US2157585A - Electric discharge device - Google Patents

Description

May 9, 1939. v. K. ZWORYKIN El AL Filed July 25, 1936 2 Sheet jnwenrs .VQ Zzhz'J-IZZL' jazz/5 gj 57150 QZZer #TTOF/VfX May 9, 1939.

ELECTRIC DISCHARGE DEVICE Filed July 25, 1936 2 Sheets-Sheet 2 455 /9 Lr/7 2/ 0071 07 5; W 7 J7 2;; INPUT 00 i I'l'l'l'l' /02 Java-@715.

v. K. ZWYORYKIN ET AL 2,157,585

Patented May 9 193 PATENT OFFICE ELECTRIC DISCHARGE DEVICE Vladimir K. Zworykin, Philadelphia, Pa., and Louis Malter, Oaklyn, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application July 25,

3 Claims.

This invention relates to grid-controlled electric discharge devices suitable for use in systems and circuits for the communication of intelligence, and has for its principal object the pro- 5 vision of a discharge tube the operation of which is characterized by a high value of mutual conductance.

- In the usual electron tube employing cathode, grid and plate electrodes, the electrons emanate from the cathode with a Maxwellian velocity distribution and approach the controlling field about the grid with various velocities. When subjected to a low retarding grid-potential the lower velocity electrons are slowed down more effectively ;than those traveling at higher velocities. Thus, while a low counter-potential field ofa certain voltage may prevent the slow-moving electrons from reaching the anode, such voltage will serve merely to retard the speed of the higher velocity electrons without blocking them. Increased increments in the grid counter-potential will cut off or block the flow of the higher speed electrons until, for some comparatively large grid counterpotential, all of the electrons will be blocked. The large grid potential variation required for controlling the electron current from saturation to cut-off is manifest in the lack of steepness (indicative of a low value of mutual conductance) in the grid potential-plate current (EgIp) curve, characteristic of the usual tube.

The present invention contemplates and its practice provides means for ensuring that the electrons approach the grid control field with a desired minimum range of velocities. Thus, whereas in discharge tubes of a known type the electrons approach the grid with a range of velocities of, say, one volt, and hence require a grid-swing of similar value for controlling the electron current from saturation tocut-off, in tubes constructed in accordance with the present invention the range of velocities of the electrons, and hence the range of potentials required to control them, may be of the order of one-tenth of a volt, more or less.

More specifically, the invention provides an electron velocity-filter intermediate the cathode and control-electrode for barring the passage of electrons whose velocities are without the limits of the desired electron-velocity range.

The filter is preferably in the form of a pair of plate-like collimating electrodes mounted to define an electron path between the cathode and control grid. One of these plates is maintained at a potential positive with respect to the other so that, unless some force intervenes, the

1936, Serial No. 92,566

electrons or most of them will be deflected and absorbed by the positive plate. To prevent this undesired condition, a magnetic field is provided, the direction of the lines of force constituting the field being parallel to the planes of the platelike filter-electrodes.

With the magnetic field in the proper direction, its effect is to deflect the electrons away from the positive plate and towards the negative plate. Under the opposing forces of the electrostatic and magnetic fields, electrons whose velocity is equal to the ratio of the electrostatic field (E) to the magnetic field (H) will pass between the filter plates in paths parallel thereto. If the velocity of a given electron is in excess of this ratio it will be drawn towards the negative plate, and if less, towards the positive plate. At certain velocities the electrons may be drawn towards one or the other of these filter plates without impinging thereagainst; these electrons, like those which pass in undeviating paths between the plates, will be subjected tothe controlling action of the grid. Electrons traveling at other velocities, higher or lower as the case may be, will impinge against the surface of one or the other of the filter plates and be effectively lost. The range of velocities of the electrons available at the side of the filter remote from the cathode (and hence subject to the controlling action of the grid) is determined primarily by the spacing of the filter-plates.

With the invention embodied in a simple three element tube the number of electrons available at the anode side of the grid may be a relatively small per cent of the total emitted by the cathode. Where necessary or desirable the loss in electron current may be effectively compensated for by electron-multiplication. The multiplier is preferably of the magnetic type, in which case the magnetic field serving to direct the secondary-electrons in the grid-anode region may also serve to direct the primary-electrons from their source, through the filter, to the grid.

Certain details of construction and operation, together with other objects and advantages, will be apparent and the invention itself will be best understood by reference to the following specification and to the accompanying drawings, wherein Figure 1 is a view in perspective of an electron discharge tube embodying the invention, portions of the tube wall and the associated magnet structure being broken away to more clearly illustrate the disposition of the electrodes;

Fig. 2 is a diagrammatic sectional view exemplifying the manner in which the device shown in Fig. 1 may be operated;

'Fig. 3 is a diagrammatic sectional view showing the invention embodied in an electron multiplier of the magnet-type;

Fig. 4 is an elevational view of cathode, filter and grid electrode assembly suitable for use in magnetic-type electron multiplier tubes.

Referring to Fig. 1, I designates an evacuated tube having a pair of oppositely located presses 3 and 5 sealed in the ends thereof. Press 5 supports a virtual source of electrons constituted by a filamentary heater element 1 contained in a metal cathode thimble 9 having a layer of electron-emissive material II on its outer end. An electron-accelerating electrode in the form of a grid I3 is likewise supported on press 5; with its meshed surface spanning the area immediately in front of the emissive surface II. The central portion of the tube contains two plate-like filter electrodes I5 and I7, disposed in spaced parallel relation on opposite sides of the long axis of the tube. The other press 3 supports a control electrode I9 and an output electrode 2I in parallel planes normal to the planes of the filter plates.

As shown in the drawings the emissive cathode surface may be rectangular in shape with its dimensions corresponding generally with those of the perimeter of the open space between the opposed filter plates I5 and I1. One of the supports, 23., for the cathode 9 terminates in press 3 and the other terminates in an external connection 25. Conductive leads 2'! and 29 supply heating current to the filament 1; wire 3| is the lead for the accelerating electrode I3. The filter plates I5 and I! are energized, respectively, through wires 33 and 35 which support them. The anode lead wire is designated 31 and the control grid lead 39.

Any convenient means may be utilized for establishing the magnetic field employed for giving the electrons passing between filter-jplates I5 and I1 the required trajectories. The device shown in perspective in Fig. 1 is constituted by a U-shape element 4| of magnetically permeable material on which is mounted an energizing coil 43 and to each upstanding portion of which is afiixed a plate 45 also of permeable material. The tube is disposed between these plates in such position that a substantially uniform. magnetic field is set up parallel to the opposed surfaces of the filter electrodes.

Referring to Fig. 2 which shows one manner of utilizing the device of Fig. 1, the upper filter plate I5 is shown connected to the positive side and the lower filter plate IT. to the negative terminal of a direct current source, exemplified by the battery 41. The accelerating electrode I3 is maintained positive with respect to the cathode by battery 49 which is shown as connected between the grounded cathode lead 25 and the midpoint of source 47. by a connection 5|. Electrode I5 is thus positive with respect to both the cathode 9 and the lower filter plate IT. The control grid I9 is preferably biased slightly negative withrespect to. the cathode as by means of athird battery 53. The-input of the device is shown connected across a high resistance 55 in the grid lead 39, and the output is taken ofi across a high resistance; 57 inseries with the anode lead 31. The cathode heater 1 and the anode 2| may be energized by any suitable or convenient means, not shown.

When the cathode heater I is energized, thermionic electrons having various initial velocities will be given off by the emissive coating I I. These electrons are drawn to and pass through the interstices in the positively charged accelerating electrode I3, whereupon they enter the area between the spaced plates I5 and IT. The electric and magnetic fields (the magnet is not shown in Fig. 2) pervading the space between these plates have a collimating effect upon the electrons, that is to say, the combined effect of these fields tends to direct the electrons in paths parallel to the long axis of the container in a manner similar to that which, in optics, a collimating lens serves to produce parallel rays of light. If the electrons were all traveling at a certain optimum velocity they would all be given parallel trajectories. As it is, the trajectories of only a portion of the electrons achieve parallelism. The electrons traveling at higher velocities are influenced more by the magnetic field than by the electrostatic field and are constrained to approach the less positively charged plate II. These rapidly moving electrons may either swerve slightly towards this plate without touching it, as indicated by line A, or they may impinge thereagainst andbe lost or filtered out as shown at B. The lower velocity electrons are influenced; more by the electrostatic field, and are drawn towards the more positively charged filter plate I5. As in the case of the extremely fast moving electrons, these relatively slow moving particles either impinge against the plate toward'which they are bent, as shown by line C, or escape it, as. indicated. at D.

From the foregoing it will be apparent that the electrons which pass between the filter plates. I5 and I1. without impinging against one or the other of them, and are subjected to the controlling action. of the grid I9, will have a narrow range of velocities. The exact range is determined to a. large extent. by the closeness of the spacing; of the filter plates. Assuming these filter plates to bespaced, say one-eighth of an inch apart and the intensity of the electric and magnetic fields, so adjusted as. to permit the passage of electrons whose velocities differ by say one-tenth of a volt, then the variation, in grid potential required to control these electrons from cut-off to saturation need only be one-tenth of a volt. Obviously such a device will, in operation, exhibit an extremely high mutual conductance and possess a useful sensitivity far greater than that of the usual gridcontrolled amplifier or other discharge tube.

As previously set forth and as clearlyshown bylines B and C in Fig. 2,, a great many of the electrons emanating from the cathode fail to reach the anode. This naturally results in a power output less than that available in tubes of similar dimensions operating without velocity filters. To compensate for this loss, one or more electronmul-tiplier stages may be provided intermediate the control grid and the output electrode.

Figs. 3 and 4 show the invention as embodied in an electron-multiplier of the magnetic type. In Fig. 3, Iitl designates an evacuatedtube of glass or Pyrex having a depending neck portion 62. The main body of the tube contains a plurality of secondarily-emissive lower electrodes 64 lying in the same plane and spaced apart along the long axis of the tube and aplurality of upper or accelcrating electrodes 66 disposed in a plane parallel to and spaced from the place in which lie the first mentioned electrodes. The lower electrodes are preferably made of silver sensitized as by an application of caesium and the upper electrodes of molybdenum, tantalum, nickel or any other metal which is easily degassed and not readily oxidized.

All output electrode 68 is mounted in one end of the tube preferably fairly close to the sets of electrodes and in a plane transverse to the tube axis.

As more clearly shown in Fig. 4 the depending neck portion of the tube has a press of the usual type sealed therein. The press supports a virtual source of electrons constituted by a filamentary heater element i2 contained in a metal thimble 'Hi having alayer of electron-emissive material 16 on its upper end. The cathode thimble is surrounded by a tubular member 18, preferably formed of a thin sheet of a ferrous metal which, as will hereinafter more fully appear, serves to limit the influence of the magnetic field upon the trajectories of the primary electrons. A lead 80 which extends through a bushing 82 in the wall of the shield 18 supports an accelerating electrode 84 and similar leads 86 and 88 support a pair of plates 9d and 92 which constitute an electronvelccity filter. A control grid 94 covers the end of the tubular shield 18 and is adapted to be energized through a lead 96 which extends from the shield 18 through the press 10 to the exterior of the glass envelope. The magnetic shield 18 terminates in an electrostatic plate 98 which extends outwardly in the plane of the lower multiplying electrodes 64.

The magnet required to operate the tube of Fig. 3 is not shown in the drawings-the omission being in the interest of simplicity. It may, however, be similar in all respects to the magnet shown in Fig. 1 and with the tube similarly disposed between the magnet plates in such position that a substantially uniform magnetic field is set up parallel to the opposed surfaces of the upper and lower sets of electrodes.

In Fig. 3 the input circuit elements connected between the control grid 94 and the cathode 14 of the tube 60 are exemplified by an input resistor I00, a grid biasing potential source I02 and a potential divider I04.

It is necessary to maintain each of the lower multiplying electrodes positive with respect to any electrode between it and the cathode. For this purpose the cathode may be connected to the negative terminal of a source of unidirectional potential, exemplified in the drawings by a resistor I06, the output electrode 68 may be connected to the positive terminal of the source through an output device such as a resistor I08 and, starting from the cathode, the next adjacent electrode 64, or first multiplying electrode, may be connected to a point I it on the resistor somewhat more positive, and each of the remaining electrodes 64 connected to successively more positive points 1 l2 and l M on the resistor.

The first upper accelerating electrode 66 immediately above the disc 98 surrounding the tubular shield l8 may be connected to a point on the resistor more positive than the point l M to which the last multiplying electrode is connected and the successive accelerating electrodes connected, respectively, to points I I8, I20 and I22 on the resistor still more positive, as clearly shown in the drawings.

The plates 98 and 92 constituting the velocity filter are connected respectively to points I24 and H26, each positive with respect to the cathode, and the accelerating electrode 84 is connected to a point [28 preferably substantially midway between the filter-plate connections.

The primary electrons once past the control grid 9d will be accelerated toward the upper electrode 66 because of the electrostatic field between it and thesurface of disc 98, and, if no magnetic field were present, they would impinge thereagainst. However, the magnetic field which is parallel to the planes of the main electrodes 64 and 65 subjects the electrons to a force component at a right angle to their instantaneous direction of travel. The direction of the component depends upon the polarity of the field. If the electric and magnetic fields are adjusted to proper values the electrons will describe trochoidal paths and will strike the first multiplying electrode which, in the drawings, is the second from the left in the lower set.

With this first multiplying electrode maintained at a potential positive with respect to the emitting electrode the primary-electrons striking it will cause the emission of secondary electrons, the number of which is dependent, in part, upon the magnitude of the potential difierence. The secondary electrons, as shown in the drawings, will in turn be accelerated toward the second upper electrode but, by reason of the magnetic field they will be diverted and focused upon the third lower electrode. Here again a multiplication, by reason of secondary emission, is secured and this process is repeated in any number of stages until the amplified stream of secondary electrons is collected by the output electrode 68 and caused to fiow in a utilization circuit exemplified in the drawings by the resistor I08 included between the output electrode 68 and the positive terminal of the potential source.

As set forth in connection with the description of the embodiment of the invention shown in Figs. 1 and 2, a magnetic field is required for the operation of the velocity-filter. The magnetic field required to give the secondary-electrons of Fig. 3 the described trochoidal paths is normally so intense that if the primary-electrons were to be subject to its direct influence they would be drawn with an overwhelming force to the less positively charged plate 92 of the velocity filter. The problem therefore present in connection with electron multipliers including the velocity filter of the invention is to reduce the intensity of this field to a value such that the primary electrons, or most of them, will be given the previously described substantially parallel trajectories while traversing the area intermediate the filter plates. This is accomplished in accordance with the invention by enclosing the area about the velocity filter in the tubular member 18 whose walls have a thickness and permeability such as to partially, and not entirely, shield the primary electrons from the efiects of the magnetic field. Ordinarily the thickness of the ferrous metal constituting the shield 18 will be of the order of of an inch, though its exact dimensions may vary in different tubes depending to some extent, upon the intensity of the magnetic field required to operate the velocity filter and multiplying stages in these tubes.

As a number of possible embodiments may be made of the above invention, and as changes may be made in the embodiments herein described without departing from the spirit and scope of the invention, it is to be understood that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the 4 greases.

prior art and the spirit of the appended claims.

What is claimed is: 1. The combination with an electric discharge tube having elements the operation of which are 1 dependent upon the presence of magnetic fields of diiferent intensity, of a magnet adapted to generate a field of an intensity sufficient to actuate the element requiring the field of greater intensity and means intermediate said magnet and the other of said elements for limiting the intensity of said field to a value sufiicient to actuate said other of said elements.

2. An electron discharge device comprising an evacuated envelope having a longitudinal axis containing an electron emissive cathode, a control grid, an anode and a pair of spaced plates mounted on opposite sides of said axis and between which the electrons travel intermediate said cathode and grid, and a magnet exterior of said envelope for directing said electrons in their passage between said spaced plates, said plates and magnet constituting elements of an electron velocity filter.

3. An electric discharge device comprising an envelope containing a cathode adapted to release a stream of primary-electrons having various velocities, a control grid in the path of said primary-electrons, an electron-filter for limiting the range of velocities of the electrons subject to the controlling action of said grid, an electronmultiplier accessible to the controlled primaryelectrons, the operation of said electron-filter and electron-multiplier being dependent upon the presence of magnetic fields of difierent intensity, means exterior of said envelope for generating a magnetic field of an intensity sufficient to operate said filter and multiplier, and means for limiting the intensity of the magnetic field applied to said electron-filter.

VLADIMIR K. ZWORYKIN. LOUIS MALTER.

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