US3009076A - Self-biased gas discharge tube - Google Patents

Description

NOV. 14, 1961 c w JACOB SELF-BIASED GAS DISCHARGE TUBE 2 Sheets-Sheet 1 Filed March 12, 1951 FIG. 2.

CARLYLE W. JACQB '/w wa attorney Filed March 12, 1951 Nov. 14, 1961 c. w. JACOB 3,009,076

SELF-BIASED GAS DISCHARGE TUBE 2 Sheets-Sheet 2 FIG.3.

United States Patent 3,009,076 SELF-BIASED GAS DISCHARGE TUBE Carlyle W. Jacob, Rochester, N.Y., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Mar. 12, 1951, Ser. No. 215,041 8 Claims. (Cl. 313-193) This invention relates to electron tubes and more particularly to gas-filled grid-controlled discharge tubes.

Thenmionic gas triodes, or thyratrons, are characterized by the fact that the grid only controls the starting of the discharge but has no etfect or further control after breakdown either to modulate, limit, or extinguish the discharge. The are can be stopped by lowering the anode voltage until the required rate of ion generation can no longer be maintained. The starting of an arc discharge between an emitting cathode and an anode depends upon the formation of positive ions in the electrode space. The are will not start until the electron current has reached a certain minimum value dependent on the anode voltage. The potential in all parts of a negative-grid-control thyratron before starting depends to a great extent on anode voltage, and for all voltages except the lowest an electron current larger than this minimum value will flow to the anode unless the current is reduced by making the grid negative with respect to the cathode.

Radio proximity fuzes conventionally contain an electric detonator which is adapted to be fired by means of a thyratron, operating as a switch, to discharge a condenser through the detonator. The fuze also contains a deferred action, or reserve, battery with a separate C voltage section to supply the necessary bias to maintain the grid more negative than the critical firing potential until an operating signal of suflicient amplitude to overcome this bias is impressed upon the grid. Both noise voltage and decay of output voltage of the deferred action type battery during the flight of a projectile may be potential causes of premature discharges of the thyratron. Because of variation of grid control ratio from tube to tube and because of these potential causes of premature detonations, the grid is biased much more negative than the starting voltage, i.e., the critical firing voltage. This reduces the sensitivity of the thyratron in that a strong firing signal is required to overcome the grid bias. Size and weight limitations are stringent in radio fuzes, and elimination of the C section of the deferred action type battery would both allow reduction in the size and Weight of the fuze and eliminate these potential causes of premature firing of the thyratron.

It is an object of the invention to provide a gas-filled grid-controlled discharges device which does not require an external grid bias voltage supply to keep the are from starting.

It is also an object of the invention to provide such a gas-filled discharge device which is extremely sensitive to weak firing signals.

It is a further object of the invention to provide such a gas-filled discharge device which has a low effective resistance to current flow and a small voltage drop, or are drop, between cathode and anode after breakdown.

The objects of the invention are attained by increasing the action of the grid in shielding the cathode from the anode and by utilizing the fall of potential in the directly heated cathode to maintain the grid sufliciently negative with respect to the cathode so that the anode potential has practically no effect in the grid-cathode region and the establishment of the arc is thus prevented until a firing signal of sufiicient amplitude to overcome this negative bias is impressed on the grid. In the preferred embodiment of the invention grid apertures of effectively smaller size than have heretofore been utilized are located opposite the high potential end of the directly heated cathode. This reduction in the size of grid apertures allows fewer lines of force from the anode to penetrate through to the cathode and thus decreases the critical firing potential below which the grid must be biased to prevent the starting of the arc. The grid apertures are provided opposite the high potential end of the cathode where the maximum biasing field exists between grid and cathode, whereby the grid is maintained at a negative bias voltage substantially equal to the fall of potential in the filament cathode. Additional bias is obtained through the flow of grid current through a grid leak resistor between the control grid and the low potential end of the cathode. In a preferred embodiment of the invention low effective resistance to current flow and low arc drop are obtained by providing at least one additional aperture in the grid out of direct line between anode and cathode whereby the lines of force from the anode do not penetrate through this additional aperture until the anode current has increased above the minimum required to start the arc. Once breakdown has occurred, ions ditfuse through, and cause current flow through, this additional aperture.

The various features of the invention will be more clearly understood from the following detailed description in connection with the accompanying drawing, in which:

FIG. 1 is a perspective view of one embodiment of the discharge tube made in accordance with the invention with a portion of the enclosing vessel and the anode broken away to show the internal electrode structure;

FIG. 2. illustrates in perspective an alternative construction of the electrode structure shown in FIG. 1;

FIG. 3 illustrates in perspective an alternative construction which is particularly adapted to provide a low effective resistance to current flow and low voltage dif-. ference between cathode and anode after the arc is started; and

FIGS. 4 and 5 illustrate in perspective alternative embodiments of the electrode structure shown in FIG. 3.

Referring to the drawings, a gas-filled discharge tube made in accordance with the invention is provided with the usual subminiature type radio tube glass envelope 10 containing an ionizable gaseous medium, for instance, one of the rare inert gases exemplified by neon and argon, or mercury vapor. The envelope 10 terminates in a press 11 in which are sealed the lead-in conductors for the electrodes and upwardly rigid rods 12 adapted to provide support for the electrode structure. Each rod 12. fits within a depression 13 provided in, and engages the outer surface of, a cylindrical metal anode 14. The rods 12 and a lead-in conductor 16 which engages and provides partial support for the anode 14 extend through snug apertures provided in circular disks 17 and 18 of suitable insulating material, such as mica, in planes transverse of the axis of the cylindrical anode 14. The mica disks 17 and 18 are made of large area to extend over the cylindrical boundary of the electrode assembly and serve as shields for the open ends of the grid. Another lead in conductor 20 extends through the press 11 and through apertures in the disks 17 and 18, and a hook 2.1 fastened thereto above the insulating disk 18 supports a directly heated filament-cathode 24 which extends through apertures in the disks 17 and 18 along the axis of the envelope 10. The cathode 2.4 is coated along the length between the insulating disks 17 and 18 with thermionical-ly active material such as barium and strontium oxides to insure a copious supply of electrons. A separate lead-in conductor 26 extends through the press 11 and connects to the lower end of the cathode 24 below the insulating disk 17.

The invention is particularly concerned with the generally tubular grid structure 27 coaxially with and disposed between the cathode 24 and the anode 14. By the word tubular in this connection is meant any form having a substantially perirnetrically complete contour whether such contour be circular or noncircular. The metallic grid 27 is closed at its upper and lower ends by the insulating disks 1 8 and 17 respectively, and a lead-in conductor 30 and a rigid rod 31 extending through the press I1 and through the insulating disks 17 and 18 engage the exterior tubular surface of the grid. A plur'a'lity of restricted passages in the form of spaced parallel narrow slits 28 having the long dimension thereof transverse of the axis of the vessel are provided near the upper edge of the grid 27 opposite the high potential end only of the cathode 24.

The influence of the grid in a gas-filled device does not extend far' from its own surface due to the formation of a positive ion sheath around the grid structure. Consequently, electrons may flow through the central part of an opening in the grid structure unless the grid is made so negative that the positive ion sheaths on opposite sides of the opening overlap. The control characteristic of the grid of a gas-filled tube is thus governed by the dimensionsof the opening through the grid. The grid must control the maximum current density. If there is one hole in the grid larger than the others, the current density through the largest hole tends to control the starting.

Many gas-filled electron discharge devices heretofore manufactured were provided with a plurality of relatively large apentures in the control grid extending the entire length of the cathode. The utilization of restricted passages 28 in the grid 27 reduces the number of lines of force from the anode which can penetrate through the cathode 24 and thus reduces the starting voltage, i.e., critical firing voltage, of the grid for a given anode potential. The slit apertures 28 are located near the top of the grid 27 opposite the high potential end of the oathode 2'4 and in the maximum biasing field between the grid 27 and' the cathode 24. Since the high potential end of the cathode 24 is approximately 1.5 volts more positive' than the low potential end when a conventional heater voltage supply is connected across the lead-in conductors 20 and 26,. a negative bias of approximately 1.5 volts existsat the slit apertures 28.

Additional negative grid bias is obtainable if an ex ternal' grid-leak resistor is utilized between the grid 27 and the low potential end of the cathode 24. Because of the initial velocities of the electrons as they leave the cathodeand the contact potential difference between the grid and the cathode, electrons reach the grid 27 even though it is negative with respect to the cathode 24. Electrons. are emitted from the cathode 24 with considerable velocity to produce around the cathode 24 a space charge of electrons that expands rapidly to the grid 27. If a grid-leak resistor of approximately one megohm is connected between, the lead-in conductors 26 and 30, electrons impinging upon the grid 27 leak away through the grid resistor and biasthe grid approximately one volt negative with respect to the low potential end of the cathode due to the voltage drop across this resistor. Since the topof the cathode 24 is approximately 1.5 volts positive; with respect to the low potential end, a negative grid bias of approximately 2.5 volts exists at the apertures 28 which shields the cathode 24 so well that the anode potential has practically no effect in the grid-cathode region. The electron current is thus maintained below the minimum required to start the discharge until a firing signal ofs'ufiicient amplitude is impressed on the grid 27.

It is possible to utilize the gas-filled discharge tube of the invention in the firing circuit of a radio proximity fuze without providing an external grid bias voltage C supply to normally maintain the discharge tube in its nonconducting state. The critical firing grid potential of gasfilled discharge tubes heretofore used in the firing circuit of. many radio proximity fuzes was approximately 2.7

volts, and a grid bias C voltage section of approximately 6.5 volts was included in the deferred action type battery to normally maintain the gas-filled discharge tube in the nonconducting state. In defective batteries the output voltage of this grid bias C section may drop from 6.5 to as low as 4.5 volts during the flight of a projectile, and as explained hereinbefore, it was desirable to bias the grid of the discharge tube approximately 3.8 volts more negative than the starting voltage in order to insure against discharge due to drift in the output voltage of the grid bias C section. I have found it convenient to provide grid apertures 28 of such size that the starting voltage, i.e., critical grid firing potential, for the discharge tube is approximately 1.8 volts. The output of the cathode voltage A section of the deferred action type battery never decreases more than 0.2 volts during the flight of a projectile, and as the elfective bias on the grid thus does not vary more than 0.2 volt, the self- -biased gas-filled discharge tube of the invention is exceptionally stable in operation when utilized in the firing circuit of a radio fuze. With a self-bias of 2.5 volts and a starting potential of 1.8 volts, it is evident that a starting signal of only 0.7 volt is required to cause breakdown. The 0.7 volt sensitivity of the discharge tube of the invention is thus considerably greater than the sensitivity of prior art discharge tubes in which the grid bias was obtained from the C section of the deferred action,

type battery.

If electron flow from cathode to grid is excessive, and if the gas-filled discharge tube receives its firing signal from a high impedance source, e.g., the cathode-anode circuit of a pentode, difficulty may be encountered in driving the grid in a positive direction to the starting voltage. The electron flow to the grid may be reduced or minimized by coating the interior of the grid, except for the inner periphery adjacent the slit apertures 28, with a suitable insulating lacquer. In such an embodiment no bias voltage from the flow of grid current through a grid-leak resistor is available, and the grid is consequently biased approximately 1.5 volts negative. This bias is still sufficient to prevent electronic flow to the anode 14 of sufficient magnitude to start the are if the width of, the slit apertures 28 is decreased sufficiently to lower the starting voltage to approximately 1.0 volt. With such an electrode structure it is only necessary to apply a firing voltage of approximately 0.5 volt to the grid to start the are, thereby maintaining approximately the same sensitivity as in the preferred embodiment wherein the flow of grid current through an external resistance. is utilized to obtain part of the grid bias.

FIG. 2 illustrates an alternative construction of the electrodes of a gas-filled discharge tube in accordance with the invention. Registering rectangular apertures 35 having their long dimension parallel to the cathode 24- are provided only opposite the high potential end of the cathode 24 both in the outer perimeter of a tubular grid 32 and in a battle plate 36 inserted between the cathode 24. and the outer perimeter of the grid 32. Although the registering apertures 35 are considerably wider than the slits 28 provided in the grid 27 of the. embodiment illustrated in FIG. 1, a plot of. the potential distribution, i.e., the electric fie-ld distribution due to a grid and anode voltage combination will reveal that the lines of force, i.e., equal potential lines which connect all points in space having the same potential, are mainly concentrated in the region between the bafiie 33 and the outer perimeter of the grid 32. Substantially few lines of force penetrate the registering apertures 35 with the result that the grid shields the cathode so well that the anode potential, has little or no efiect in the grid-cathode region. Sufficient electron current will not flow to start the discharge until a firing signal of sufficient amplitude'to overcome the self-bias developed by the upon the grid 32.

Once the arc has started a large percentage of the ions tube is impressed are unable to pass through the grid apertures if the slits 28 are made sufiiciently small. The small grid apertures 28 may impede the free flow of ions and so increase the tube internal resistance and are drop between cathode and anode as to introduce serious limitations in the operation of the tube. Positive ion flow from the plasma to the surrounding electrodes and walls constitutes the chief loss of ions from the arc. FIG. 3 illustrates an alternative electrode structure in which restricted passages are provided in the grid only opposite the high potential end of the cathode in order to .reduce the starting voltage and to provide self-bias for the discharge tube, and yet the voltage difference between cathode and anode after starting is a minimum. In this modification a semicylindrical anode is positioned symmetrically with respect to the grid but outside of a direct line between the cathode and the slit apertures through which electronic flow passes for initiating the breakdown of the tube. A generally tubular grid 40 parallel with and surrounding a directly heated cathode 24 is provided with a plurality of spaced parallel slits 28 opposite the high potential end of the cathode 24 and also with a plurality of relative larger apertures 41 diametrically opposite the slits 28 and along the length of the cathode 24. A semicylindrical metallic anode 43 is disposed opposite the apertured slits 28 coaxial with the cathode 24. The anode 43 is thus out of direct line with the apertures 41, the grid 40 serving as an electrostatic shield and segregating the cathode 24 from the anode 43 and preventing the field from the anode 43 from extending through the apertures 41 to the cathode 24. The only effect that the anode potential has in the grid-cathode region is determined by the lines of force that penetrate the apertured slits 28. Prior to the breakdown of the tube the electrons emitted by the cathode assume a distribution such that the electron density is greatest near the cathode and less at a greater distance from the cathode except at the grid apertures 28. This results in the formation of a cloud of electrons on the outside of and adjacent the grid apertures 28, and only the electrons in this cloud are aflected by the field from the anode and are attracted thereto. Once the electron current through the slit apertures 28 has reached a certain minimum value, depending upon the anode voltage, ionization of the gas occurs and many ions difiuse throughout the envelope and initiate a discharge through the apertures 41. In this manner the magnitude of the discharge current is made entirely independent of the size of the slit apertures 28, and the effective resistance to current flow after starting, as well as the voltage drop between anode and cathode, are minimized.

In order to positively insure that ions will initiate a discharge through the apertures 41, a very narrow slit 44 isrprovided in the grid 40 connecting one restricted passage 28 and an aperture 41. The slit 44 is narrower than the restricted passages 28 to allow the positive ion sheaths on opposite sides of the slit 44 to overlap and prevent the establishment of the arc therethrough. The current density through the restricted passages 28 thus controls the starting, but once the arc is established ions flow through the slit 44 and cause the discharge to follow along the narrow slit 44 in a manner similar to the action of a fuze cord to initiate the discharge through the aperture 41. A chain of closely spaced small perforations (not shown) may be utilized instead of a continuous narrow slit 44 to insure the initiation of the discharge through the aperture 41.

FIG. 4 illustrates a modified electrode structure in which the semicylindrical anode 43 of FIG. 3 is replaced with a cylindrical metallic anode 45 having the inner periphery thereof opposite the apertures 41 covered with a coating of suitable insulating material 46 to prevent the lines of force from the anode 45 from penetrating through the apertures 41 to the cathode 24.

Still another alternative grid structure which can be utilized to provide self-bias and a low starting voltage in 6. a gas-filled discharge tube and yet allow the cfiective resistance to current flow and voltage drop between anode and cathode after starting to be a minimum is illustrated in FIG. 5. Slit apertures 28 are provided in the generally tubular grid 50 only opposite the high potential end of the cathode 24. A relatively larger aperture 51 of generally rectangular shape is provided below the slit apertures 28 along the remainder of the cathode 24. One upright wall of a shielding bracket 53 having a U-shaped cross section is fastened to the grid 50 opposite the low potential end of the cathode 24 and the second upright Wall is fastened to said grid 50 between the slits 28 and the aperture 51 in order to provide two rectangular passages 55 in planes transverse of the plane of the aperture 51. This grid structure prevents the lines of force from the cylindrical anode :1-4 from penetrating successively through the openings 55 and 51 to the cathode, but once electronic flow through the slits 28 has caused ionization of the gas, ions diffuse downward and initiate the main discharge through the inner central aperture 51 and the two rectangular passages 55.

While the embodiment of the present invention constitutes a preferred form, it is to be understood that other forms might be adopted all coming within the scope of the claims which follow.

I claim:

1. An electron discharge device comprising an enclosing vessel containing an ionizable medium, a directly heated cathode extending along the axis of the vessel, a tubular grid provided with a plurality of spaced parallel slits opposite the high potential end only of said cathode and with at least one aperture diametrically opposite said slits, said grid surrounding and in substantial parallelism with said cathode throughout its length, insulating means electrostatically shielding the ends of said tubular grid, a semicylindrical anode encompassing and in substantial parallelism with said grid and said cathode opposite said slits, said slits being sufficiently small to prevent the passage of sufiicient electrons therethrough to the anode to ionize said medium, and a plurality of lead-in conductors extending through said vessel respectively from said anode,

cathode and grid.

2. An electron discharge device in accordance with claim 1 wherein said anode comprises a sylindrical conducting member having a coating of insulation covering the interior surface thereof opposite said aperture.

3. An electron discharge device in accordance with claim 1 in which said aperture is rectangular and extends parallel to the axis of the vessel along the length of said 'd. 4. An electron discharge device comprising an enclosing vessel containing an ionizable medium, a directly heated cathode extending along the axis of the vessel, a tubular grid provided with a plurality of spaced parallel slits opposite the high potential end only of said cathode and with a larger aperture along the remainder of the length of said cathode, said grid surrounding and in substantial parallelism with said cathode throughout its length, insulating dis'ks electrostatically shielding the ends of said tubular grid, a cylindrical anode surrounding and in substantial parallelism with said cathode and said grid, a shield spaced from said grid and opposite said larger aperture whereby the field from said anode does not extend to the cathode through said larger aperture, and a plurality of lead-in conductors extending through said vessel respectively from said anode, cathode and grid.

5. An electron discharge device in accordance with claim 1 in which said shield comprises a member U- shaped in cross section with one leg of the U secured to the grid opposite the low potential end of the cathode and the other leg secured to the grid between said slits and said larger aperture and with the bottom of the U opposite said larger aperture whereby a pair of openings are provided between the larger aperture and the anode in planes transverse of the plane of said larger aperture.

6. An electron discharge device comprising an enclosing vessel containing an ioniza-ble medium, a directly heated cathode extending along the axis of the vessel, a generally tubular grid parallel to the axis of the vessel and surrounding and in substantial parallelism with said cathode and provided with an aperture opposite the high potential end only of said cathode, a partition in said grid with an aperture of approximately the same size as and spaced from the aperture in said grid, insulating disks extending in planes coincident with the end of said grid and electrostatically shielding the ends of said tubular grid, a cylindrical anode surrounding and in substantial parallelism with said cathode and said grid, and a plurality of lead-in conductors extending through said vessel respectively from said anode, cathode and grid.

7. An electron discharge device comprising an enclosing vessel containing an ionizable medium, a directly heated cathode extending along the axis of said vessel, discharge control means surrounding and enclosing said cathode but electrically insulated therefrom and provided with at least one restricted passage opposite the high potential end only of the cathode through which electrons may pass, and an anode outside of said control means and symmetrically positioned with respect to said passage whereby the application of a voltage source to said cathode develops a bias between said control meansand said cathode which shields said cathode from-said anode so that the field of said anode does not extend to said cathode and the electron current to said anode is limited to a value below the minimum required to cause breakdown.

8. An electron discharge device in accordance with claim 7 in which said control means is also provided with a relatively larger passage out of direct line between said cathode and said anode and through which current may be conducted after the arc has been started by electronic flow through said restricted passage;

References Cited in the file of this patent UNITED STATES PATENTS 1,921,004 Samuel Aug: 8, 1933' 2,061,254 Rockwood, Jr Nov: 17, 1936 2,292,382 LeV-an Aug. 1:1, 1942 2,436,835 Stutsman Mar. 2; 1948 2,460,794 Selvidge Feb. 1, 1949 2,553,184 Gehrke May 15, 195 1

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