US2153918A - Dissector tube - Google Patents

Description

April 1939- P. T. FARNSWORTH 2 D DISSECTOR TUBE Filed Dec 31, 1935 INVENTOR,

PH/LO T. FARNSWORTH BY AMPLIFIER ffi 1 I ATTORNEYS.

Patented Apr. 11, I939 nrssso'ron orth, San Francisco, Gellfl, signer, by mesne assientmto Fe aan rth Television & Radio Corporation, Dover, DeL, a

' I corporation of Dela Application December 81, 1935, Serial No. 56,976

other-collector of electrons having an elementaryv areeges" coinper'eiwith the area' the cathode. The ;di ss ec i; of thisfparticular invention" diflfrs gieylqesilhfits class inthat the eleo v 'is'undistorted anti in focus only im medihtely ediacent, to the electron collector, cno that",t he,iother1portions thereof may he so dis" torted esprecticelly to lose their image charace The primcry ohj'ect, of the invention is to pro vide. .e"f devic wherein the portion of the image undergoing dissection at any instant is sharply in rocustregarcilessof the degree of deflection of the or" electrons-es a. whole} Secondary oh" ject'fs are; 'lfo muses. means oftiissection wheree w ap ere greatly' re ducegh to proector..tuh e. wherein theoptical inflow hen h l'th ldb eeps em or viewiis .p ected becomes, inefiect, a portion oi meme e to provi we for the analysis-or dissection on imege whereintherequir'eci elecge et u ko ad n plex corrective circuits.

My my ntion ipossesses numerous qtherz objects ewiiteey dva i. e: o h h, o:

d i izine y novel me i-fi s he .nderstoos tha vnimtmethod is QPRli et er-en r usi findfzfihfil 'lrkdfii i 7 r siiiqedthi' fl .V i' :45 linens w 'th -e pe msof he.'--.- 1 e.=- wormi -compon nt nt et gqw e n nt-:sim1iet n;-as m r m trv r usot r sted mu in tep e on.

wunot being .made-Eoonducting; end? e. iqregoing; will he set f orth in the moti n r s c fi ao er ii tenir 1 Figure 3 is a line diagram showing the general conformation of the electrostatic field produced in the dissectortube-, a.nd theresultent electron paths produced in the absence of the magnetic in television scanning hy means accurate focus of the electroh image at the scan ning aperture in spiteof the varying strength of the resultant field produced by the focusing and deflecting coils. Where theirnage is stationery oithe dissector tuhe one of the major problems is to maintain it is possible to produces, plane'focus having en accuracy and resolution comparable to wfirs't' 7 class N optical image. In producing such e; focus magnetically the field is e djustecl' so'that eitch'fjl'ii electron, in its path from th 'cathode 'toth'" anode, accomplishes an integral umber of, turns along a. helical path andreturnsto tengencywith the'lin of force throuehjtsloriQin in'the plane of the aperture. The ce'irom the source" of the electron to its rec-a1 surface is directly. 1 proportional to the me n i filocity of the electron elong..,itspe;th and inversely proportional tothe;

strength a: the resultant'zneeheticfield. gen- '1 eralvitllis'eclyiseble to loca te' thejipqll'ectoror scum-Z 25 ed for focusing ind deflecting H we s ea .witharesultantsavingofuptofiftyper has two aspects, both of which involve passing the electron stream through an electrostatic field of non-uniform gradient, and both of which contribute to the final result. The photoelectric cathode is a unipotential surface, and the electrons collected to form the picture current all arrive at the same point. It is obvious, therefore, that the final velocity of all of the arriving electrons must be-the same, since each has fallen through the same potential. By making the field stronger adjacent the edges of the cathode than it is at the center, however, the initial acceleration of the peripheral electrons may be increased, and hence, although their final velocity is the same, their mean velocity is greater than for the central electrons, and their focal point is therefore farther along their line of fiight even under the action of the combined deflecting and focusing field.

Considering the other aspect of the invention. the electrostatic field is so devised as to form a type of electrostatic lens which tends to concentrate the entire emission from the cathode in the immediate locality of their point of collection. The tendency of the magnetic field is to spread out the concentrated beam to form an electrical image, but the particular electrons which are being collected at any instant will, as a result of. the concentrating action of the'field, be traveling along substantially the same paths which they would take were the deflecting field not present. These natural paths of the electrons from all other points will cut, more or less sharply, across the electro-magnetic field, and hence will be acted upon thereby. The function of the magnetic field may therefore be considered as being not so much to deflect desired electrons toward the scanning aperture as to deflect the undesired electrons away therefrom. The electrons collected are therefore almost unaffected by the deflecting field. with $11 01 the extremely msgneticfieldswonldbenecemaryinorderto getcomplete efiect alone a condition might, perhaps, achievedatwhlchanyvalueoffocusing whatsoeverwouldbesufldmttodiverttheunqiired' where magnetic-focusing alone is of deflecting current and seventy five per of power, a saving equipment cost and with an increase in overall definition.

Considering the invention in detail, the dissector tube, which is the heart of the apparatus, comprises a cylindrical evacuated envelope i, provided'at one end with a window 2 through which the optical image of the field of view may be projected, and at the other end with a reentrant stem 3 throush which is brought a lead 4 connecting to-the cathode I, The active surface 1 of the cathode is, as nearly as possible, optically plane, and it .is surrounded by a cylindrical flange 8 so that the entire cathode forms a shallow cup' or dish. I prefer to form the entire cathode of silveaand to oxidize and deposit caesium on the surface I in the well known manner to form a photoelectric surface which is as uniform and sensitive as it is possible to construct.

Starting at some distance from the rim of the cathode is a conductive film I formed on the interior of the envelope wall, and extending to a point closely adjacent to the window 2. In practice this coating is extremely thin, and its thickness is grossly exaggerated in the figure merely in order to show it at all. The preferred method of forming the film is to evaporate some metal, such as nickel, within the exhausted tube, protecting the window 2 and the gap ll between the cathode and the film by means of shields temporarily inserted in place before the final construction of the tube. This results in an additlonal film beneath the metal of the cathoa. No attempt has been made actually to show the film in this portion of the tube, since it coacts with the cathode itself in establishing the electrostatic field and its production as a separate film is partly for the sake of convenience, and partlyinordertoeffectasavinginsllvenfor with the surface film the'fiange I on the oathode need not beso long as would otherwise be anodeaandthemscebetweenar'eanintssdependent;andthemafliematicsrequiredtodeiive fm-mthepicturecurrentisplseedasclose window 2 asisstructurallyfeasible. lectormaytakeanumber of forms, being that shown in the schematic whereinthecollector ilissurrotmdedbyashield ",thelatteratanodepotentiahandsuppliedwifl: snaperture llwhichdefinestheelementaryarea, or the cross-section of the electron stream which reachesthecollecto'r. Inl'igureiamoreelabcrate structm'eis shown involving the useof an electron multiplier or multipactosfor proper tionally increasing the number of electrons reachlnstheexternalcircuit..'lhisstrdctureisdescribed and claimed in my copending application, Serial No. 10,604 above-referred to. Theactionofthisdeviceisnotgermanetothepmaioaors ent invention, and it should therefore sumce to state that-the central electrode or anode t2, the

shield l3 and aperture i4 exercise the same functions as in the more simple structure. The two semicylindrical plates l5 are operated at a mean potential corresponding substantially to that of the shield l3, and hence efiectively perform the same function, i. e., they are not at materially different potential from the surrounding anode structure.

In the simplified circuit shown in Figure 2 there is shown a focusing coil l'l, which is a solenoid surrounding the entire dissectof and supplied with acurrent from a source It regulated by a rheostat IS. The tube is encompassed by the necessary deflecting coils, as shown in detail in my former application, Ser. No. 321,805, one pair of coils being represented in the diagram by a single coil supplied by a sawtooth wave low frequency oscillator 22. The lateral deflection is achieved by a similarly disposed pair of coils,-

with axes at right angles to the coil 20, which are also represented in the diagram by a single coil 23 supplied by a high frequency sawtooth wave oscillator 28.

The collector electrode l2 connects to an output-resistor 25 and through a potential source 21 to the cathode 5. The shield I3 is operated at slightly different potential from the collector. and connects to the anode cylinder 9. An output amplifier 28 connects across the output resistor and supplies whatever transmission medium may be used with the device.

In operation an image of the field of view to be transmitted is focused by the lens upon the sensitive surface I of the cathode 5. These electrons are attracted toward the anode by the potential applied thereto, which may vary within wide limits but is conveniently of the order of 600 volts. Except that the values of the currents used in the coil system are less there is but one difference, from the operator's standpoint, be-

tween the present device and that disclosed, for

instance, in my former application, Serial No.

321,805. This difference lies in the positioning of the deflecting collswith respect to the deflection produced by them when the image is in focus. It is well known that where no focusing field is used, the axes of the deflecting coils to produce, say, a vertical deflection, must be horizontal. With a uniform focusing field, and without the use of electrostatic compensation as described in the present application, the axes of the deiiecting coils coincide with the direction of the deflection produced thereby, i. e., to produce a vertical deflection the axes of the coils must be vertical. With the setup as here described the focusing coil may be made somewhat shorter than is necessary to produce a completely uniform field, and the axes of the deflecting coils will-be made somewhat oblique In practice, for a vertical defiection, the deflecting coil axis is somewhat "nearer the vertical than the horizontal. The

coils for producing the horizontal deflection are mounted normally to the vertical deflecting coils;

their displacement from the horizontal is similar to the displacement of the vertical deflection coils from the vertical. The exact adjustment is readily found by experiment.

Before considering the action of the electrostatic field, it'must be pointed out that the entire WDW end of the tube is, when the tube is in actual operation, at anode potential. This is brought about by secondary emission from the.

insulating surface of the envelope.- When the the central electrons reach;athiscsameiivelecityamp beam of electrons is defiected, certain of the peripheral electrons strike the walls of thetube, closely adjacent the angde fllm, witlr sumcient velocity to knock outsecondaryelect 'ons Elec; rons fr m h s p rtion ti-the w ll-ate llected. a by the node, thus pla i sa more rosltivaehars I up n he w l and ea in themloeitv-ot th striking electrons and consequentlm-progressively-.. increasing the number .911 scc hsiama and to the point where thewalls re ch, thean e,-,p tential- When -Penalti s es ehli h the emi t d elec rons arereeat rahtcslta the. the and: a condition of equilibrium is, established. 1 not ce ai e act y. what-sta t ..-t emr ees may be some leakage or the positive cha v the film along the glasswalls as it see t gili 0 llsh atthe. u is concerned, the anode form oil a deep cylindric metal cylinder. m

The action of the electrostatic. field produced by the char ed-surf es pf ,thstahed tan atheas cathode are lllustratedialjfigure 3. The hea line 3; indicates "the shape 9;- the-gredlansection l of the negatively. chargedicathod H face, whil the heavy line tashowsaheshapeo the op surface that is positively-charge, plicity the collector, ls. ,9mitted; .;hut. noted that t is element is eateatdean wit ht o e u whereaihenshie 'iia aat gh 9. walls reduces materially-che po, ntia, siadi be neglected forihe insist nt. v

The dotted lines 33 indicate app xi inatel th tween the two surfaces; liluuipoteK1,,3a,,v not shown. since their: wouldvunne essaril can 1 fuse the diaaran rtiiutn'lt aahtzt a lilit eidt from the shaptoi thei lmtrodesith tst tfi will he strone-sadiacentiimperialist :ei' sith cathode andrelativelnweake .afr t mid @3 Mn 3 will be recognizedxthahaestron .flel iipdt'iifitmil 1 g to a high potential gradient, whereas the weake field correspondsitoratlowionenhence thematic which the 'peripherallieledtr nsoapnmaoh vfihel final velocity willcbagneater. thamzthait at which The field adjacentl-tliehcathodeeis convergent; but as it approachesethee anogla it istartsmta. diverge. ThBifdl'CRQbiflQbM'Dbfiifl-I electrons-tat: any moment isl in thetdireetionebfirtheilinassofe-t.. force. The electrons asttheyi-starti alongtcthir paths tend till-fallow; thet llriessorcforceacbut. than. velocities which-*theyficquireiprevent mdoingtz;

"so. By the time tlieyihavei'enteredi'thatdivergent.c

portion of the.'fieldxttheware'ttravelirig withilapa proximately wiper centiiofetheimfinalrs velocitym of following the pathsvhidicatedrhyuthetconvengin as .s lines 83, tendingntoicoricentrate atea" point closely? adjacent the collecting apertureto'; an. to .su t

As has been statedi 'tlies': effect-1:pifcthevcollector. upon the field is relatively unimportant, butsuchs effect as it does have is to decrease the divergence of the field. It will be recognized that the mutual repulsion of the electrons, in the stream would necessarily prevent point concentration, but practice has shown that the concentration which can be achieved is excellent, the beam of electrons which started out with the characteristics of an image over four inches in diameter being brought down to a spot one-half inch or less in size. .Where only a small point on the periphery of the cathode is illuminated, so that the repulsive effect of the other electrons is not present, the approximation to the aperture may be considerably closer.

Whena stream of electrons of this character is passed through the magnetic focusing field the concentration of the beam is, of course, destroyed. Those electron paths which are substantially parallel to the fleld are affected by it in a normal way, and are brought to a focus in the plane of the aperture, traversing the paths which are little different from what they would be were the electrostatic field straight and uniform. The peripheral electrons, however, cut sharply across the focusing fleld, and are accordingly deflected in spiral paths and brought into tangency with the line of magnetic force through their origin considerably closer to their source than they otherwise would be. They are, however, deflected from the aperture, and since they do notenter into the picture signal their ultimate paths and points of collection are immaterial.

When the deflecting field becomes eil'ective, however, so that the peripheral electrons are passing substantially parallel to the resultant fleld, these in turn are brought to a focus at the aperture but little affected by the magnetic fleld, while the central-electrons and those from the opposite edge of the cathode cut sharply across' the magnetic field and are diverted. The higher mean velocity of these peripheral electrons, due

to their quicker acceleration, brings their focal plane closer to the aperture than it would he were the centralelectrons.

The effect may be considered from another angle; the magnetic field serves to focus the electron stream, not in a plane, as would be the 'fromtheareaunderscansiontodiifusesndfail to fall within the aperture; the only-dimculty which arises from'a failure of exact focus is thatsomeofthe electrons from adiscentelementary areas-will also be collected. The result of a failure of exact focus, therefore, is merely the equivalent of using a slightly larger collecting aperture, and not a tendency toward a dead level of illumination such as would occur if the direction of the electrons were random. A much greater tolerance as to strength of focusing field than would otherwise be the case is therefore permissible.

traveling at the same mean velocity as ency of all of the electrons is to fall into the aperture and it is therefore unnecessary to force the desired electrons into line magnetically, but merely to divert the undesired electrons from the aperture sumciently so that they will notcause trouble.

In order to determine the correct proportion for an electrode structure, other than the example given, to obtain the effect here described, the best method is one of cut and try. An approximate solution may be obtained mathematically if the efl'ect of necessary incidental structures which destroy symmetry be neglected. The mathematics, however, are extremely complex and the experimental solution is relatively easy. A perfectly plane cathode, without the surrounding flange, gives maximum difference in strength of field between the periphery and the axis of the structure. This arrangement, however, exercises almost no concentrating effect. The deeper the cathode cup, the closer to the emitting surface the point of concentration falls,'while the deeper the anode cup the greater the. peripheral velocity in comparison to the axial velocity. By laying out graphically a proposed electrode structure, and tracing in the lines of force, the approximate paths of the electrons may be predetermined. The optimum structure may, atthe first approximation, be taken as the shallowest cathode cup which will give concentration at the point desired. The spacing between anode and cathode naturally affects this design. Only the final adjustment, in order to get most satisfactory effects, need therefore be constructed and tried.

I claim:

1. The method of electrical Picture analysis which comprises the steps of forming an electron streamhaving thecha'racteristicsctanelectncal lmageatitssourcepnllandsubjectingsaid streamtothecombinedactionofelectricand magnetic fields, one of said flelds bdng propor tionedtodh'ectallelectronscomponentofsaid atanyoneinstantandtodiverttherunainder from stream at a prcelected'point of collection, and

3. The method of electrical picture Nil-lids whichcomprisesthestepsofformingane'iectron streamhsvingthecharacteristiesotsnelectrieal image'atitssouroeonly,passingsaidstream' throughan electric held of non-uniformgradient toward a substantially axialpcint of collection, said field being steeperin gradient adJs'cent the edgesofthesourcethanadjacentthecenter thereof, wherebythejperipheraleloctronsofsaid streamapproachtheirflnalvelocirymorequictly axial point of collection, the initial acceleration of the peripheral electrons of said stream being greater than that of the more central electrons of said stream and the terminal velocities of all of said electrons being' substantially equal, and passing said electrons through a substantially constant longitudinal magnetic field and a transverse magnetic field varying in such manner as to bring the paths of electrons from all parts of said source successively parallel with the resultant of said 0 fields.

PHILO T. FARNSWORTH.

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