US2459778A - Image dissector - Google Patents

Description

C. C. LARSON IMAGE DISSEGTOR Jan. 18, 1949.

2 sheets-smet 1 Filed July 9, 1945 INVENTOR CHRISTIAN C. LARSON ATTORNEY Jan. 18, 1949. c. c. LARsoN 2,459,778

IMAGE DISSECTOR.

Filed July 9, 1945 2 SheetsASheet 2 ATTORNEY Patented Jan. 18, 1949 IMAGE mssEc'roR Christian C. Larson, Fort Wayne, Ind., assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application July 9, 1945, Seria-l No. 603,981

lThis invention relates to television picture signal generating tubes, and more particularly relates to image dissector tubes of the type where the picture signal is amplified by dynamic electron multiplication.

An image dissector tube as used conventionally comprises a photosensitive cathode on which an optical image is focused for converting it into an electron cloud representative of the optical image. A shield rprovided with a minute scanning aperture is spaced from the photosensitive cathode. The electron cloud is cyclically deflected across the scanning aperture so that at any instant electrons from a selected elemental area of the photocathode are able to enter the aperture. It is conventional practice to multiply the electrons passing through the scanning aperture by means f an electron multiplier.

It has been suggested to multiply the electrons which have passed through the scanning aperture of an image dissector tube by dynamic electron multiplication. To this end the electrons are oscillated repeatedly between two spaced secondary electron emissive surfaces so that secondary electrons are liberated at a ratio greater than unity each time a primary electron impacts one o f the electrodes. A dynamic electron multiplier of this type is commonly referred to as a multipactor and may be arranged to be self-oscillating. When the electrons which are multiplied in the multipactor are allowed to oscillate continuously between the secondary electron emissive surfaces, a saturation current will iinally be obtained so that the electron stream in the multipactor is no longer representative of the light characteristics of a selected area of the photocathode. Therefore, it is necessary to interrupt the oscillation of the electrons in the multipactor periodically by some means such as a high fre# quency oscillation generator. When a multipactor is periodically interrupted in this manner, it tends'to be unstable unless it is driven by waves at a suitable frequency impressed from an eX- traneous source on the secondary electron emissive surfaces. Furthermore, a special oscillation generator or some other means must be provided for periodically interrupting the dynamic multiplication of the selected electrons.

It is an object of the present invention, theree fore, to provide an image dissector tube where the picture signals developed .by the dissector are multiplied by dynamic electron multiplication, without the necessity of periodically interrupting the electron multiplication.

A further object of the invention is to provide 11 cmins. (o1. 17e-7.20

i ranged for converting an optical image into an electron cloud representative of the optical image. A secondary electron emissive surface is provided which is spaced from the cathode and has an area that is small in comparison with the area oi the cathode. Electron deflecting means are provided for cyclically sweeping the electron cloud across the secondary electron emissive surface.'

There is also provided means for continuously oscillating electrons emitted from successive elemental areas of the cathode between the secondary electron emissive surface and the cathode,

so that electrons from each selected area impact the secondary electron emissive surface a predetermined number of times. A target is provided for collecting the ampliiied electron stream which has been swept across the secondary electron emissive surface. Thus, an ampliiied train of picture signals is derived which is representative at any instant of the light characteristics of a selected elemental area of the optical image.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the accompanying drawings:

Fig. 1 is a schematic representation of an image dissector tube and associated electiic circuits embodying the present invention;

Fig. 2 is a front elevational, fragmentary view taken on line 2-2 of Fig. 1 and illustrating a portion of an electrode of the image dissector tube having an aperture therein and an adjacent secondary electron emissive surface;

Fig. 3 is a schematic side view of the photosensitive cathode and a spaced electrode of the image dissector tube of Fig. l illustrating repeated traversals of selected electrons between the two electrodes;

Fig. 4 is a front elevational view similar to Fig. 2 of a modification of the apertured electrode with an associated secondary electron emissive vwire arranged adjacent the aperture;

optical window 3. Electrode is arranged opposite photosensitive cathode :l in envelope 2. Anode 5, which preferably is ring-shaped as shown, is arranged midway between photosensitive cathode l and electrode 5 so as not to interfere with the normal path of an electron cloud moving" from cathode l toward electrode 5.

Photosensitive cathode 4 is arranged for converting an optical image projected thereon into an electron cloud. For the purpose of projecting an optical image of an object schematically represented at 6 through window 3 upon photosensitive cathode il, there is provided lens system l. Photosensitive cathode 4 preferably vis also made secondary electron emissive in a manner well known in the art. It may be pointed out that most photosensitive materials are also good secondary electron emitters.

Electrode 5 is provided with aperture 8. Surface I I), which may be formed on electrode 5, is' made secondary electron emissive and lis arranged adjacent aperture 8, as illustrated particularly in Fig. 2. The area of surface IIJ is small in comparison with the area of photosensitive cathode 4. In order to deect the electron cloud developed 'by photosensitive cathode 4 in two directions normal to each other, there are provided line scanning wave generator II and field scanning wave Vgenerator I2 arranged for developing sawtooth deleoting currents at the lin'e'scanning frequency and eld scanning frequency, respectively. Scanning wave generators II and I2 are connected, respectively, with scanning coils I3 and I4 all of conventional construction.

Focusing coil I5 energized by a suitable current source such, for example, as battery I 5 is arranged for providing a magnetic eld to focus electrons forming an electron cloud secondary electron emissive surface I5 as well as on photosensitive cathode 4. An antiresonant circuit 2t', including variable condenser 2l and inductor'22, is connected in parallel between photosensitive cathode i and electrode 5. The mid point of inductor 22 is connected by a center tap through blocking -condenser 23 to anodeV 6.4 Ground is connected to the center tap of` inductor 22 as illustrated. A voltage source such, for example, as battery 2li is connected across potentiometer 25 and has its negative terminal grounded as shown. Positive potential is supplied to anode 5 through a suitable tap on potentiometer 25 connected between anode 6 and condenser 23. This potential may be of the order of magnitude of 500 volts. Oscillation generator 25 is arranged for developiiig a sinusoidal voltage wave outputl which is connected across inductor 21 inductively coupled to inductor 22. i

Image dissector tube yI operates as follows. Light from object 6', which is focused on photosensitive cathode 4 by optical system liberates photoelectrons therefrom which form an electron cloud. The photoelectrons are attracted by the positive potential applied to anode t through y battery 24. The magnetic focusing .eld develtoward electrode 5.

4 oped by focusing coil I5 guides the photoelectrons At the same time the magnetic delecting eld developed by the sawtooth currents iiowing through delecting coils I3 and ifi sweeps the photoelectrons across secondary electron emissive surface Il) and aperture 8 in two directions normal to each other. Secondary electron emissivesurfacelflll Ashould be arranged so that thephotoelectrns are first swept across surface i0 `and then into aperture 8, that is, the

f line scanning eld should direct the electrons from surface I5 into aperture 8.

The photoelectrons liberated from cathode 4 are accelerated by the positive potential applied to anode 5 until they reach the median plane of -anode 5 Whereafter they are decelerated again so that they reach electrode 5 with substantially zero velocity. However, it should be pointed out that the photoelectrons will impact electrode 5 with a small velocity, corresponding to their initial velocity which depends upon the wave length of the light focused on photosensitive cathode 4 and upon the work function of the photosensitive material of cathode 4. The initial velocity ofthe electrons, however, is normallyinsufcient to impart enough energy to the electrons to liberate .secondaryl electrons from surface I0.

Oscillatlon generator 25 impresses a high frequency sinusoidal voltage wave upon inductor` 22. The half period of the frequency ofthe voltage wave developed by oscillator generator :zii should approximately equal the transit timey of an electron, between cathode 4 and electrode 5. .at a certain interval within each cycle of the wave `developed by oscillation generator 25 a photoelectron liberated from cathode 4 will be accelerated toward electrode 5. The accelerated electron should impact surface IEJ with sucient energy to release secondary electrons at a ratio greater than unity.

Due to the deflecting eld developed by the sawtooth currents iiowing through deflecting coils E5 andfI/I, electrons from successive `elemental areas of cathode 4 will be directed toward secondary electron emmissive surface I0. The electrons from all but, the selected areas are` collected by electrode 5. Accordingly, electrode 5 with the exception of surface l0 should .bel

face` I5 by primary electrons from a selected elemental area of cathode Il are again attracted by the positive potential of anode 6. Before or at the time the electrons have reached the median plane of anode 5 the alternating potentials on cathode ai and electrode 5 have reversed so that the electrons are now attracted by cathode which they will impact with suicient energy to liberate secondary electrons at a ratio greater than unity. In this manner the electrons from a selected elemental area of cathode i are impacted repeatedly between surface l5 and cathode This continues until the deiecting iieid developed by the sawtooth currents iiowing through deflecting coils I3 and I4 has swept the multiplied electron streamacross surface it and intoaperture 8. The electrons from the selected areas are thus multiplied by dynamic electron multiplication.

kThe secondary electrons liberated fromI sur- The number of impacts which the electrons from a selected elemental area of cathode 4 will carry out depends upon a number of factors, such as the size of surface Ill, the speed with which the electrons are swept across surface lll and the electron transit time between cathode 4 and electrode 5. Preferably, the electrons from a selected cathode area make five traversals between 'cathode 4 and surface l0 before they are deflected into aperture 8.

The transit time of the electrons preferably should be so short that these five traversals take place during the time one picture element is scanned. It may be assumed, for example, that 30 complete pictures or frames are scanned in one second and that each picture consists of 700 lines. Each line will accordingly consist of 700 picture j elements, and, therefore, 7002 30 or about 15,000,000 picture elements are scanned per second corresponding to a frequency of 15 megacycles Iper second. Since the electrons are required to make ve traversals during the time interval one picture element is scanned, the frequency developed by oscillation generator 26 should beabout 75 megacycles per pecond with the above assumptions.

Referring now to Fig. 3, there is illustrated the path 30 along which an electron from a selected elemental area 3| travels between cathode 4 and secondary electron emissive surface l before it is deflected into aperture 6. It will be understood that the electron path has been shown schematically only. The alternatingr accelerating potentials applied to cathode il and electrode 5 as well as the distance between the two electrodes should be such that the electrons travel from cathode 4 to surfacel and back againin the required time. It will be appreciated that the velocity of the electrons is also dependent to some extent on the magnetic focusing eld developed by coil |5. As shown in Fig. 3, the electron impacts surface I6 as well as cathode' 4-twice and, accordingly, the multiplied electron` stream deflected through aperture 3 is equivalent to that obtained by a fourstage static electron multiplier.

It should be pointed out that at the time the amplied electron current, representative of cathode area 3|, travels .through aperture 8, some-.electrons from adjacent cathode areas will also be deflected into aperture 8. Some of these electrons have been liberated from cathode area 32 and have made three traversals between cathode 4 and surface i0 before they reach aperture 8. At the same time some electrons will reach aperture 8 which have been liberated from cathode area 33 and which have traversed the space between cathode 4 and electrode 5 once only.

It may be assumed that each primary electron liberates five secondary electrons from either surface l0 or cathode 4. The electron current represented by those photoelectrons which have been liberated from area 33 may be assumed to be unity. The electrons which originated from area 32 have impacted-surface i0 and cathode 4- twice, and, therefore, this multiplied electron current passing through aperture 8 is equivalent to4 52 or 25. On the other hand, the electrons which have originated from area 3| have impacted surface l0 and cathode 4 four times, and thus the multiplied electron current from area 3| is 54 or 625. Hence, the current of themultiplied electrons from area 32 is 4% of thev current represented by the multiplied electron stream originating from area 3|. 'The electron current of the photoelectrons liberated from area 33 is '.l6% of that of the electrons liberated from area 3|. These three electron currents will be superimposed to constitute the output signal. However, it will be evident that blurring of the reproduced image due to the picture signals containing components from adjacent cathode areas will be negligible.

The multiplied electron stream passing through aperture 8 is represented at .any instant of the light characteristics of a selected cathode area. This electron stream may be further multiplied lby static electron multiplier 35 comprising a plurality of multiplier stages 36 and electron collector 3l.l The iirst multiplier stage 36 is kept at a potential that is positive with respect to electrode 5 by means of a suitable tap connected to poten,- tiometer 25. The succeeding multiplier stages 36 are maintained at increasingly positive poten-l tials by means of taps connected to potentiometer 25. Electron collector 31 is kept at a potential that is positive with respect to the last multiplier stage 36 and is connected through load resistor 40 to the positive pole of battery 24. Bypass condenser 4| connected between ground and the positive pole of battery '24 is arranged to provide a low impedance path for alternating current. The amplied output signal is developed across resistor 40 and may be coupled through condenser 42 to a television preamplifier. 1

Referring now to Figs. 4 and 5, there is illustrated a modied electrode 45 corresponding to electrode 5 of Figs. 1 and 2` Electrode 45 is provided with aperture 46. "Wire 41 extends across the surface of electrode 45'which faces photosensitive cathode. 4. Wire 41 has a portion 48 arranged adjacent aperture 46 which is made secondary electron emissive. In some cases electrode 45 and wire 41; may be used'with advantage in image dissector tube l illustrated in. Fig. 1 in steadof electrode '5 and secondary electron emisu sive surface I0.

Referring-now to Fig. 6, in which like components are designated by the same reference nu'- merals as were used in Fig. 1, there is illustrated image dissector. tube 50 which is essentially iden. tical to image dissector tubey illustrated in Fig. 1'.v However, instead of electrode 5, there is provided electrode 5| having an area that is small in come parison -with that of photosensitive cathode 4; Electrode 5| is providedwith aperture 52 and secondary electron emissive surface 53 arranged adjacent aperture 52. There is further provided collecting electrode 54 arranged to collect at any instant electrons from allbut a selected area of photosensitive cathode 4. Collecting electrode 54 is provided with aperture 55 which may be larger than aperture 52 in electrode 5| to permit passage of the multiplied electron stream there; through. Electron collector or target 56 is arranged behindapertures52 and 55 for collecting the multiplied electron stream which has passed through apertures 52 and 55. It is to be under stood that target 56 may also be arranged in the space occupied by aperture 52 and electrically insulated from electrode 5|.

Electrode'5l is connected to antiresonant circuit '20 asis electrode 5 of image dissector tube Collecting electrode 54 may be supplied with a` potential that is positive f against the average potential-of cathode 4 and electrode 5| by means of a'suitable-tap on potentiometer 25. Targetv 56 may' be kept at 'a still higher positive potential by connectin'git'to potentiometer 25 through loadl resistor 40. The output signal is developed across load resistor lil and may be coupled to a television preamplifier through coupling condenser 42. It is also feasible to provide a static electron multiplier, such as illustrated at 35 in Fig. 1, between electrode 5i and target 56.

Image dissector tube operates in the same manner as image dissector tube I. Electrons from successive selected elemental areas of cathode 4 are directed toward secondary electron emissive surface 53 where they lliberate secondary electrons by impact. These electrons are repeatedly o'scillated between surface 53 and cathode 4 before they are; swept across aperture 52 and collected by target 56. It will be understood that the electrons from successive selected elemental area-sof cathode li are oscillated continuously between surface 53 and cathode 4. It is not necessary, however, to interrupt the dynamic multiplication of the electrons because the selected electrons are deiiected into aperture 52 after a predetermined number of impacts on surface 53 and cathode 4.

It is also feasible to dispense with oscillation generator 2t and inductor 2l. In that case the image dissector tube of the invention as illustrated in Figs. 1 and 6 will operate in the manner of a multipactor and develop an alternating current oscillating in antiresonant circuit 20 at a frequency whose one-half period approximately equals the transit time of an electron between cathode it and surfaces 53 or Hl, respectively. For example, the operation of the tube of Fig. 1 is as follows, -but it is to be noted that the tube of Fig. 6 operates in similar fashion.

The photoelectrons liberated from photosensitive cathode l will leave the cathode positively charged, and, therefore, a current will flow from cathode l through the left hand portion of inductor 2'2 and its center tap to the negative terminal of battery 24. The current owing from photosensitive ca'thode 4 to battery 24 develops a potential drop which appears on cathode 4 and electrode 5 in such phase as to accelerate the photoelectrons causing them to impact surface l with a nite velocity. Accordingly, secondary electrons will be liberated from surface Il) at a ratio greater than unity.

Now electrode has acquired a negative charge deilciency due to its loss of electrons and, accordingly, a current will flow from electrode 5 through the right hand portion of inductor 22 and its center tap to the negative terminal of battery 2t. This current will again cause a potential drop appearing on cathode 4 and electrode 5 in such a phase as to accelerate the secondary electrons toward photosensitive cathode 4. `Then the cycle repeats again until the electrons from a selected area are passed through aperture 8. In that instant electrons from the next selected area of cathode 4 will be directed toward secondary electron emissive surface I0 so that theoscillation of the electrons continues without interruption;

A multipactor arranged as an oscillation generator is disclosed, for example, in the United States Patent No. 2,071,516` granted to Philo T. Farnsworth on' February 23, 1937. The multipactor has been found to start oscillating when a photoelectric current is initiated in the tube.

While there has been described -what is at present considered the preferred embodiment-of the invention, it will be obvious to those skilled in the art that various changes and modifications may. be made therein without departing from` the invention, and it is,l therefore, aimedy in the appended claims to cover all such changes. and'` modifications as fall within-the true spiritand` scope of the invention. What is claimed'is:

l. A television picture signal generatingLtube4 comprising a photosensitive cathode arranged'Y for converting an optical image into an electron. cloud representative of said optical image, anAll electrode spaced from said cathode and having a secondary electron emissive surface, electron deflecting means for cyclically sweeping said` electron cloud across said surface, means forA continuously oscillating electrons emitted from successive elemental areas of said cathode vbe-l' tween said surface and said cathode so that elec-y trons from each .of said elemental areas impact said surface a predetermined number of times and thereby produce an amplified electron stream, means cooperating with said deilecting means for sequentially selecting said amplied electron stream from said electron cloud, andl a" target arranged for collecting said selected lelectron streams thereby to derive an amplied train' of picture signals representative at any instant of the light characteristics of an elemental area of said optical image.

2. A television picture signal generating tube comprising a photosensitive cathode arranged ior converting an optical image into an electron cloud representative of said optical image, 'an' electrode spaced from said cathode and havingl a secondary electron emissive surface covering an area that is small in comparison with the area oi' said cathode, electron deflecting means for cyclically sweeping said electron cloud simultaneously in two directions normal to each other across said surface, means for continuously'oscillating electrons emitted vfrom successive ele-'1 mental areas of said cathode between said sur!! face and said cathode so that electrons from each of saidelemental areas impact said surface repeatedly and thereby produce an ampliedelecpicture signals representative at any instant ofthe light characteristics of an elemental area of said optical image. "ff

3. A television picture signal generating tube comprising a photosensitive cathode arranged l for converting an optical image. into an electron' cloud representative of said optical image, an electrode spaced from said cathode and having a secondary electron emissive surface covering an area that is small in comparison with the area of said cathode, electron defiectlng means' for cyclically sweeping said electron cloud simultaneously in two directions across said surface, means for continuously oscillating electrons areas impact said surface apredetermined number of times and thereby produce an amplied;

electron stream, a collector for collecting 'all electrons except those forming said amplified electron stream, and a target arrangedfor co1-` lecting the amplied electron stream which has been swept across-said surface;thereby to derive an amplified train of picture signals "representa--V tive at any instant. of the .lightcharacteristics of.

Aa selected elemental area of said optical' image.

4. A television picture signal generating tube comprising a photosensitive -cathodearranged for converting an optical image into an electron cloudA representative of 'said optical image, an electrode having an 'aperture therein and being spaced from said cathode, a secondary electron emissive surface formed on said electrode adjacent said aperture' and having an area that is small in comparison Awith the area of said cathode, electron delecting means for cyclically sweeping said electron cloud successively across said surface and said aperture'means for continuously oscillating electrons emitted from successive elemental areas of said cathode between said surface and said cathode so that electrons from each of said elemental areas impact said surface a predetermined number of times and thereby produce an amplified electron. stream for passage through said aperture, and a target I, for collecting the amplified electron stream which has passed through said aperture, 'thereby to derive an amplified train of picture signals representative at any instant of the light characteristics of a selected elemental area of said optical image.

5. A television picture signal generating tube comprising a photosensitive cathode arranged for converting an optical image into an electron cloud representative of said optical image, an elecn trode having an 'aperture therein and beilg spaced from said cathode, a secondary electron emissive surface arranged adjacent said aperture and having an area that is small in comparison with the area of said cathode, electron deflecting means for cyclically sweeping said electron cloud simultaneously in two directions successively across said surface and said aperture, means for applying operating potentials to said cathode and said electrode lto continuously oscillate elec trons emitted from successive elemental areas oi said cathode between said surface and said cathode so that electrons from each selected area impact said surface a predetermined number of times before they are swept thereacross to pass into said aperture, an electron multiplier arranged behind said aperture for further ampli fying the electron stream which has passed through said aperture, and a target for collecting said amplied electron stream, thereby to derive an amplified train of picture signals rep resentative at any instant of the light characteristics of a selected elemental area of said opti cal image.

6. A television picture signal generating tube comprising a photosensitive cathode arranged for converting an optical image into an electron cloud representative of said optical image, an electrode having an aperture therein and beingr spaced from said cathode, a secondary electron emissive surface arranged adjacent said aperture and having an area that is small in comparison with the area of said cathode, electron deflecting means for cyclically sweeping said electron cloud simultaneously in two directions normal to each other successively across said surface and said aperture, means for applying operating potentials to said cathode and said electrode to continuously oscillate electrons emitted from successive elemental areas of said cathode between said surface and said cathode so that electrons from each selected area impact alternately said surface and said cathode a predetermined number of times before they are swept thereacross to pass into said aperture, electron collecting means 10 forcollecting electrons from all but said selected area, and a target for collecting the amplified electron stream which 'has passed through said aperture, thereby to derive an-ampliiier train of picture signals representative at any instant of the light characteristics of a selected elemental area of said optical image.

7. A television lpicture signal generating tube comprising a photosensitive cathode arranged for converting an optical image into an electron "cl'oud representative of said optical image, an electrode having an aperture therein and being spaced from said cathode, a secondary electron emissive surface arranged adjacent said aperture' and having an area that is small in comparison-with the area of said cathode, electron deiiecting means for cyclically sweeping said electron-cloud simultaneously in two directions normal'to each other successively across said surface and-said aperture, means for applying operating Apotentials to said cathode and said electrode to continuously oscillate electrons emitted'irom successive lelemental areas of said cathode between said surface and said cathode so that electrons from each selected area impact said surface a predetermined number of times before they are swept thereacross to pass into said aperture, electron collecting means for collecting electrons from all but said selected area, an electron multiplier arranged behind said aperture for further amplifying the electron stream which has passed through said aperture, and a target for collecting said amplied electron stream, thereby to derive an amplifier train of picture signals representative at any instant of the light characteristics of a selected elemental area of said optical image.

8. In a television image dissector tube having an evacuated envelope, a photosensitive cathode mounted in said envelope, an electrode in said envelope spaced from said cathode and provided with a scanning aperture, said electrode having a secondary electron emission ratio of less than unity, a secondary electron emissive surface formed on said electrode adjacent said aperture and facing said cathode, said surface having a secondary electron emission ratio of more 'than unity and also having an area that is small in comparison with the area of said cathode, and means for impressing an alternating voltage upon said cathode and said electrode.

9. In a television image dissector tube having an evacuated envelope, a photosensitive cathode mounted in said envelope, an electrode in said envelope spaced from said cathode and provided with a scanning aperture, said electrode also having a secondary electron emission ratio of less than unity, a linear member extending across the surface of said electrode facing said cathode comprising a portion adjacent said aperture having a secondary electron emission ratio of more than unity, and means including a source of alternat ing current coupled to said cathode and to said electrode for producing an oscillating field between said cathode and said electrode.

10. In a television image dissector tube having an evacuated envelope, a photosensitive cathode mounted at one end of said envelope, an electron collecting electrode having an aperture mounted at the opposite end of said envelope, said collecting electrode having a secondary electron emission ratio of less than unity, a second electrode mounted adjacent said collecting electrode and having a scanning aperture in alignment with said collecting electrode aperture, a secondary aasa've 1 l electron emissive surface formed on said second electrode adjacent said scanning aperture and facing said cathode, said surface having an area thatis small in comparison with the area of said cathode and having a secondary electron emission ratio of more than unity, the remaining surface of said second electrode having a secondary eleca second electrode mounted on the cathode side K of said collecting electrode in alignment with said relatively large aperture and itself being provided with a relatively small scanning aperture, a sec- 12 ondary electron emissive surface formed lon said second electrode adjacent said scanning aperture and facing said cathode, said surface having an area that is small in comparison with the .area of said cathode and having a secondary electron emission ratio of more than unity, the remaining surface of said second electrode having a secondary electron emission ratio of less than unity, and

an oscillation generator coupled between said cathode and said second electrode.

CI-mISTIAN C. LARSON.

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

UNITED STATES PATENTS Y Number Name Date 2,071,515 Farnsworth Feb. 25, r193'? 2,213,547 Iams Sept. 3, 1940 227,103 Orthuber Dec. 31, 1940 James July 14, 1942

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