US2509053A - Space current device employing mutually bombarded electrodes - Google Patents

Description

May 23, 1950 c. J. CALBICK 2,509,053

SPACE CURRENT DEVICE EMPLOYING MUTUALLY BOMBARDED ELECTRODES Filed Nov. s, 1947 33 F/G.Z 3f 25 1 1 1 24) 37 36 23 28 /NVEN TOR C. J. CALB/CK BY ATTORNEY Patented May 23, 1950 UNITED STATES- PATENT OFFICE SPACE CURRENT DEVICE EMPLOYING MU TUALLY BOMBARDED ELECTRODES Chester .1. Calbich, Cbatham, N. J. amignor to neu Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York I Application November 8, 1947, Serial No. 784,835

11 Clainu. (c1. zso-z'rs) This invention relates to electrode structures for vacuum tubes, and more particularly to electron emitting structures suitable for use in cathode raytelevision receiver tubes.

It is an object of this invention to increase the emission density and stability of cathode structures for cathode ray tubes.

It is another object of this invention to utilize in an electrode structure for a cathode ray tube theprinciple of mutual bombardment in vacuo of two emitting electrodes.

It has been heretofore proposed to utilize in a vacuum tube the principle of mutual bombardment in vacuo of two electron emitting electrodes. If two electron emitting electrodes are enclosed in an evacuated space, and a suitable alternating voltage is applied between them, they will, if one is heated sufllciently by a starting filament or other means, each emit enough electrons to bombard the other, so that the initial heating mechanism can be dispensed with. Various structures making use of this principle are disclosed in Patent 1,705,377, issued March 12, 1929, to E. Y. Robinson. The present invention is concerned with a specific cathode structure for cathode ray tubes which makes use of this principle and yields a high electron emission density In accordance with the invention, there is provided, by way of example, an electrode structure for a cathode ray tube comprising a cathode member having a substantially flat electron emitting outer surface and an electron emitting inner surface which is a surface of a revolution, such as one which is a paraboloid or one which includes a portion of the inner surfaceof'a hollow sphere, an auxiliary cathode having a rounded or semispherical electron emitting end adjacent the inner surface of the cathode member and having a starting filament near or around it, and a cylinder surrounding the auxiliary cath-' ode to concentrate electrons from the starting filament upon the rim of the open end (away from the fiat surface) of the cathode member. The cathode member is preferably of tantalum and the auxiliary member of tungsten. Parts of the fiat surface of the cathode member can be carburized if desired to make the treated portions more inactive than other portions thereof. An alternating potential is applied between the cathode member and the auxiliary cathode. The starting filament is energized to cause the inner surface of the cathode member to be bombarded with electrons. The electrons emitted by this inner surface bombard the auxili- 2 ary cathode during one-half of the cycle and when this emission has reached a certain value, this auxiliary cathode becomes sumciently heated to bombard the cathode member (causing emission from its flat outer surface when it has heated sufliciently) during the other half of the cycle. At a properly selected time after the establishment of mutual bombardment, the starting filament is turned off. This electrode structure can be used with suitable anodes to produce a focused beam of electrons in a cathode ray tube in accordance with well-known practice.

The invention will be more readily understood by referring to the following description taken in connection with the accompanyin drawings forming a part thereof in which:

Fig. 1 is an enlarged cross-sectional view of electrode structure in accordance with the invention; and

Fig. 2 shows a portion of a cathode ray tube in which the electrode structure of Fig. l is mounted.

Referring more particularly to the drawing, Fig. 1 shows, by way of example for purposes of illustration, an electrode structure ill capable of emitting electrons with high electron emission density. The electrode structure I comprises a cathode ii, an auxiliary cathode 12, a starting filament l3, and a cylinder l4 surrounding the members I2 and I3 and serving to concentrate electrons from the starting filament 'upon the rim of the open end of the cathode II.

The cathode II is made from a cylindrical piece gf alrefractory metal such as tungsten,

- a m enum, osmium and tantalum whi h thermionic electrons adequately at temperatures considerably below their melting points. Other metals which melt before producing appreciable electron emission densities (such as silver, gold, platinum, etc.) cannot be used for this purpose unless the surfaces thereof are treated in some manner (for example, by coating with oxides of barium and strontium) to render them thermionically more active. The interior of the cathode Ii is hollowed out to produce a paraboloidal inner surface IE (or one which includes a portion of the inner surface of a hollow sphere).

The auxiliary cathode I2 is a post of a refractory metal (such .as that of which the cathode H is made) and the end thereof projects into the cavity of the cathode II. The tip 16 of the auxiliary cathode I2 is preferably rounded as shown. in the drawing. Connected to the auxiliary cathode I! by the support member I7 is the cylinder I4, preferably of molydenum, which surrounds the auxiliary cathode l2 and the end of the cathode H near the member l2 and serves to concentrate electrons from the starting filament it upon the rim of the open end it of the cathode II. The cylinder i4 also serves to prevent bombardment, especially during starting, of the ring it which supports the cathode II by means of a plurality of support members IS. The cylinder i4 also acts to a limited degree as a re.- diation shield, and also as a grid to prevent electrons from the post i2 being drawn into the space to the right of the cathode Ii whose righthand surface 23 is furnishing the beam current of the cathode ray tube. The starting filament i3 comprises a coil of wire of a suitable material such as tungsten, surrounding and having one end 2i connected to the post [2 and having the other end 22 thereof connected to a terminal 23. Heating current is applied to the starting filament l3 by means of the terminal 23 and the terminal 24 which is connected to the end 2! of the filament It by means of the cylinder i4, the connector l1 and the rod l2. The ring it is fastened to terminal 25 which is thus electrically connected to the cathode II by means of this ring and support It.

The outer" surface 20 of the cathode H prefer ably has a composited surface. By composite surface is meant one whose electron emission varies locally over the surface in a controlled manner. This control may be by means of different materials or by localized chemical treatment which produces surface areas of difl'erent thermionic properties or which affects their thermal emissive power so that their local temperature is changed or it may be by thermal means (such as those disclosed in the Robinson patent, for example) or by a combination of one or more of these. In some applications it is desirable or essential that the emitting area be sharply defined geometrically or that a portion of the area should not emit electrons. For example, if on the fiat end 20 of the cathode H (which by way of example is of tantalum or some other thermionically more active material, such as carburized, thoriated tungsten), a shallow depression 33 is ground and if a small thin disc 23 of tungsten is then welded or otherwise fastened in this shallow hole, the center of the cathode is thermionically inactive compared to the rest of the cathode surface. The result is an annular emitting area on the end of the cathode H. Obviously, the principle of the example can be generalized (within practical limits) to cover the making of composite emitting surfaces the activity of which differs in local areas in any desired manner. If desired, the outer surface of the tungsten disc 28 can be sandblasted to greatly increase its emissivity. If, in addition, this shallow hole 38 is made slightly concave and the tungsten disc 23 is welded to the tantalum cathode ll only at its periphery, the tungsten is radiatively cooled to a temperature much below that of the tantalum. This example is given to illustrate the general principle that effects due to variation of emissivity can be increased by some degree of design control of thermal conduction.

It has been discovered that if the cathode ii and auxiliary cathode 12 are made of different materials, superior operating results are produced. By way of example, the cathode II can be made of tantalum and the auxiliary cathode I2 of tungsten.

Surrounding the cathode I l is the annular ring 21 which can be placed at the same potential as the cathode H by means of the terminal (or at some other potential, if desired). If placed at the same potential, the field lines in the vicinity of the cathode are more nearly parallel to the axis if the first member of the electron optical system following the cathode is a disc. The surrounding ring 21 is not necessarily fiat. It can be conical, paraboloidal or any other convenient shape (electron optics, in general, requires that these shapes should be axially symmetric. Moreover, e surface H in the cathode need not be fiat but can, for example. be a concave spherical surface for electron optical reasons.

Reference will now be made to Fig. 2 wherein the cathode structure it) of Fig. 1 is shown P sitioned in the butt end of a cathode ray tube 30. Associated with the cathode structure l0 can be any suitable electron optical arrangement. By way of example. positioned parallel to and adjacent the annular electrode 21 is a modulatine electrode 3| followed by a first accelerating anode 32 which in turn is followed by a second accelerating anode cylinder 33. Members similar to the electrodes 3i, 32 and 33 are shown in connection with a cathode having a fiat surface in Patent 2,260,313 issued October 28, 1941 to F. Gray and the operation of such electrode members is also disclosed in the Gray patent. It is obvious, however, that the cathode structure It shown in Fig. 1 is suitable for use in a variety of electron gun structures. In some of these structures, the annular member 21 surrounding the cathode can be omitted or placed to the right of the plane of the surface 20.

A source 34 of alternating voltage is connected by means of the transformer-35 to the terminals 25 and 24 or, in other words, between the cathode ii and the auxiliary cathode i2. The frequency and voltage of the wave applied to the terminals 25 and 24 are not critical; by way of example, they can be 60 cycles and 1000 volts respectively. An alternating source 36 of the order of 6 volts at 60 cyc s is connected between the terminals 24 and .4 to provide heating current for the starting filament i3. The members 3t, 32 and 33 are placed at appropriate potentials to form and accelerate a cathode ray beam. Suitable potentials for these members are set-forth in the Gray patent. Connected in either the primary or secondary circuit of the transformer 3 (by way of example, it has been shown in the primary circuit) is a resistance 31, the purpose of which will be described more fully below.

The operation of the arrangement shown in Figs. 1 and 2 will now be described. Heating current is applied to the starting filament it to cause it to emit electrons to the surface 15 of the cathode H when the phase of the high voltage alternating wave from the source 34 and the transformer 35 is such that the cathode ll is positive with respect to the starting filament and the auxiliary cathode i2. These electrons heat the cathode H and, when its temperature has been raised sufficiently high, it also emits electrons which, during the opposite phase of the alternating wave when the auxiliary cathode I2 is positive with respect .to the cathode ii, bombard the auxiliary cathode l2 and raise its temperature sumciently high so that it also mits electrons. These electrons are in effect added to those emitted by the starting filament, so that the temperature of the cathode rises more rapidly,

. causing rapidly increasing thermionic emission of electrons which, on the alternate half cycle,

bombard the auxiliary cathode l2, causing its temperature also to rise rapidly and its emission to increase. When this emission has reached can be said that mutual bombardment is establish'ed and the current in the starting filament is turned off by any suitable circuit. One such starting circuit is disclosed in the Robinson patent mentioned above. The tempertaure at which the cathode II operates can be adjusted to any value above a certain minimum by control of the magntiude of the alternating bombarding voltage. The minimum value is determined by the goemetry of the mutual bombardment space. At any i value of bambarding voltage above a, certain minimum, the temperatures of'the cathode II and auxiliar cathode I! will rise until the bombarding current is limited by space charge. When the applied bombarding voltage is symmetrical, i. e. where it has the same amplitude in both directions, the temperatures will rise until the bombarding current from the auxiliary cathode l2 to the cathode H is thus limited. The applied bombarding voltage may have different amplitudes in the two directions, as may be secured, for example, by inserting a copper oxide rectifier in the primary circuit of the transformer 35 supplying voltage to the terminals 25 and 24. In particular, if the amplitude of the alternating bombarding voltage applied during the half cycles when the auxiliary cathode I2 is positive with respect to the cathode II is much less than that applied during the alternate half cycles, the temperature will rise until the bombarding current from cathode H to the auxiliary cathode I2 is limited by space charge, the bombarding current in the reverse direction being limited by the temperature of the auxiliary cathode l2. The importance of this application of asymmetrical bombarding voltage is that it reduces the minimum temperature of stable operation. A second desirable effect is that heating of the cathode ll tends to be localized around is closed end 20, since the right-hand end of auxiliary cathode I2 is at a. much higher temperature than its lefthand end, because of heat loss due to thermal conduction and radiation. Because of this localized heating, the useful end 20 of cathode H is at a higher temperature than the open end.

The minimum temperature of stable operation can be further reduced by insertion of the series resistance 31 in either the primary or secondary circuit of the transformer 35 supplyin the bombarding voltage. In general, with an asymmetrical structure such as the mutual bombardment cathode of Fig. 1, best operation is secured by using a compensating asymmetry of either voltage or material or both. When the auxiliary cathode is tungsten' and the cathode is tantalum, for example, efiects are obtained similar to those obtained by an asymmetrical voltage wave when the two mutually bombarded cathodes are of the same material.

For a particular structure such as that of Fig. 1, an appropriate circuit can be designed to yield a low minimum temperature. This has the advantage that the life of the post constituting the auxiliary cathode I2 is greatly increased compared with that under conditions of simple spacecharge stabilization. Above the minimum operating temperature, the temperature can be simply controlled to any value below the melting point of the cathode II by control of the voltage applied to the primary of the transformer 35.

Other advantages and features of mutual bombardment are: (1) cathodes of large or small area massive enough to give long life and sumcient mechanical strength can be heated to high temperature with a resultant very high electron emission density; (2) this heating is done by high voltage and relatively smallbombarding current which is in sharp contrast to conductively heated cathodes of large area; (3) as a consequence of the small current and also the symmetrical distribution, magnetic fields due to heating are extremely small (very much smaller than in the best indirectly heated cathodes); (4) by proper control methods (some of which have been set forth above) absolute stability and freedom of temperature adjustment within the mutual bombardment region can be achieved; (5) if high emission density is not required, the temperature range of stable operation is sufiicient to permit extremely long life by lowering the temperature; (6) long life can be achieved by making the auxiliary cathode suificiently massive and by operating at a sufliciently low temperature; (7) in case it is desired to subject cathodes to chemical manipulation, as in thoriatien and carburization processes, operating conditions can be specified by which this action can be achieved which .is in considerable contrast to directly heated,

radiatively shielded cathodes; (8) the power source required and auxiliary starting equipment are simple; (9) wide latitude of geometrical design for the achievement of special purposes is possible and, in particular, adequate shielding either for radiative purposes or to prevent bombardment current from entering the useful region of the electron discharge device is easily achieved; (10) composite cathodes and composite cathode surfaces (such as that produced in the front surface of the cathode II by the insert disc 26) can be used, such cathodes and cathode surfaces being important for special applications; (11) the cathode can act as a getter, producing an exceptionally high vacuum as the tube operates, tantalum cathodes probably being the best for this use; (12) ion spots can be eliminated from the fluorescent screens of cathode ray oscillograph or television tubes because a pure metal cathode which does not emit appreciable numbers of negative ions (the cause of the ion spots) can be used instead of an oxide-coated cathode which emits large quantities of such ions; and (13) mutual bombardment can also be used in the production of high temperature per so, as is required in a high temperature vacuum furnace. The principal disadvantage of the mutual bombardment method is the fluctuation of temperature over the cycle of its bombardment but this is of small importance when the useful current is space-charge limited as in most electron discharge devices. In any event thefluctuation is small and can be reduced by increasing the mass per unit area of the cathode ll.

Various modifications of the embodiment described above can be made without departing from the spirit of the invention the scope of which is indicated in the claims.

What is claimed is:

1. A space current device comprising an evacuated container enclosing a first thermionic electrode member having a first electron emitting surface and a second electron emitting surface, said second surface including a concave surface,

a second thermionic electrode, at least a portion 7 oftheolectronemittingareaofsaidsecondelectrode being positioned within the space bounded by said concave surface, and means for applying an alternating potential wave between said first and second electrode members.

2. The combination of elements as in claim 1 in which said concave surface is paraboloidal.

3. A space current device comprising an evacuated container enclosing a cathode member having a first electron emitting surface and a second electron emitting surface, said second surface being concave, and an auxiliary cathode member comprising a metal rod having an end thereof adjacent said second surface, at least a portion of the electron emitting area of said auxiliary cathode member being positioned within the space bounded by said concave surface.

4. A space current device comprising an evacuated container enclosing a cathode member having a first electron emitting surface and a second electron emitting surface, said second surface being concave, an auxiliary cathode member comprising a metal rod having an end thereof adjacent said second surface, at least a portion of the electron emitting area of said auxiliary cathode member being positioned within the space bounded by said concave surface, and a shielding member surrounding at least a portion of said auxiliary cathode and said cathode memher.

5. A space current device comprising an evacuated container enclosing a cathode member having a first electron emitting surface and a second electron emitting surface, said second surface being concave. an auxiliary cathode member comprising a metal rod having an end thereof adjacent said second surface, at least a portion of the electron emitting area of said auxiliary cathode member being positioned within the space bounded by said concave surface, and a starting filament surrounding said auxiliary cathode.

6. A space current device comprising an evacuated container enclosing a cathode member having a first electron emitting surface and asecond electron emitting surface, said second surface being concave, an auxiliary cathode member comprising a metal rod having an end thereof adjacent said second surface, at least a portion of the electron emitting area of said auxiliary cathode member being positioned within the space bounded by said concave surIace and an annular member surrounding that portion of said cathode member remote from said auxiliary cathode memher.

'7. A space current device comprising an evacuated container enclosing a cathode member having a first electron emitting surface and a second electron emitting surface, said second surface being concave and an auxiliary cathode member comprising a metal rod having an end thereof adjacent said second surface, at least a portion of the electron emitting area of said auxiliary cathode member being positioned within the space bounded by said concave surface, said cathode and said auxiliary cathode being of different refractory metals which emit thermionic electrons at temperatures considerably below their melting points.

8. A space current device comprising an evacuated container enclosing a cathode member having a first electron emitting surface and a second electron emitting surface, said second surface being paraboloidal and said first surface being flat, and an auxiliary cathode member comprising a metal rod having a rounded end thereof adjacent said second surface, at least a portion of the electron emitting area of said auxiliary cathode member being positioned within the space bounded by said paraboloidal surface and the longitudinal axis of the auxiliary cathode coinciding with the longitudinal axis of the paraboloidal surface.

9. A space current device comprising an evacuated container enclosing a cathode member having a first electron emitting surface and a second electron emitting surface, said second surface being concave, and an auxiliary cathode member comprising a metal rod having an end thereof adjacent said second surface, at least a portion of the electron emitting area of said auxiliary cathode member being positioned within the space bounded by said concave surface, said cathode being made of tantalum and said auxiliary cathode being made of tungsten.

10. A space current device comprising an evacuated container enclosing a cathode member having a first electron emitting surface and a second electron emitting surface, said second surface being concave, and an auxiliary cathode member comprising a metal rod having an end thereof adjacent said second surface, at least a portion of the electron emitting area of said auxiliary cathode member being positioned within the space bounded by said concave surface, said cathode having a first surface the electron emission of which varies locally thereover in a controlled manner.

11. A space current device comprising an eves-- uated container enclosing a cathode member having a first electron emitting surface and a second electron emitting surface, said second surface being concave, and an auxiliary cathode member comprising a metal rod having an end thereof adjacent said second surface, at least a portion of the electron emitting area of said auxiliary cathode member being positioned within the space bounded by said concave surface, said first surface of said cathode having a small disc of tungsten set in the center portion thereof leaving a free space under the disc for cooling purposes, the remainder of said cathode being of a refractory metal which emits thermionic electrons but which is other than tungsten.

CHESTER J. CALBICK.

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

UNITED STATES PATENTS Number Name Date 1,585,766 Chubb May 25, 1926 1,617,684 Holst et al Feb. 15, 1927 2,143,262 Farnsworth Jan. 10, 1939 2,231,610 Becker Feb. 11, 1941 2,355,795 Glass Aug. 15, 1944 FOREIGN PATENTS Number Country Date 519,111 Great Britain June 9, 1939

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