US2141322A - Cascaded secondary electron emitter amplifier - Google Patents

Description

Dec. 27, 1938. c, THOMPSON 2,141,322

CASCADED SECONDARY ELECT RON EMITTER AMPLIFIER Filed June 25, 1955 4 Sheets-Sheet 1 V////////ZW//////////////////////% INVENTOR HARRY C. THOMPSON Dec. 27, 1938. H. c. THOMPSON CASCADED SECONDARY ELECTRON EMITTER AMPLIFIER 4 Sheets-Sheet 2 Filed June 25, 1935 INVENTOR HARRY -C .THOM PSON Dec. 27, 1938. H. c. THOMPSON 2,141,322

CASCADED SECONDARY ELECTRON EMITTER AMPLIFIER 4 Filed June 25, 1935 4 Sheets-Sheet 3 INVENTOR HARRY CJ'IHOMPSON m ATTORNEY 0 00 00 00 0 0 0 0 0 0 0 0.0)0N 0W0H0N 0H0N0H4NM NA n0 10n0 0 0 0N0N0 0H0N0 Dec. 27,1938. H. c. THOMPSON 2,141,322 CASCADED SECONISARY ELECTRON EMITTER AMPLIFIER Filed June 25, 1935 4 Sheets-Sheet 4 70 l -|:::E T 21 INVENTOR HARRY C. THOMPSON ATTORNEY I Patented 27, 1938 CASCADED SECONDARY ELECTRON I EMITTER AMPLIFIER Harry Thompson, Montclair, N. J., assignor,' by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application June 25,1935, Serial No. 28,227 r Claims.

My invention relates to electron discharge devices, and more'particularly to electron discharge amplifiers utilizing secondary electron emission to multiply or increase an electron stream from a primary electron source, such as a photocathode or a thermionic cathode.

An electrode bombarded by high speed electrons will emit secondary or' impact electrons.

The-ratio-of the number of impact electrons to I the number of impinging electrons, which depends on the character of the surface and also on the impact velocity of the impinging electrons, may be greater than unity. For example, a ratio of three or more secondary or impact electrons to one impin'gingelectron is readily obtainable with metal surfaces treated in known ways and Sub? jected to discharges at potentials of 300. to 400 volts. Since under such conditions the number of impact or secondary electrons emitted exceeds the number of impinging electrons, the electron stream is multiplied into a greater stream of-secondary or impact: electrons.

secondary or impact electrons in turn impinges with sufficient velocity upon a second electrode, particularly one with a suitably treated surface, the ratio of emission of I impact electrons. to impinging electrons at the second electrode may also be greater than unity, hence a multiplication or increase of current may occur at each impact of an electron stream upon each electrode of a row or series of electrodes. The degree of multiplication for any surface depends in general upon the velocity of the impacting electrons, which is determined mainly bythe difference in potential between the source of electrons. and the secondary emitter upon which the electrons impinge. For any particular emitter'surface the rati'o'of emitted electron current to received electron current is equal to or only slightly greater than unity at a difference of potential which may for convenience be calledthe critical voltage. As the difference of potential is increased, the emitted electron current increases, at first rapidly,. and then more and more slowly, until a, difference of potential, which may be called the saturation. voltage, is reached, beyondwhich an increase in difference or potential causes practically no increase in the emitted current. In general, curves showing the ratio of secondary electrons to pr mary electrons plotted against velocity of theprimary electrons in 1 volts rise rapidly at first, reach a maximum be- If this stream of.

(on. 179-1 1 I impinges on an emitter of secondary electrons, the secondary electrons thus produced impinge upon another secondary emitter, and so on to the output electrode. These devices have proved to be rather unreliable and ineflicient, probably because of incomplete and defective control and utilization of the secondary electron streams. Secondary electrons appear to be emitted in all directions at thepoint of impact of an impinging electron, and while they have on the average initial velocities greater than those of primary electrons emitted 'by a primary electron source, some have very high and others have very-low initial velocities. The stream of secondary electrons is more diffuse and more difi'icult to direct to agiven target by an electrostatic field than a stream of primary electrons from a cathode, and in the secondary emission devices heretofore known, particularly those depending on electrostatic fields to direct the discharge, the field dis tribution favorable for directing the electron streams from one emitter to the, next is unfavorable for removing the impact electrons from the emitters with maximum efliciency. Another difiiculty encountered in the prior devices which attempt to cause secondary electron streams to flow in succession to several secondary emitters merely by maintaining the emitters at progressively higher potentials arises from a tendency of the secondary electrons from any impact emitter to go directly to the highly positive output electrode, instead of going only to the emitter which is next higher in potential. For these and other reasons, amplifiers or electron multipliers of the secondary electron emission type as heretofore made have been relatively ineflicient and erratic inoperation and have not been used commercially.

One object of .my invention is to provide an electron'discharge device utilizing secondary electron emission to increase the ratio between change charge devices and between change of output current and change of incident light intensity in photoelectric discharge devices in a more efiicient and reliable manner than has heretofore been feasible.

Another object of my invention is to provide an amplifier or electron multiplier of the secondary electron emission type in which the secondary electron stream from each emitter is directed .ac-

curatelyto the desired place, usually the place of optimum potential for the desired result, and is not dispersed to points of, greater or less than optimum potential with the resulting decrease 0 net gain of the device.

A further object is to provide an amplifier or electron multiplier of this type in which there is practically no loss of secondary electrons and in which the best conditions for amplification or other desired results can easily be obtained by external adjustments.

Another object of my invention is to provide a secondary electron amplifier or multiplier which is efficient and reliable in operation and in which the amplification obtainable in one stage is very great as compared with the amplification obtainable with a thermionic amplifier of the usual type.

My invention has numerous other objects and advantages, some of which, with the foregoing, will appear from the-description of some illus'- trative embodiments of my invention. The embodiments described are merely illustrative, since many changes and modifications in them can be made within the scope of my invention.

In accordance with my invention, primary electrons from a photocathode or thermionic cathode are directed to'an impact surface preferably sensitized to have high secondary emissivity or sensitivity, where the impinging electrons produce secondary or impact electrons in a ratio greater than unity; substantially all of the secondary electron stream thus produced is then directed to another similar surface of higher positive potential, where again the impinging electrons produce secondary electrons in a ratio greater than unity; and. so on, until substantially all of the greatly increased stream of secondary electrons produced at the last impact is collected by an output electrode. In accordance with my invention, I restrict and direct the secondary electron streams thus produced so that the major portion of each secondary electron stream will impinge upon the sensitized emitter surface at the desired place and under conditions favorable for the desired results. maintain at each emitting surface, preferably by means of one or more non-emitting field elec-' trodes, an electrostatic accelerating .potential gradient having a component substantially normal to the surface from which the secondary electrons are emitted and favorable for the removal or the escape of the secondary electrons, as I have found that such a field favorable for escape is necessary for high emciency. I also use, in conjunction with such a potential gradient, a magnetic field to direct each secondary electron stream along a fairly definite and predetermined path to the part of the emitting surface most favorable for securing the desired results. This magnetic field may be of such extent that its intensity and direction are substantially constant throughout the space occupied by the device. The direction of the magnetic field is parallel to the plane of the secondary emitting surfaces, and transverse to the directions of the electrical fields and to the average electron paths, hence the electrons are caused to follow arcuate paths between points which differ in potential. Substantially no interference or interaction between the high velocity impinging electron stream and the low velocity emitted electron stream has been observed, as the magnetic field causes the emitted impact electrons to move out of the stream of impinging electrons as soon as the emitted elecplates having a secondary emission ratio greater lows an arcuate path which ends on the succeeding plate. Instead of a row of plates I may use a continuous sheet or strip of high resistance extending from near the cathode to near the output electrode and sensitized on the surface exposed to electron impact. Along this high resistance strip there is a rising positive potential gradient from the end nearest the primary cathode to the other end which is at a positive potential somewhat lower than the positive potential of the output electrode. With a suitable electrostaticaccelerating field at the surface of this strip and under the influence of the uniform magnetic field parallel to the surface of the strip the electron stream from the primary cathode is concentrated upon an area. of the sensitized strip which is near the primary cathode and is at a potential sufficiently positive with reference to the cathode to give a secondary emission ratio greater than unity; the secondary electrons from that area are concentrated on a succeeding area more remote from the cathode and of higherpositive potential, at which again the ratio of secondary emission is greater than unity; and so on, until the stream of secondary electrons from near the high potential end of the strip are concentrated upon the output electrode. The length of the arcuate paths of the electron streams and consequently the distances between the areas of impacts of the streams upon I prefer to the strip can be controlled by adjustment of the electrostatic and magnetic fields and thereby the electron streams can at will be directed to those areas of the strip at which the conditions are most favorable for obtaining the desired results. The uniform magnetic field parallel to the emitting surfaces permits the use of a high potential gradient favorable for accelerating or drawing the secondary electrons away from the areas of secondary emission and thus high efficiency of secondary emission is obtained. With the continuous strip emitter there can be no loss of secondary electrons, since all of them must strike somewhere on the strip and the most favorable conditions for the desired results, as, for example, optimum amplification of the current, can easily be obtained by external adjustment of the values of the magnetic field and of the electrode potentials.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation will best be understood by reference to the following description taken in connection with the accompanying diagram- Figures 3, 4, and 5 are longitudinal sections,

greatly exaggerated in thickness, of forms of high resistance strip emitters useful in tubes of the type shown in Figures-1 and 2; Figure 6 is a longitudinal section showing a modified a,'141,sas

form of field electrodes; Figure '1 is a longitudinal section of another modification using a series or row of emitter plates, each connected to an external resistor; Figure 6 is a fragmentary section showing the emitterplates connected to an internal resistor; Figure 9 is .a plan view of an emitter plate structure useful in tubes of the type shown in Figure 6; Figure 10 is a longitudinal section along the section line l6i0 of Figure 9; Figure 11 is a fragmentary view showing an emitter of which a part is a resistance strip and the remainder a row of emitter plates; Figures 12 and 13 area cross-section and longitudinal section of a cylindrical form of tube of the type shown in Figure 7; Figure 14 is a diagrammaticcross-section of a tube which is in general similar to that shown in Figure 1, but of cylindrical construction and connected to operate as a combined detector oscillator and amplifier; Figure 15 is a diagrammatic plan view showing how a tube such as shown in Figure 14 may be modulated electromagnetically instead of by a control grid; and Figure 16 is a longi-- tial of the output electrode. This strip or sheet tudinal section of a form of tube having a stem at one end andresembling the conventional form of vacuum tube. I Y

The drawings illustrate diagrammatically various embodiments of my invention in an amplifying tube, the electrodes of which are enclosed in a sealed envelope 20, preferably of glass, and either highly evacuated, or containing residual gas at a pressure too low to permit a self-sustaining discharge in the tube. In operation there may be a difference in potential of several hundred volts between some of the electrodes in the tube, hence the tube is usually highly evacuated. The desired potentials forthe electrodes of the tube may be obtained from any convenient source, as, for example, a voltage divider comprising a battery 2i and resistor 22, indicated in Figure 1. The amplified current may be utilized in an output circuit 23, which is indicated diagrammatically. I

In the tube shown in Figure 1, a modulated light beam, indicated by a broken line, passes through the wall of the envelope 20 and falls upon a photocathode 24 within and near one end of the envelope and preferably concave or semi-cylindrical so as to produce a concentrated and preferably focused beam or sheet of primary electrons constituting the modulated input current. At the other end of the tube an output electrode or anode 25, which may be made roughly cup-shaped so as to reduce the effects of any secondary electron emission from its interior, collects the output current. a The output electrode is connected through the output circuit 23 to the positive end of the voltage divider or resistor 22.

Interposed between the cathode 24 and the output electrode 25 is a secondary electron emitting electrode or surface comprising a high resistance sheet or strip emitter 26 connected to the resistor 22 so that 'the end near the cathode is at ,a somewhat higher positive potential than the cathode 24' and the other end is at a positive potential somewhat lower than the potenis of metal, preferably with its surface sensitized to be capable of emitting secondary electrons copiously when bombarded, and has a potential gradient along it. A voltage difference of 600 or more volts between the ends of the strip is often desirable in order to secure the voltage drop or distribution necessary to obtain secondaryemission at a ratio greater than unity at several places along the strip, hence the ohmic resistance of the strip should be as high as feasible to keep the current which flows in the strip as small as possible;

The modulatedstream of primary electrons I produced by the modulated light beam falling on the photocathode 24 is directed along an arcuate path, as indicated by arrows, to an area on the strip emitter or electrode 26fwhich is sufllciently positive with respect to the cathode to emit secondary electrons in a ratio greater than unity. The stream of secondary electrons thusproduced is greater than the stream of primary electrons, and is directed along a similar arcuate path, also indicated by arrows; to another area further along the strip and at higher positive potential, where again the ratio of secondary emission is greater than unity, producing a still greater stream of secondary electrons, and so on, until the very much greater stream from the high potential end of thestrip is delivered to the anode or output electrode 25. In effect there is a series of secondary electron emitting cathodes in cascade, and the gain in anode current may be approximately the nth power of the ratio of secondary electrons to impinging electrons where the ratio is the same at each impact and 11. is the numberlof impacts.

For efficiency there should be at the surface of the strip 26 a potential gradient favorablefor the escape of the secondary electrons. To obtain such. a favorable potential gradient, I provide means, such as a field or accelerating electrode 21 positioned above and extending along the emitting strip 26 and maintained positive with reference to it for producing at the emitting surface an electrostatic field the same in magnitude and direction atevery part of the emitting surface, and with a component normal to the emitting surface for drawing the emitted electrons away from the surface. Since the field electrode 21 is unipotential, and there is a potentialdifiz'erence between the ends of the strip 26, the field or accelerating electrode, which is preferably about the same length and width as the strip 26, is set with one end close to the high potential endof the strip 26 and the other end at such a distance from the low potential end of the strip that the desired kind of electrostatic field is obtained. The preferred setting is such as to make the equipotential surfaces of the field between the accelerating electrode 21 and the strip 26 substantially parallel to the accelerating electrode 21.

The spacing of one end of the field electrode 21 from the low potential end of the strip 26 may be as much greater than the spacing of the other end from the high potential end of the strip as the difference of potential between the field electrode and the low potential end of the strip is greater than that between the field electrode 21 and the high potential end of the strip 26. For example, where the difference of potential between the strip 26 and the field electrode 21 at the low potential end of the strip is about ten times the difference of potential between it a and the other endof the strip, as may be the is about ten times that at the high potential end.

To guide the electron streams tothe desired areas on the strip. emitter 26 I provide a magnetic field of which the lines of force are parallel producing such a magnetic field is an electromagnet 28 having polepieces 29 shown in Figure 2 substantially coextensive with and mounted at opposite sides of the emitter electrode 26. The strength of the magnetic field, which may be varied at will by a rheostat 30, need not be very great, and is well within the range of field strength obtainable with a permanent magnet, which may be used if desired.

' The emitter or resistance strip 26 may to advantage be a very thin film of metal, such as nickel, molybdenum, or tungsten on an insulating foundation such as quartz, mica, pyrex glass, or a ceramic, such as high temperature porcelain. A thin film of metal on an insulating foundation 3|, as indicated in Figures 3, 4, and 5, makes a mechanically rigid emitter with a resistance high enough to give the desired voltage drop along the strip Without passing excessive current,'and the surface of such a film can be sensitized so as to have high secondary electron emissivity. Where'the voltage is applied at the ends of the strip, as shown in Figure 1, all of the output cur-' rent fiows into the strip at one end, and in order to maintain throughout the strip a substantially uniform voltage drop during operation of the tube when more secondary electron current is drawn from the part of the emitter strip near the anode than from the part near the cathode the strip should be of lower resistance per unit length at the end adjacent the cathode and may be made thicker at that end, as shown in Figure 3. Where, as shown in Figure 4, the strip is connected at several points along its length to corresponding points on the resistor 22, the greater part of the secondary electron output current flows from the part of the strip near the anode, and that part may to'advantage be increased in thickness by steps 32, as indicated. In some cases the insulating foundation 3| may to advantage be backed by a metal plate 33 grounded to high frequency through a condenser, as shown in Figure 5. In this case the insulating foundation may be a layer of glaze or enamel on the metal plate 33, or. may be a thin layer of highly refractory insulation.

The surface of the high resistance metallic film emitter 26 may be sensitized with caesium or similar metal, in much the same way as photoelectric cathodes for photo-tubes are made, by depositing a thin film of caesium on oxidized silver, as emitters having oxidized silver surfaces thus sensitized with caesium have been used successfully in electron multipliers of the secondary electron emission type. For example, a thin film of silver, deposited on the surface of the metal plate or of the high resistance film by electroplating by cathode discharge, or in other ways, may be wholly or partially oxidized and then exposed in vacuum to caesium vapor to obtain a film of caesium on oxidized silver. I

A molybdenum emitter, either a plate or a high resistance film of molybdenum on quartz, may be oneness sensitized with lithium borate by grinding anhydrous lithium borate to a powder fine enough to remain in suspension for an hour or more in a nonaqueous vehicle such as amylacetate, diethylcarbonate, or acetone containing a few drops of a nitrocellulose or similar binder. If the emitter is dipped into this suspension, it will be wetted on the surface and when the vehicle is evaporated. by slight heating if necessary, the finely powdered lithium borate will be uniformly distributed over the surface of the emitter. Good secondary electron emissivity is obtained with a deposit of lithium borate so thin that when the emitter is observed directly, there appears to be no deposit at all on it, but when observed in light incident on it at a small angle the surface has a slightly cloudy appearance. Various compounds of alkali and of the alkaline earth metals, for example, borates or silicates of barium and of beryllium may be used if they can be heated to the proper temperature for maximum sensitivity without destroying the underlying metallic film. The molybdenum surface may also be sensitized with barium peroxide in substantially the same way as with lithium borate.

Figure 6 shows a tube in general similar to that shown in Figures 1 and 2, but smaller in diameter because a grounded grid 35 mounted parallel to the emitter strip 26 and interposed between the field electrode and the strip permits a marked decrease in the angle of the field electrode to the emitter strip. This grid 35 is preferably of uniform pitchand may be made, for example, of wire mesh. The field electrode 36 is like the field electrode 21, but extends beyond the cathode 24 and has a window in registry with the cathode, the end portion 3'! beyond the window being curved, as shown, to decrease the effects of charges on the glass wall. The grid 35 is further from the field electrode at the low potential end of the strip than at the other end, and has in conjunction with the field electrode the effect of I a variable mu grid in the accelerating field, thus modifying the distribution of the equipotential surfaces of'the field to produce at the surface of the emitter strip the same kind of electrostatic potential gradient as is obtained in Figure 1 but with the field electrode much more nearly par: allel to the emitter strip than in Figure 1.

Figure 7 shows a tube essentially like that shown in Figures 1 and 2, with a series or row of plate emitters 40 set edge to edge in the same plane or surface and connected in parallel to points-on the resistor 22 chosen to impress on the emitters 40 positive potentials which increase progressively and by steps from the emitter nearest the cathode to the emitter nearest the output electrode. Preferably the width of the plate emitters is also progressively greater. The differences of potential between the cathode and the first emitter, and between each emitter and the succeeding one, should be great enough to cause each emitter to have a secondary emission ratio greater than unity. With this form of tube the electrostatic accelerating field and the magnetic guiding field are usually fixed at values which cause substantially all of each electron stream to impinge on the succeeding emitter under the most favorable conditions, although if desired, these values may be varied to modify the action of the tube and to direct only'a part of each electron stream to the succeeding emitter, the remainder of the stream being dispersed to other surfaces or through the openings between the emitter plates to the non-emitting lower surfaces of the plate emitters. A screen grid 4! may be'mountedin front of the output electrode.

' and may be attached to the field electrode, as

essary for the emitter assembly, instead of alead for each plate emitter, as in Figure 7. Figure 9 shows the emitter assembly inplan view, with the plate emitters mounted edge to edge on min- Y resistance metal coiled on aninsulating support sulating foundation 5, such asglass, quartz, or similar material, like the foundation 3| in Figure 3, and with the internal resistor 45 extending along one side of the foundation. The resistor 45 may, for example, be a fine wire of high rod carried by the foundation 45. If the length of the resistance wire is great,'it may be first coiled, and the coils again coiled, much like the double coiled filaments used in low wattage incandescent lamps. The resistance between emitters may be varied by varying the pitch of the coils to compensate for the difierences in secondary electron current from the different emitters, so that the desired'voltage differences between'the emitter plates will be maintained.

Figure 10 indicates another convenient way of mounting the plate'emitters 40 edge to'edge on a layer of refractory insulation, such as a ceramic enamel on a conducting plate 41,-which may be grounded directly or through a capacity, in which case all the emitters are grounded for high frequencies 'due to capacity between the emitters and the conducting plate- 41, so that modulation of their potentials by the signals is reduced.

Figure 11 shows diagrammatically a portion of the tube shown in Figure 7, modified by replacing the first three emitter plates by a strip emitter, connected as in Figure 1. The first impacts occur on the emitter strip 25 and the remainder on the plate emitters 40.

Figures 12 and 13 show a tube of the same general type as that shown in Figure '7,'but of cylindrical construction. Theelectrodes of the tube are enclosed in a tubular glass envelope lengthwise of which a substantially uniform magnetic field is producedby a winding around the tube. For example, a magnet 5|, consisting of two annular coils, one near each end of the tube and connected in series, may be used.. The electrodes of the tube are mounted on a tubular insulating foundation 52 concentric with the envelope and having in one side an elongated longitudinal slot or window 53 through which the modulated light beam can' reach the interior of the insulating foundation. A semi-cylindrical photocathode 54 is mounted inside the tubular member to be parallel to and adjacent one edge of the window, and a collector electrode or anode 55 is mounted inside the tubular member 52 near the other edge of thewindow. The dis-' field having a potential gradient which is normal to the emitter surfaces and tends to draw the electrons directly away from the emitters and radially inward of the tube is produced-by a. field electrode 51 mounted'inside the tubular member 52, and of about the same length as the plate emitters. This field electrode may conveniently be made as a sheet metal cylinder considerably smaller than the tubular member. The accelerating field of the field electrode should have substantially the same relation to the emitters 56 as the accelerating field in the tube shown in Figure 7 has to the emitters 40. For structural reasons-I prefer to mount'the field electrode-coaxial with the member 52,-and to obtain the dey sired field configuration by a tubular grid 58 concentric with and surrounding. the field electrode '51. The grid 58 is of progressively varying pitch,

preferably of sheet metal, such as polished nickel, i

and mounted as shown in Figure 12 adjacent the window 53 so as to refiect and concentrate the modulated light beam on the photocathode. This metal reflector is. preferably grounded bya connection to the cathode. I

The tubular member 52 which carries the electrodes may be supported'in the envelope 50 in various ways, as, for example, by mica discs or end spacers, shown in Figure 13, and supported from a stem or the wall of the bulb by support rods or similar means such as are used in conventional radio tubes.

In operation, the modulated light beam falling on'the reflector 59 is reflected to the photocathode 54 from which the beam of primary electrons fiows in an arcuatepath, as indicated by interter, producing secondary electrons in a ratio nections of the corresponding electrodes in Figure 7.,

Figure 14 shows another form of cylindrical construction of a tube of the general type shown in Figure 7. In this particular'modification, the.

electrodes are enclosed in a tubular bulb 50 in which a uniform magnetic field extending lengthwise of the tube is produced by means of a field winding 6|. The primary electron source in this tube is a thermionic beam-forming electrode consisting of a straight cylindrical cathode. 62,

of the usual equipotential or indirectly heated type coated with oxides of barium and strontium.

The discharge from the thermionic cathode is converged into two diametrically opposite beams or sheets by means of a beam-forming electrode comprising a helical grid 53 surrounding and coaxial with the cathode and preferably of the conventional type with twoside rods, and a pair of semi-cylindrical sheet metal focusing plates 64 mounted parallel to the cathode and of such size and position as to have between their edges two diametrically opposite slots parallel to the oathode. The focusing plates 64, which are electrically connected to the grid 63, may conveniently be attached to the side rods of the grid.

The two focused electron beams are directed 40 rupted lines in Figure 12, to the first plate emit- 1 along two rows of plate emitters 85 and 86 ex tending radially and in opposite directions from the cathode, with each emitter 65 electrically connected to the corresponding emitter 66. As indicated diagrammatically in Figure 14, there is at the outer end of each row of plate emitters an anode 61, the two anodes being electrically connected. The accelerating field in this particular type of tube is. produced by a tubular field electrode 68 coaxial with the cathode and of a diameter slightly greater than the length of the two rows of plate emitters and their anodes. The field electrode may be circular in cross-section, as shown, or may be modified in cross-section, if a modified field is desired. As indicated by the arrows. one of the beams from the cathode is directed along one row of plate emitters 65 and the other beam is directed along the other row of plate emitters 66. The electrical connections of the various electrodes to the voltage divider 22 are similar to the connections in Figure 7.

The plate emitters 65 and 66 are shielded from deposits of material from the cathode, since the plate emitters are out of registry with the slots in the beam-forming electrode, and it has been found that material vaporized or sputtered from the thermionic cathode travels in substantially straight lines from the cathode to the partsof the field electrode 68 in registry with the slots.

Due to this construction, the secondary electron emissivity of the plate emitters is more constant than in a tube where deposits from the cathode can reach the emitters.

The tube 01' Figure 14 may be used as a combined detector oscillator and amplifier tube by connecting a tuned circuit 69 to a pair of the emitter plates at which the current is great enough to produce the desired results. With such a connection the current oscillates by virtue of the negative resistance through these emitters. whereby a beat or intermediate frequency may be obtained in the output circuit.

As diagrammatically indicated in Figure 14, the Y a point of connection of the plate emitters to the source of voltage may be varied so that the potentialsof the emitters with reference to one another and to the cathode may be varied at will. For example, the potential of one of the emitters may be made such that the current to it and also the output current passes through a maximum, on the far side of which the emitter has negative resistance. Use may be made of this property to set up stable oscillations.

The discharge in this form of tube may be modulated by connecting an input circuit 10 to the grid 63 or, if desired. the grid and focusing plates 64 may be main ained at a constant bias and the discharge modulated electromagnetical- 1y, as diagrammatically shown in Figure 15, by connecting the input circuit to an electromagnetic modulating coil H encircling the tube, and pref erably interposed between sections of the field winding GI.

,Figure 16 shows in longitudinal section a modified form of tube which is in general like the tube shown in Figure 7, but is single ended, like the conventional receiving tube, and of considerably less length than an equivalent tube constructed as shown in Figure 7. In this modification the electrodes are enclosed in an envelope or bulb having a stem 16 at the lower end. The electrodes are carried upon and supported by an oblong slab of insulation 11 mounted to extend lengthwise of the bulb and carrying on each side a row of plate emitters iii. The anode or collector I9 is mounted on the slab near the upper trated and more or less focused beam which, by

the conjoint action of an accelerating field produced by a field electrode M, in the form 0! a sheet metal strip bent to-surround the slab I1 and the electrodes carried by it, and having ad- J'acent the end. of the bulb a window through which the modulated light beam can reach the cathode, and of a transverse magnetic field extending through the tube parallel to the plate emitters and conveniently produced by a magnet such as shown in Figures 1 and 7, is directed along an arcuate path to the first emitter on the side of the slab ll opposite the anode 19. The discharge, as indicated by arrows, follows arouate paths along, the emitters on' this side of the slab to the emitter at the lower end of the slab 11, then around the lower end of the slab, and then back along the plate emitters on the other side of the slab to the anode.

The field electrode BI is preferably symmetrically disposed about the emitter plates and the accelerating field is modified in much the same way as in Figure 12 by means of a grid 82 which increases progressively in pitch from the oathode to the anode. This grid may conveniently be of the ladder type, consisting of parallel side rods with the grid rods extending transversely of the siderods, the spacing between the grid rods increasing progressively, as indicated in Figure 16.

Details of construction of this tube are not shown, as the slab 11 may be supported from its edges by support rods extending from the stem IS, in accordance with common practice in the tube art, and the field electrode 8i may also be carried from the stem on support rods. The grid may be supported from the slab II by means of wires or insulating tie rods, such as are commonly used in the art.

The electrical connections of the tube are substantially the same as shown'in Figure 7. In

operation, the electron beam impinges on the first emitters, the discharge then passes along the emitters on one side of the slab, then around the lower end of the slab to the emitters on the other side, and thence back to the anode. -The potentials of the emitter plates are so chosen that multiplication of the discharge occurs at each plate.

This particular modificationoi the tube has the advantages of compactness and of permitting the modulated light beam to be directed into the tube through the end of the bulb.

While I have indicated thepreferred embodiment of my invention of which I am now aware and have also indicated only one specific appllcation for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. An electron discharge amplifier comprising a primary electron emitter, an output electrode for receiving an electron discharge initiated by said emitter, a surface adjacent said emitter and said output electrode and having a plurality of 7 areas each constructed"-'to=-liberate electrons on impact and maintained at different voltages which are positive with respect to said primary emitter, a unipotential accelerating electrode extending along said surface from said cathode to said anode and spaced from said surface with the end near the anode closer to said surface, and means for establishing a magnetic field parallel to said surface for directing. the electron discharge from one of said. emitter areas to another emitter area which differs from it by a voltage sumcient to cause the impact electron stream from said other emitter area to be greater on the average than the electron discharge to it.

2. An electron discharge amplifier comprising an electron emitting cathode, an output elec-. trode, means for providing between said cathode ters, means for establishing a .magnetic field parallel to the surface of said emitters and transverse to the path of electron fiow along said surface to direct the major part of the electron stream from each of said emitters to another emitter of higher positive potential, and a tuned circuit connected to one of said emitters.

3. An electron discharge amplifier comprising an electron emitting cathode, a surface having a plurality of regions, each constructed to emit impact electrons in a ratio greater than unity, a unipotential accelerating electrode extending longitudinally of and set at an acute angle to said surface for producing an electrostatic field having a component normal to said surface and a magnetic field parallel to said surface and transverse a straight line through said regions for directing the electron flow from said cathode to a region of said surface and directing substantially all the resultant fiow of impact electrons from each of said regions to another region until the desired amplification is obtained,

and an output electrode for collecting the ampli- 4. An electron discharge device comprising means including a cathode and an anode for proclined to said emitter to be closer to said emitter at said anode than at said cathode for producing an electrostatic field having a component normal to said magnetic field for causing the discharge betweeen said cathode and anode to flow in cascade along arcuate paths to successive points on said emitter.

5. An electron discharge device comprising means including a cathode and an anode for producing a modulated'electron stream, a secondary electron emitter adjacent the'path of said discharge between said cathode and said anode and comprising an'insulating sheet, a film of metal on one surface of said sheet, a sensitizing coating on said film for emitting impact electrons, means for connecting the ends of said strip to a source of voltage, means for producing along said emitter a. magnetic field and a unipotential accelerating electrode coextensive longitudinally of said emitter and spaced from and inclined at an acute angle to said emitter to produce an electrostatic field normal to said magnetic field for causing the discharge between said cathode and anode to flow in cascade along arcuate paths to successive points on said emit-' ter. a

6. An electron discharge device comprising means including a cathode and an anode for producing a modulated electron stream, a sec- 'ondary electron emitter adjacent the path of said discharge between said cathode and said anode and comprising a surface having a plurality of regions each constructed to emit impact electrons in a ratio greater than unity, ac-

celerating means comprising a field electrode extending along said surface, a grid connected to and extending from said cathode between said field electrode and said surface and having a pitch which increases progressively from said cathode for producing an electrostatic field which increases in intensity from said cathode to said anode, and means for producing a magnetic field. parallel to said surface and normal to the electrostatic field between said field electrode and said emitter for directing the electron fiow from said cathode to a region of said surface and directing substantially all the resultant fiow of impact.

electrons to other of said regions in succession until the desired amplification is obtained, and an output electrode for collecting the amplified fiow.

7. An electron discharge device comprising means including a cathode and an anode for producing a modulated electron stream, a secondary electron emitter extending alongside a line from said cathode to said anode and comprising a sheet of insulation, a row of emitters mounted on said sheet, a resistor extending lengthwise of said sheet, connections between said emitters and successive points on said resistor, circuit terminals at the ends of said resistor, means for producingat said emitter a magnetic field and a unipotential accelerating electrode extending longitudinally of and inclined to said emitters to produce an electrostatic field normal to said magnetic field for causing the discharge between said cathode and anode to flow in cascade along arcuate paths to successive points on said emitter.

8. An-electron discharge device comprising a thermionic cathode, a pair of beam forming electrodes on opposite sides of said cathode for form ing the discharge into two electron beams, two impact electron emitters adapted to emit electrons on impact of an electron stream and positioned to extend from said cathode in opposite directions in alignment with said beam forming electrodes, means for directing said beams in opposite directions and in cascade to a plurality of points along said emitters to amplify said discharges, and means for collecting said amplified discharges.

9. An electron discharge device comprising a tubular member having an opening in the side,

. an electron emitting cathode mounted inside said V mounted on the inner wall emit impact electrons in 8 c memberadjacent one side of said opening, a plu- 4 rality of emitters constructed to emit impact electrons in a ratio greater than unity and of said member in a row with one end adjacent said cathode, a cylindrical field electrode inside and coaxial with said member, a cylindrical grid coaxial with said field electrode and of progressively increasing pitch from said cathode and connected to said cathode, means forproducing a magnetic field lengthwise of said member to direct the discharge said rows or emitters extending around the of the row of emitters to unity positioned in a row on each side of said slab, an electron emitting cathode at the upper end of said slab near the upper end of the row 01 emitters on one side of said slab, ananode mounted adjacent the upper end of said slab near the upper end of the row of emitters on the other side of said slab, a field electrode spaced from and encircling said slab lengthwise from said cathode'to said anode and grid of progressively increasingpitch extending around said slab between said field electrode and said emitters, and means for producing a magnetic field parallel and transverse to said slab for directing the discharge from said cathode in cascade along the emitters on one side of said slab, around the lower end ci' said slab, and back along the row oi emitters-on the other side of said slab to said anode to amplify said discharge.

. HARRY C. THOMPSON.

lower end oi. said slab, a 1o

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