US2070816A - Gas-filled discharge tube - Google Patents

Description

Feb. 16, 1937. c. J. R. H. VON WEDEL GAS-FILLED DISCHARGE TU1 3E Filed Jan. 20, 1931 INVENTOR CarZJK/Y 1 0/2 iVedeZ BY 11M ATTORNEYS Patented Feb. 16, 1937 UNITED STATES GAS-FILLED DISCHARGE TUBE Carl J. R. H. von Wedel, Newark, N. J., as signor to Electrons, Inc., a corporation of Delaware Application January 20, 1931, Serial No. 509,902

11 Claims.

This invention relates to gas filled discharge tubes having auxiliary electrodes for controlling the operation of the tube An object of the invention is to provide, in a gas filled tube, suitable control electrodes or grids which will insure more dependable. and accurate regulation of the output of the tube, and such that the desired operating characteristics will be maintained during the life of the tube.

More particularly the construction is designed to overcome the undesired effect of grid emission, which is caused by a number of reasons hereinafter set forth.

A further object is to provide an efiective control for higher operating plate voltages without increasing the discharge resistance in an unde sired way which wouid increase the losses and cause heating of the tube and electrodes.

I accomplish the foregoing objects by the use of a grid of suitable material which does not become emissive and having such configuration as to give suitable control of the electron stream and prevent secondary emission due to ionic bombardment.

Referring to the accompanying drawing,

Fig. 1 shows in perspective a suitable form of .grid embodying the principles of the invention;

Fig. 2 is a cross-section through the center of the grid shown in Fig. 1;

Fig. 3, is a comparison of the effect of ionic bombardment tending to cause grid emission between different forms of grids;

Fig. 4 shows a tube embodying the invention,

and

Fig. 5 is a modification of the invention.

With regard to the general behaviour of grid controlled gaseous discharge tubes of relatively low internal resistance, only the starting point of general ionization of the gas in the tube can be controlled, and the grid has no further control after the main discharge starts until the plate current supply is discontinued and the discharge extinguished, or until the plate supply passes through zero in the case of alternating current supplied to the plate circuit of the tube. The main discharge in such a tube will start, however, at a point where the product of electronic speed (or voltage acceleration of the electrons on accountof the tube field between the discharge electrodes) and the current, or their number per unit of time, arrives at a certain value dependent on the gas used and its pressure in the tube.

The starting point furthermore depends on the type of energy absorbed by the atoms of the gas present and how these atoms respond in their different stages of excitement with regard to absorbing and reradiating the received energy obtained from the electronic current. Thus as to how and when the main ionization of the gas is introduced is also a question of the kind of work done by the electronic current in the tube. High speed electrons of smaller number will introduce the general ionization in a different manner from low speed electrons of larger current, which means the ionizable atmosphere present will, according to the kind of work done, respond similarly with different atoms of said atmosphere if impurities are present, or may respond with different stages of excitement of similar atoms. In one case large amounts of energy can be absorbed without causing suitable reradiation for excitation of nearby atoms and in the other case the absorbed energy may readily be reradiated and communicated to nearby atoms. In the first case more work must be done or higher losses occur in the tube.

In grid controlled gaseous discharge tubes, particularly those operating on alternating current, the negative potential of the grid in case of ascending plate voltage first acts to deflect the electrons emitted by the incandescent cathode, that is, it will retard these electrons and shield the plate field. When the voltage is rising on the plate, however, occasionally some higher speed electrons will escape and as soon as they pass the grid they are accelerated by the plate field. It may also be said that a positive field is only a less negative field and that the electrons escape towards it being repelled by the more negatively charged space. This plate field, being of the order of some hundred volts, will accelerate these electrons far above the ionization potential of the gas, but as pointed out above, it may not induce the main discharge; its energy will be absorbed or reradiated and there may not be sufficient energy available to keep ionization going on until the atoms near the plate also become ionized. However, if a larger number of electrons escape, even if no general ionization occurs, ions are created at a certain distance from the grid and these are attracted by the grid and start to move toward it.

Here is the point where trouble with the grid will start. As a matter of fact we wish to obtain control of the number of electrons passing the grid and coming from the incandescent cathode and we wish to do this with the static field of the grid in such a way that we control their number safely up to the point where the numtia and slow speed compared to the electrons,,

ber of these electrons emitted by the cathode do the necessary work to induce general ionization. This is the only way to keep the control conditions constant, so that we may be assured that a given plate voltage will start the main discharge if the grid has a certain negative voltage with respect to the cathode. We must therefore avoid disturbances introduced by such first created positive ions between grid and plate, be-- cause we have no control over their action.

These positive ions moving toward the grid acquire from the field in the tube diflerent velocities because they are created at diiferent distances somewhere in the space between grid and.

plate, either directly by the impact of electrons or indirectly by first taking up reradiated energy from other excited atoms. In flying towards the grid the ions may become neutralized there if their speed is very low, and since they are not numerous, they will cause only negligible grid currents to flow. With ions having higher speeds, however, and the grid materialbeing hot or made from substances which will readily emit electrons, they will start to cutout secondary electrons, one or more at each impact, and these are accelerated under the plate field and increase or cause general ionization which is now not under control. This is particularly likely to happen if the ions strike the grid at a 45 angle or thereabout, because the greatest number 0: secondary electrons are then cut out.

Furthermore if the grid wires are round and very thin, some positive ions start to travel around them in a planetary orbit until they finally fall on the wires and neutralize their charge. This seems to be one of the most undesired actions causing considerable irregularity of the grid control because these ions, with large inerby doing so will neutralize the grid field for a large number of electrons coming from the cathode and a large amount of them will pass the grid causing general ionization of the tube at an undesired time, and out of control of the grid.

Referring to Figs. 1 and 2, there is shown a grid with a plane portion 1 having openings defined by vanes or ribbons 2 which are perpendicular to the plane portion. The grid is adapted to be mounted so that its vanes are parallel to the main discharge path between the cathode and plate electrodes of the tube. Since the vanes present only their edges to the discharge path, those ions and electrons which strike the grid fall upon a surface which is substantially perpendicular to their path and are neutralized by the grid, thus causing substantially no secondary emission. The electron stream tending to pass through the relatively large slots 3 is controlled efliciently by the static field set up by the relatively large side surfaces of the vanes. A convenient way of making the grid of Figs. 1 and 2 is to cut slots in a metal sheet so as to form parallel metal strips, and then twist these strips 90 about their axes so that the edges of the strips are exposed in the direction of the discharge path.

Fig. 3 illustrates how a grid composed of round wires 4 will cause secondary emission of electrons created by ions falling on a multitude of surfaces at angles to the discharge path, as compared with a grid composed of vanes presenting only small surfaces which are perpendicular to the ionic stream. Thus, when ions strike the surface of the grid at angles other than the perpendicular, the impact of the ions may cause electrons to be thrown oil from the grid surfaces,

causing the grid to become emissive and interfering with the proper control of the tube. Also, the positive ions created before the main discharge starts are prevented from traveling in orbits around the grid wires by the extended surface of the fiat vanes 2.

The grid should be made of material which will not readily emit electrons under bombardment, and should have good thermal conductivity so that it will remain relatively cool. It further should have a low discharge resistance in the direction of the discharge path. Also, ifthe grid is not sufliciently heat conductive, certain parts of its surface will heat up because, even though the amount of heat in the discharge path may not be great due to the low density of the gas, its temperature is very high, and the heating of parts of the grid facilitates the disturbing secondary emission.

It is not essential, however, that the grid be a good conductor of electricity since only a very small current carrying capacity is necessary to establish its operating potential. It is not necessary, therefore, that the grid be made of metal,

particularly in the case of high voltage tubes, and

materials which have less electrical conductivity, and which do not readily become emitters, may be used. Substances such as carbon or graphitized carbon mixed with substances such as beryllium oxide, .aluminum oxide, zirconium oxide or uranium oxide. A carbon-grid according to the invention may be prepared by winding a linen or cotton strip on a metal support so that it forms a grid structure similar to Fig. l with parallel ribbons exposingtheir si es to each other and their edgesiii the direction of the discharge path. Thisstructure can first be parchmentized and dipped in a solutionof a colloid or fine mixture of graphite with zirconium oxide and uranium oxide or aluminum oxide. It is then placed in an oven and carbonized in a similar way isdeveloped by the art in making carbon filaments for lamps. The whole grid structure can thereafter be heated in a hydrocarbon or cyanogen atmosphere to carbonize its whole surface as far as it is exposed to bombardment, and contains metal parts for support. ,Ail highly emissive incandescent electrodes sputter or vaporize to a certain extent during the process of manufacture and when the tube is in service, particularly in the case of cathodes coated with oxides of the alkaline earth metals. The coating is sputtered onto or condenses on the grid surface and tends to make the same more emissive. Therefore, it has been found desirable in practice to make the grid of a material which will neutralize the emissivity of deposits of vaporized cathode material. In case the grid is metallic, such metals should be used the hydroxides of which are definitely acid. such as tungsten or tantalum or at least more acid than the hydroxide of molybdenum. Such metals have the propery of combining with the alkaline earth metals on their surface and forming for instance, tungstates (because always some oxygen is freed in the tube) which are not emissive or are poor emitters.

Fig. 4 shows a discharge tube having an envelop 6 with reentrant end portions 7 and 8 supporting the electrodes, the tube being filled with gas, preferably an inert gas such as argon, although other ionizable atmosphere may be employed, such as mercury vapor, or a mixture of mercury vapor and inert gas. The cathode as- 5 sembly 9 comprises an open ended can iii enclosing a plurality of emissive filaments ii, preferably of the oxide coated type. The resultant compound from a mixture of an alkaline earth metal, such as barium, calcium or strontium oxide and the oxide of a metal the oxy-acld of which is less acid than titanic acid, such as aluminum oxide or zirconium oxide, comprises a desidable coating. The space directly between the upper open end of the cathode and the anode I4 is surrounded by a grid l2 of a form particularly suitably for high voltage tubes. A circular carbonizedmetal disk or shield II by which the discharge path is deflected to obtain a shielding action to prevent back-arcing from the anode lin the event, it becomes negative-4s placed close to the anode l4, approximately within the mean free path of the electrons, with a given gas pressure in the tube, so that any electrons freed from the anode will not strike an arc, but will travel at such short distances only that ionization re--' sulting therefrom is insufficient to introduce the main discharge. The electron stream from the cathode passes out through the slots between the vanes 2 in the cylindrical side portion of the grid. The vanes extend radially, and their edges point in the direction of the electron stream, so that the sides of the vanes do not present angular surfaces to the electron path.

The grid is surrounded by a flanged annular shield l5 which extends in close proximity to the wall 6 of the tube, thus providing a good heat radiating surface near the cooler portion of the tube and also intermediate the ends of the tube where the heat radiated to its wall will not affect or tend to break the seal of the envelop. This extending shield also cooperates with the negative charge which is normally present on the tube wall and shields the portion of the tube above the grid from that below and thus prevents any discharge around the grid.

The plate supporting wires l6 are preferably enclosed with protective insulation or shielding means l'l to prevent ionic bombardment of the conductors. 1

Fig. 5 shows a construction which is adapted for high plate voltages of different value, and in which two grids 2i and 22 are placed in the discharge path in such manner as to reduce the discharge resistance of the tube. In this case the grid 22, which is near the cathode, may be made of metal if desired, and the grid structure 2|, which is near the anode 23, should preferably be made of non-emissive carbon and oxide compounds, such as mentioned hereinbefore The grid structure 2i surrounds the plate 23 and serves to partially shield or neutralize the lines of force from the plate and also serves as a shield for the plate lead-in conductors i6 and their insulating covering I! from ionic bombardment. The grid, or at least that portion of it behind the plate, is placed within a distance from the plate approximately equal to the mean free path of the electrons. The grid is preferably connected to the plate circuit by means of a current limiting resistance 23 and variable connection 24 to the plate supply transformer 25, and

' its potential can therefore be made more or less and load applied thereto. Grid 2| may, however,

tained at a negative potential with respect to the cathode, and its potential may be controlled by potentiometer 29 connected to the grid by conductors 21 and 28. The potentiometer is connected across a source of direct current 30 whereby the potential on the grid may be adjusted, the source of direct current being connected to the cathode throughthe secondary winding 3| of a transformer 33. The primary wind ig 34 of the transformer is connected, in series I ith a phase shifting impedance 32, to the plate supply, to impress an out of phase alternating potential on the grid 22 to control the starting point of the tube throughout the entire alternating current cycle.

Any of the foregoing grid structures may have diiferent shapes other than that shown, such as a ribbon wound in the form of aspiral, or parallel ribbons arranged in criss-cross relation, so as to obtain small surfaces exposed in the direction of the discharge and substantially perpendicular thereto, with larger surfaces parallel to the discharge path. i

What I claim is:-

1. A grid controlled gaseous discharge tube comprising a thermionic cathode, an anode, and an electrostatic'control electrode therebetween, disposed in an ionizable atmosphere of substantially constant density throughout the tube, the control electrode comprising spaced apart strips with relatively thin edges disposed in the discharge path between said anode and cathode with their edges substantially perpendicular to the paths of ions impinging thereon and with their larger lateral surfaces parallel to said paths, said strips being spaced from the, anode a distance greater than the mean free path of said gas filling.

2. A grid controlled gaseous discharge tube comprising an envelope, a rare gas atmosphere therein, a thermionic cathode, an anode, an impervious shield disposed between said cathode and anode transverse to the direct path therebetween and extending close to the wall of said envelope, said shield having an opening therein, and a series of thin spaced apart vanes bridging said opening and forming a starting control grid, said vanes presenting only their edges to the ions im pinging thereon.

3. A discharge tube comprising an envelope, an ionizable atmosphere therein, an electron emissive cathode, an anode, a shield disposed between said cathode and anode obstructing all straight discharge paths therebetween, a grid surrounding said shield, and means surrounding said grid and cooperating with said shield and grid for electrically shielding that portion of the envelope containing the anode from that portion thereof containingthe cathode.

4. A discharge tube as defined by claim 3 in which the first-mentioned shield is spaced from the anode a.--.distance determined by the mean free path ofthe atmosphere in said tube whereby back-arcing is prevented.

5. A grid structure for a gaseous discharge tube comprising a central, conductive member, an outer conductive member surrounding the central mem-. her but spaced therefrom, both said members being impervious to the passage of electrons and ions therethrough, and a. control grid comprising' a series of vanes disposed in the space between saidmembers and providing therein a series of slots defined by the lateral surfaces of said vanes.

6.- A grid controlled gaseous discharge tube comprising an anode, a thermionic cathode. a filling of inert gas adapted for a low voltage gaseous arc discharge between the cathode and anode, and a control electrode the potential of which determines the starting of said discharge, said control electrode comprising spaced apart strips disposed in said discharge path with their thin edges perpendicular thereto and exposing their larger lateral surfaces substantially parallel to a small portion of'the length of said path, said strips being located a distance from the anode greater than the mean free path of said gas filling.

'L'A grid controlled gaseous discharge tube comprising a substantially cylindrical envelope, an, anode and a thermionic cathode disposed therein and aligned on the axis of said envelope, an inert gas filling providing a gaseous are discharge path between the anode and cathode, and athird electrode disposed therebetween with a plane portion of said electrode perpendicular to said axis and extending close to the envelope wall, said electrode comprising means for controlling the-starting of the gaseous arc discharge between the anode and cathode including a series of vanes spaced apart and so disposed relative tothe ions created in the tube prior to the starting of said discharge that said ions strike the edges of the vanes perpendicularly or pass thevanes substantially parallel to their lateral surfaces, said vanes being spacedirom the anode 'a distance greater than the mean free path of saidgas filling.

8. A grid controlled discharge tube comprising an envelope containing an ionizable atmosphere,.

spaced from each other compared to the space a between the anode and the member near it, both said members restricting the available space for the passage of said discharge from anode to cathode, a starting control grid disposed within said restricted space, and a lead-in conductor for said grid sealed in the envelope wall at a point remote from all conductive parts of the other electrodes.

9. A grid-controlled gaseous discharge tube comprising an envelope, an ionizable atmosphere therein, a thermionic cathode, an anode, conductive control means. having an electron impervious portion thereof spaced from the anode within the mean free path distance in said atmosphere and a perforate portion thereof spaced from said anode a distance greater than the said mean free path, said impervious portion being disposed in the direct path between said anode and cathode to lengthen substantially the discharge path therebetween.

'10. A discharge tube comprising an envelope containing an anode, a thermionic cathode and an ionizable atmosphere adapted in combination to produce a low resistance arc discharge from anode to cathode, a starting control grid,

a discharge impervious conductive shield disposed between the anode and grid, and a conductive shield disposed between the grid and charge current after it is started, said tube comprising an envelope containing an ionizable atmosphere, an anode and a thermionic cathode disposed therein to provide suflicient space from anode to cathode for'an arc discharge therebetween, discharge impervious conductive shields restricting the cross sectional area of said envelope at two or more points between the anode and cathode, said points being spaced apart a distance greater than the mean free path of electrons in said atmosphere, and starting control means disposed between said shields.

CARL J. R. H. von WEDEL.

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