare earths may be used if present in a very thin film on a supporting surface of other material of higher melting point. Generally, although alkali earths may be used, they are among the less satisfactory possible metals for the formation of the cathode. Uranium zirconium car bide is a satisfactory material.

While it is convenient to form the cathode as a tube of constant diameter, this is not essential and the cathode tube need not be of uniform diameter from end to end of the tube.

Envelope 1, when the device is assembled, will have the air exhausted from it and a suitable gas under pres sure will be introduced into and retained within the envelope under pressure. The operation of the device of the present invention depends on the ionization of this gas. The actual pressure chosen depends generally upon the size of the cathode structure. The permissible range of pressure of gases present in the envelope is from 5-100 mm. of Hg. Experience has shown that the slotted cathode of this invention can operate both at higher and lower gas pressures than can an unslotted cathode. Thus, the use of the slotted cathode of this invention increases the operating range of the permissible gas present.

The invention is not limited in this use to a particular gas. The noble gases are among those which may be used. Mercury vapor or others may be used.

What is claimed is:

1. In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode longitudinally spaced from said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being slit and formed with a cavity therein facing in the direction of the anode.

2. In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode longitudinally spaced from said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being slit and formed with a cavity therein facing in the direction of the anode, and said envelope containing inert gas under pres sure of from 5-100 mm. of mercury.

3. In combination in an electric discharge tube an airimpervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode longitudinally spaced from said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being slit and formed with a cavity therein facing in the direction of the anode, the length of said cavity being of the order of twenty (20) times its diameter.

4. In combination in an electric discharge tube an airimpervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being longitudinally slit and formed with a cavity therein facing the direction of the anode.

5. In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being longitudinally slit and formed with a cavity therein facing in the direction of the anode, said slit being approximately inch in diameter, and said envelope containing inert gas under pressure of from 5-400 mm. of mercury, the length of said cavity being of the order of twenty (20) times its diameter.

6. In combination in an electric discharge tube an airimpervious envelope of insulating mate-rial, an anode extending thereinto, a hollow metallic cathode longitudinally spaced from said anode having an open end facing said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being slit and formed with a cavity therein facing in the direction of the anode.

I. In combination in an electric discharge tube an air-impervious envelope of insulation material, an anode extending thereinto, a hollow metallic cathode having an open end facing said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being slit and formed with a cavity therein facing in the direction of the anode, and said envelope containing inert gas under pressure of from 5-100 mm. of mercury.

8. In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode having an open end facing said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being slit and formed with a cavity therein facing in the direction of the anode, the length of said cavity being of the order of twenty (20) times its diameter.

9. In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode longitudinally spaced from said anode having an open end facing said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being longitudinally slit and formed with a cavity therein facing in the direction of the anode.

It). In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode having an open end facing said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being longitudinally slit and formed with a cavity therein facing in the direction of the anode, said slit being approximately inch in diameter, and said envelope containing inert gas under pressure of from 5-100 mm. of mercury, the length of said cavity being of the order of twenty (20) times its diameter.

11. In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode longitudinally spaced from said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being longitudinally provided with a narrow slit opening into its interior and formed with an extended opening therein facing in the direction of the anode.

12. In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being longitudinally provided with a narrow slit opening into its interior and formed with an extended opening therein facing in the direction of the anode, and length of said cavity being of the order of twenty (20) times its diameter.

13. In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode longitudinally spaced from said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being longitudinally provided with a narrow slit opening into its interior and formed with an extended opening therein facing in the direction of the anode, said slit being in communication with said extended opening.

14. In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow metallic cathode longitudinally spaced from said anode, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being longitudinally provided with a narrow slit opening into its interior and formed with an extended opening therein facing in the direction of the anode, said slit being in communication with said extended opening, and length of said cavity being of the order of twenty times its diameter.

15. In a cold cathode gas filled lighting tube, an anode at one end of the tube and a cathode at the other end, said cathode being somewhat cylindrical and generally hollow, and means extending through a major portion of the length of the cathode for permitting positive ions from the exterior of the cathode to enter the generally hollow portion of the cathode and to cause secondary emission of electrons from the interior of the cathode.

16. The structure of claim 15 further characterized in that said cathode is formed of a perforated metal.

17. In combination in an electric discharge tube an air-impervious envelope of insulating material, an anode extending thereinto, a hollow cathode of refractory metal alloy, at lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being slit and formed with a cavity therein facing the anode.

18. In combination in an electric discharge tube an airimpervious envelope of insulating material, an anode extending thereinto, a hollow cathode of a conducting compound, a lead-in wire in electrical connection with said cathode and extending to a point outside of the envelope, said cathode being slit and formed with a cavity facing in the direction of the anode.

19. The method of producing a hollow cathode with perforations distributed at random over the entire surface of the enclosing shell by the process of sintering in a suitable form a coarse powder of refractory material under conditions of temperature and pressure such that irregularly spaced holes between the particles are left in the cathode structure, which holes provide numerous paths for the injection of positive ions into the cathode cavity.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Skellett 313-192 X Seibt 313189 Lubcke 313-348 X Krieger 313186 X Butler 313341 Depew 3133 Depp 313185 Townsend 313185 Story 313-184 X JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.