US3324348A - Cold cathode tube whose pressure is below the critical pressure - Google Patents

Description

June 6, 1967 G. w. BAKER 3,324,348

COLD CATHODE TUBE WHOSE PRESSURE IS BELOW THE CRITICAL PRESSURE Original Filed May 24, 1962 PASCHEN CURVE VOLTAGE UM (BREAKDOWN! INVENTOR. PRESSURHmm. of Hg) x D|STANCE(cm.) GEORGE w. BAKER FiG.4

United States Patent 3,324 348 COLD CATHODE TUBlfWI-IGSE PRESSURE IS BELOW THE CRITICAL PRESSURE George W. Baker, Stamford, Comm, assignor to York Research Corporation, Stamford, Conn., a corporation of Connecticut Continuation of application Ser. No. 197,423, May 24, 1962. This application June 2, 1966, Ser. No. 554,898 6 Claims. (Cl. 315168) This application is a continuation of Ser. No. 197,423, filed May 24, 1962, now abandoned.

This invention relates to an improved cold cathode electron discharge tube which is adapted to allow large currents to pass therethrough so as to provide a current for triggering a device such as an explosive.

It is desirable to accurately control the time at which an explosive is to discharge and more particularly it is absolutely essential to prevent accidental discharge of an explosive. The use of the explosive may be in any application ranging from underground explorations to detonations of military devices. Cold cathode tubes are also useful in industrial control apparatus.

In such devices, it is desirable to provide an electron discharge tube which will respond to a predetermined trigger or start signal voltage and which will not in fact respond to signals of magnitudes smaller than a predetermined trigger voltage.

Electron gaseous discharge tubes featuring a cold cathode have been found highly satisfactory in this type of application since they will not operate unless and until the predetermined signal voltage is reached.

Such electron discharge tubes are filled with a gas which has a predetermined breakdown voltage. This breakdown voltage may also be considered as a trigger or ignition voltage causing ionization of the gaseous molecules in the tube and allowing high current to flow. The actual breakdown voltage depends upon many factors, such as the type and pressure of the gas utilized, the nature of the cathode material, and the configuration and relative spacing of the electrodes in the tube.

In general, when a voltage is initially applied between two electrodes in a gaseous atmosphere, no free electrons or ions are flowing therebetween and it is expected that a relatively high voltage is necessary to cause gaseous discharge or breakdown. When the electric field is uniform, as in the case of ideal plane parallel electrodes, the breakdown voltage depends upon, among other things, the number of molecules between the electrodes, the gas pressure and the distance between the electrodes. The breakdown voltage is a minimum for a certain product of pressure and distance and increases With either an increase or decrease of this product.

These considerations are expressed definitively as the Paschen curve. This curve indicates that the breakdown voltage depends upon the total number of molecules of gas between two electrodes for a given material and a given gas. However, the Paschen curve is applicable to plane parallel electrodes.

As shown in Patent 2,592,556, there is provided a gaseous discharge tube featuring cathode, grid and anode elements in which the grid to anode spacing, multiplied by pressure of gas used is such as to fall at a point on the Paschen curve to the left of its lowermost portion.

In this above-noted patent, the grid and anode are spaced one from the other and a relatively high voltage may exist between the grid and anode before breakdown occurs. The grid acts to shield the anode from the cathode. However, in the utilization of this tube in circuitry which is used to elfect triggering action, the electrical signal source connected to the grid (or one of the grids if more than one grid is used) may become disconnected or opened, at which time the shielding effect of the grid from the anode is eliminated. Discharge between the cathode and the anode may result, detonating the explosive device perhaps at an inopportune time.

It is an object of this invention to provide a cold cathode tube in which breakdown is intended to occur between the cathode and anode.

It is a further object of this invention to provide a cold cathode tube in which breakdown will not occur if the grid should become disconnected from the remainder of the circuit.

It is a still further object of this invention to provide a cold cathode tube which will quickly operate when a predetermined signal voltage providing large amperage Still another object of this invention is to provide a circuit utilizing an improved cold cathode tube which will not operate it the grid should become disconnected from the remainder of the circuit.

It is still another object to provide a novel grid arrange ment in a cold cathode tube which may be used in a circuit that is more reliable than heretofore available.

Other objects and various further features of novelty and invention will be pointed out or will occur to those skilled in the art from a reading of the following specifi cation in conjunction with the accompanying drawings. In said drawings, which show, for illustrative purposes only, a preferred form of the invention:

FIG. 1 is a circuit arrangement in which my novel tube is utilized;

FIG. 2 is a perspective view showing the arrangement of the parts of my novel tube;

FIG. 3 is an exploded view of the parts of FIG. 2; and

FIG. 4 is a graph of the Paschen curve for helium.

Referring first to FIG. 4 there is shown a conventional Paschen curve for helium. This curve is utilized in determining the proper spacing of the electrode elements of the gaseous tube as shown in FIG. 2. The tube shown in FIG. 2 comprises an outer envelope 5 and a base seal not shown. The cathode 2 of this tube comprises a hollow metal sleeve 6, a conductive planar element such as a flat plate '7 attached tangentially thereto, and a small cup 8 attached to sleeve 6 by means of conductors 9 (FIG. 3). Sleeve 6 houses an anode 4 which is positioned substantially at the center thereof in axial extending relationship. The sleeve 6 and the plate 7 elfectively tend to shield anode 4 from the remaining elements of the tube. The keep-alive electrode 1 is a bent section over cup 8 as shown, while the grid comprises an inverted L-shaped wire, the horizontal arm of which is positioned over the cathode and anode.

The spacing between anode 4 and sleeve 6 of cathode 2 is such that when multiplied by the pressure of the gas, the resulting point falls to the left of the lowermost part of the Paschen curve. The effective spacing between the grid 3 and the cathode is such that when multipled by the pressure the resulting product falls to the right of the lowermost point of the Paschen curve.

The cup 8 is the active part of the cathode. In the embodiment shown, a wire mesh is pressed over potassium chloride capable of providing bright spot emission to form a solid pill in a manner which is conventional in the art, which pill is securely positioned in the bottom of the cup 8. The pill itself is conventional in the art, a form of which is shown in the said Patent No. 2,592,556 or in Patent No. 2,185,189. A slight amount of radioactive material is provided within envelope 5 so as to increase the number of ions available for a quick start. This, in conjunction with the use of the keep-alive electrode, provide a quantity of ions within the envelope 5 producing a glow current which allows the tube to trigger in very short intervals of time.

As shown in FIG. 1 the tube is connected so that breakdown or triggering occurs when a voltage of approximately 250 volts is applied to grid 3. A storage device such as a condenser 11 is charged through switch 14 to a supply 13 of approximately 2500 volts. Once condenser 11 is charged, switch 14 may be opened so that the voltage supply 13 is disconnected from the condenser.

One side of the load 12 is connected to the condenser 11 and the other side to the anode 4. The cathode 2 of the tube is connected to the other side of condenser 11. A high voltage supply 16, for example, 700 volts, is applied to keep-alive electrode 1 over a very high resistance 17 which may have a value, for example, of 12 megohms. The keep-alive electrode produces a glow current in the tube. This current provides a potential difference between keep alive electrode 1 and cathode 2 of approximately 150 volts since most of the voltage drop is across high resistance 17.

The voltage between the anode 4 and either of the elements grid 3 or cathode 2 will not produce detonation because the anode 4 will be shielded by sleeve 6 and plate 7 of the cathode 2.

The pressure varies within the envelope from between 4 /2 millimeters to millimeters of mercury (Hg) and when high voltage is desired, 6 millimeters of pressure is preferred, whereas when low voltage is desired 14 millimeters of pressure is preferred. Helium gas is utilized in the tube described herein.

It is important to recognize that the primary purpose of this invention is to reduce the probability of accidental gaseous breakdown. If, for example, a lead connecting to the keep-alive electrode 1 were disconnected, or opened, it would then be more ditficult to initiate gaseous breakdown. If, for example, the lead to grid 3 were to become opened, there would be no change in the voltage requirements for gaseous breakdown between cathode 2 and anode 4 since in all cases, the cathode shields the anode. If either the cathode or anode circuits were opened, there could be no current flow through the load.

While the foregoing description sets forth the principles of the invention in connection with specific apparatus, it is to be understood that this description is made only by way of example and not as a limitation of the scope of the invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A cold cathode tube comprising an envelope,

said envelope filled with a gaseous medium,

an anode,

cathode means spaced from said anode at a distance such that the product of said distance and the pressure of said gas falls to the left of the lowermost point of the Paschen curve,

a grid,

said grid being spaced from cathode at a distance, re-

spectively, such that the product of said distance and the pressure of said gas falls to the right of the lowermost point of the Paschen curve,

said cathode surrounding said anode and thereby shielding said anode, said cathode means including solid pill-shaped emitter means coupled thereto. 2. An electrical circuit comprising a cold cathode tube having an envelope, said envelope filled with a gaseous medium, an anode, cathode means spaced from said anode at a distance 4 such that the product of said distance and the pressure of said gas falls to the left of the lowermost point of the Paschen curve,

a grid,

said grid being spaced from cathode at a distance, re-

spectively, such that the product of said distance and the pressure of said gas falls to the right of the lowermost point of the Paschen curve,

said cathode surrounding said anode and thereby shielding said anode, said cathode means including solid pill-shaped emitter means coupled thereto,

voltage supply means,

a load means,

said voltage supply means being connected to one side of said load means, the other side of said load means being connected to said anode, the other side of said voltage supply means being connected to said cathode,

and means to provide a triggering voltage to said grid.

3. The circuit of claim 2 in which said cold cathode tube further includes a keep-alive electrode,

voltage supply means including a resistance connected to said keep-alive electrode to efiect a glow current within said tube.

4. A cold cathode tube comprising an envelope,

said envelope being filled with a gaseous medium,

an anode,

solid cathode means surrounding said anode including a conductive, tubular element positioned concentrically surrounding said anode, said tubular element being spaced from said anode at a distance such that the product of said distance and the pressure of said gas falls to the left of the lowermost point of the Paschen curve, said cathode means including solid pill-shaped emitter means coupled thereto,

a grid,

said grid being spaced from said cathode at a first distance, such that the product of said first distance, and the pressure of said gas falls to the right of the lowermost point of the Paschen curve.

5. The cold cathode tube of claim 4 further including a keep-alive electrode formed in an inverted L shape, the horizontal part thereof being positioned over the top of the other elements within said envelope.

6. In an electron discharge device of the cold cathode type having an active cathode surface element and anode and grid elements positioned within an envelope filled with gas, the invention comprising,

a cathode sleeve surrounding said anode and connected directly to said active cathode surface, the distance between said anode and cathode sleeve being such that the product of said distance and the pressure of said gas falls to the left of the lowermost point of the Paschen curve, said active cathode surface element including solid pill-shaped emitter means coupled thereto. Y

References Cited UNITED STATES PATENTS 2,592,556 4/1956 Germeshausen 313 JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, Examiner.

D. E. SRAGOW, R. SEGAL, C. R. CAMPBELL,

Assistant Examiners.

Claims (1)

1. A COLD CATHODE TUBE COMPRISING AN ENVELOPE, SAID ENVELOPE FILLED WITH A GASEOUS MEDIUM, AN ANODE, CATHODE MEANS SPACED FROM SAID ANODE AT A DISTANCE SUCH THAT THE PRODUCT OF SAID DISTANCE AND THE PRESSURE OF SAID GAS FALLS TO THE LEFT OF THE LOWERMOST POINT OF THE PASCHEN CURVE, A GRID, SAID GRID BEING SPACED FROM CATHODE AT A DISTANCE, RESPECTIVELY, SUCH THAT THE PRODUCT OF SAID DISTANCE AND THE PRESSURE OF SAID GAS FALLS TO THE RIGHT OF THE LOWERMOST POINT OF THE PASCHEN CURVE, SAID CATHODE SURROUNDING SAID ANODE AND THEREBY SHIELDING SAID ANODE, SAID CATHODE MEANS INCLUDING SOLID PILL-SHAPED EMITTER MEANS COUPLED THERETO.

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