f l - O.OV347. f Hidz,.. (1)

Transcription

1 214 PHLlPS TECHNCAL REVEW VOL. 15, No. 7 THE FOCUSNG OF TELEVSON PCTURE-TUBES WTH FERROXDURE MAGNETS by J. A. VERHOEF : : Magneic elecron lenses n Philips developed Ferroxdure *J, a ceramic permanen-magne maerial. One of he mos imporan applicaions of his maerial, which akes full advanage of is special properies, is in focusing magnes for elevision picure-ubes... For he focusing of he elecron beam in elevision picure-ubes, magneic lenses are commonly used. The focusing acion resuls from he magneic field which has roaional symmery abou he axis of he lens. The focal lengh f is given by z= +00 f l - O.OV347. f Hidz,.. (1) z=-oo where V is he poenial difference raversed by he elecrons before hey ener he lens and Hz is he field srengh on he axis of he lens 1). f an elecromagneic lens is used, boh V and Hz vary somewha wih he supply volage. The circui can be designed such ha hese variaions largely compensae each oher. This is of course no longer possible when permanen magnes are used because only V hen varies wih he supply volage. Neverheless, permanen magne lenses are usually preferred nowadays o he elecromagneic ype of lens. There is no need o supply he considerable power which is required o energize an elecromagneic lens, so ha he power pack can be simpler. A he same ime one source of hea in he apparaus is eliminaed. Finally, he cos of manufacure of permanen magne lenses is considerably less han' ha of elecromagneic lenses. n spie of he above-menioned advanages, he convenional permanen magne lens )las.he serious disadvanage ha he field is spread ou along he *) Also known by he name "Magnadur". 1) Wih he consan used, he formula is expressed in raionalized Georgi unis. V is expressed in vols, zand f in meres and H. in A/m, (1 A/m = X 10-2 oersed). The validiy of eq. 1" is limied o he case of a "hin" lens, i.e. a lens in which. he axial' exen of he field is. 'small 'compared wih he objec and image disances. f he lens. is no hin,..oher erms mus be added o he expression (1). AsH. appears, in hese erms oo, only in even powers, he focal lengh is always independen of he sign of H., he imporance of which will be seen presenly. axis much furher han ha of an elecromagneic lens of he same srengh. The reasons why such an exended field is undesirable will firs be discussed. The fundamenal difference beween elecromagneic and permanen magne lenses will hen be considered more closely. Finally i will be shown how he sray field of permanen magne lenses can be considerably resriced. will become eviden ha, for his purpose, a magneic maerialof high eoereiviy is necessary. Some of he "Ticonal" magne seels saisfy his requiremen. However, an imporan economic advanage is gained by he use of Ferroxdure, he new permanen-magne ceramic, one of whose disincive properies is a high coerciviy 2). n addiion o he low cos of Ferroxdure, is use leads o a very simple consrucion. Some consequences of a srong sray field The elecron lens is siuaed on he neck of he ube beween he ion rap and he deflecion coils (fig. 1). Especially when a shor-neck ube is used 3), hese hree componens are so close ogeher ha he field from a convenional permanen magne lens sill has a considerable value a he ion rap and he deflecion coils (fig. 2). The resul is, ha if.he posiion or srengh of he lens is changed, he angle hrough which he ion rap deflecs he' elecron beam is also changed. The beam hen goes eccenrically hrough he ube, so ha he raser, which is "wrien" by he elecron beam on he screen of he ube is no longer well-cenered. Furhermore, par of he beam will now be cu off by he diaphragm (fig. 3), so ha he brighness 2) J. J. Wen, G. W. Rahenau, E. W. Gorer and G. W. van Ooserhou, Ferroxdure, a class of new permanen magne maerials. Philips ech. Rev. 13, 194,-208, ) The reasons why he neck of a picure ube should be kep shor, and means by which his may be achieved were discussed recenly in his Review. See J. L. H. Jonker, A shor lengh direc-view' picure-ube, Philips eeh. Rev. 14, , 1953 (No. 12). '

2 JANUARY 195'1 FOCUSNG WTH FERROXDURE MAGNETS 215 Fig. 1. Television picure-ube (diagonal = 36 cm). Mouned on he neck are: 1 ion rap magne, 2 elecron lenses, 3 deflecion coils. is 0 /2 i- --- r--- T jf' i ,r' ' r---r-...._ "':1""';1 l~ l--- e 2,5x/04.A/m Hz 11 ",~ 2,0 7675:' 1,0 0,5 o -0,5-1,0-1, 5-2,0 : '~ : li : i i, i f, ~ 1 i 1 ;~ va ; ij ~ r---r- r-- - ':)..-{ "- -.~ ~ -r / : e- 1--"- /~ '... y,-1/ 6 1"_/ i li U ~! / o cm _z Fig. 2. The form of he field along he axis of a picure-ube, due o a) an elecromagneic lens, b) a convenional permanen magne lens, and c) a permanen magne lens of he new ype. The convenional permanen magne lens (b) gives rise o a field wih a considerable "ail", which is sill quie srong a he ion rap T and a he deflecion coils D. i diminishes. f he field of he focusing magne exends o he deflecion coils, a roaion is caused as well as a disorion of he picure, which changes when he lens is adjused. By a renewed adjusmen of he ion rap magne and of he deflecion coils, Fig. 3. Schemaic represenaion of an ion rap. The elecron beam emied by he cahode 1 always conains negaive ions which would damage be screen of he ube. A local magneic field, perpendicular o be plane of he drawing, bends he elecron beam so ha i passes hrough he aperure in he diaphragm 4. The ions (large dos) are negligibly affeced and hus fall on he anode 3. 2 is he conrol grid. hese disorions can be correced again, bu he muual ineracion beween he elecron lens and he ion rap magne and he deflecion coils is very undesirable; apar from he inconvenience of he successive adjusmens, he adjusmen for he opimum ligh spo on he screen is upse. Elecromagneic and permanen-magne elecron lenses An elecromagneic lens consiss of a coil enclosed in a sof iron casing provided wih an inernal air gap (fig. 4a). The srengh of he lens depends on

3 216 PHLPS TECHNCAL REVEW. VOL. 15, Noó a he curren hrough he coil. A permanen magne lens of convenional consrucion consiss of an axially magneized ring of "Ticonal" or a similar magne seel, provided wih pole pieces (fig 5(l)' he line inegral of he field hrough he lens along he axis. From Maxwell's heory, in he case of permanen magnes, z= +00 f Hzdz = 0, z=-oo since no elecric currens are flowing. For an elecromagneic lens on he oher hand we have z= +00 f Hz dz n i. z=-oo The srengh of a lens is deermined by he form of Hz2 (see Eq. (1) and foonoe 1)), so ha, as far as is focusing acion is concerned, i is of no consequence ha he line inegral of he field of he permanen magne is zero. Wih a Ferroxdure lens he imporance of his fac is even more sriking han wih a convenional lens, as will be seen presenly. Fig. 4. a) Schemaic represenaion of he consrucion of an elecromagneic elecron lens showing he lines of force. The coil 1 is surrounded by a sof iron casing 2, which has an air gap. The srengh of he lens is conrolled by varying he curren hrough he coil. b) The form ofhe field along he axis ofhe lens shown in (a). The srengh of such a lens can be varied by means of an adjusable sof iron ring. The more his ring bridges he gap beween he pole pieces (he more he air gap is shor-circuied), he weaker he lens. n figs. 4 b and 5b are ploed he field srenghs along he axis for he wo ypes of lens. The essenial difference is ha wih an elecromagneic lens he field srengh is nowhere negaive, while ha of he permanen magneic lens necessarily has boh posiive and negaive values. Considering he area beween he curve and he z-axis as posiive above he axis and negaive below i, he oal area for a permanen magne lens amouns o zero, whils for an elecromagneic lens i is proporional o n-i (n is he number of urns of he coil and i is he curren). This area is, in fac, a measure of ~- _h ()758 Fig. 5. a) Schemaic represenaiou of he consrucion of a permanen magne lens showing he lines of force. 1 annular piece of magne seel, 2 pole pieces, 3 moveable sof-iron ring by means of which he srengh of he lens may he varied. b) Form of he field along he axis of his lens. The algebraic sum of he areas under he curve above and below he z-axis is here zero.

4 JANUARY FOCUSNG WTH FERROXDURE MAGNETS 217 A beer physical picure of he essenial difference beween. an elecromagneic lens and a permanen magne lens may bc ' obained as follows. Firs consider firs he field of a cylindrical film of curren (of lengh L and diameer D), c.g. ha -which resuls from passing a curren hrough a coil of one layer of hin wire( of diameer small compared o D). This field can he calculaed by elemenary mehods. nfig. 6 he field along he axis ~l 2 -"L (,--_-"", Fig. 6. Calculaed disribuion of he axial field of eylindrieal eurren films of lengh L and diameer D. The posiion of he eurren film is shown by he skech jus under he graphs. The greaer he parame~r D/L, he more he field is spread ou along he axis. Along he abseissa is ploed H./i, where i is he curren per uni lengh of he curren film. f i is expressed in A/m, hen H is also given in A/m (in raionalized Giorgi unis, 1 A/m Ri 1/80 oersed). 3 magne is concerned, is he same as he effec of wo curren films of equal srengh per uni lengh flowing in opposie direcions along he inner and ouer surfaces. The superposiion of he fields of hese curren gives he field of he magneized ring (fig. 8). Some idea of he effec of a sof iron casingplaced round he curren,coil, in he case of he elecromagneic lens, can he obained by regarding he oal field as due o he superposiion of he original field of he coil and he field of he casing, which becomes magneized by he field of he coil. Due o he casing he field becomes more concenraed and less spread ou along he axis (see he lines of force in fig.4a). f he casing is made wihou an air gap, a field would sill exis ouside he casing (fhl dl = wi remains valid!) The form of he field of a permanen magne lens wih pole pieces can be visualized by considering he lens as a magneiç circui wih wo parallel air gaps - a narrow gap on he inside and a wide gap on he ouside. The field in he laer gap is responsible for he negaive pars of he field on he axis (fig. 5.) The negaive par of he field canno be avoided, because he value of he line inegral along he axis mus always be zero. Should one ry o avoid he negaive field, for example by making he ouer air gap smaller wih he aid of pole pieces, hen a he same ime he useful field of he inner air gap would he weakened. Closing one of he air is ploed for differen values ofhe raio D/L. can be seen ha for consan lengh L, he field is more concenraed when he curren film diameer D is smaller. Neglecing he influence of he sof-iron casing of he elecromagneic lens, he field of he whole coil can he obained by he superposiion of he fields of he separae curren films wih differen values of D/L. The resuling field will be roughly bell-shaped, as in he curves of fig. 6. For a permanen magne lens he siuaion is differen. Neglecing he influence of he pole pieces, he lens has he form of a ring-shaped magne. is assumed ha he ring is uniformly magneized in an axial direeion and ha he magneizaion is due o circular aomie currens which lie in a plane perpendicular o he axis of he magne (fig. 7). The elecrie charges, excep hose which move a he inner and ouer surfaces of he ring, neuralize each oher's influence ouside he magne iself. The ne effec of he whole magneizaion, as far as he space ouside he Fig. 8. Theoreical disribuion of he field along he axis of a homogeneously magneized ring (curve a), obained by superposing he fields due o he curren films on he inner (curve b) and ouer surfaces (curve c), where D/L is 3.46 and 6.33 respecively. (These values apply o he Ferroxdure rings used for foeusing in elevision picure-ubes.) Curve d is he measured field of such a ring. The maxima of he heoreical and he measured eurves have been made coinciden. The sligh deviaion beween he wo curves is aribuable o inhomogeneous magneizaion in he axial and radial direcions. The curves are symmerical on eiher side of.he verical axis. gaps would have he resul ha he field on he axis would disappear compleely. The ask of he pole pieces in he convenional permanen magne lens is principally o enable he lens srengh o be varied by means of he moveable ring (fig. Sa), and he smoohing O'1of roaional asymmeries of he magneizaion of he magne ring. Fig. 7. The magneizaion of a uniformly, axially magneized ring can be hough of as originaing in circular' aomic currens in he plane perpendicular o he axis of'he ring. The field ouside he magneized maerial is hen equivalen o ha of wo curren films flowing in opposie direcions along he inner and ouer surfaces of he ring. Focusing wih Ferroxdure magnes,.., T~ reduce he sray field; a lens hash 'een dével- -öiied wlîich consiss of wo fla rings of Ferroxdure, coaxially mouned, and axially magneized in

5 218 PHLPS TECHNCAL REVEW VOL. 15, No. 7 opposie direcions (fig. 9). Apar from he cenral region, he fields now largely neuralize each oher (fig. 10). The oal area under he curve Hz = f (z), of course, remains zero, for his is he case for any combinaion of permanen magnes. As has already been poined ou, his is no in any way opposed o he saisfacory operaion of his lens, because he srengh of he lens is deermined exclusively by he form of Hz2 (doed line, fig. lob). reduce he magneizaion o zero is abou 2.4 X 10 5 A/m (3000 oersed), while ha of Ticonal E amouns o only 0.53 X 10 5 A/m (660 oersed).) Compared wih Ticonal E, Ferroxdure is herefore much less demagneized by he field of he magneic poles which esablish hemselves a he end faces of he ring. Wih a given volume of magneic maerial and a given inernal radius, if he axiallengh L is made smaller (and so he radial dimension h greaer, Fig. 9. Two phoographs of a focusing uni made np of wo Ferroxdure rings (1 and 2) axially magneized in opposie direcions. The disance beween he rings is adjused by he screw 3. 4 is a guide pin, 5 an adjusable sof iron ring wih which he elecron beam can be cenered. The plae 6 is of aluminium. The focusing uni seen in fig. 1 is of slighly differen consrucion. As he value of (BH)max is much smaller for Ferroxdure han for he magne seels commonly used in convenional permanen magne lenses, e.g." Ticonal" E, i is no surprising ha a greaer oal volume of Ferroxdure is needed han of Ticonal "E", o produce a lens of a given srengh 4). n spie of his fac and in spie of he division of he maerial ino wo separae rings, he whole lens is no longer han he convenional lens, hanks o he fla form of he rings. This fla shape is possible owing o he grea coerciiviy of Ferroxdure. (The field srengh which would be necessary o 4) A. Th. van Urk, The use of modern seels for permanen magnes, Philips ech. Rev. 5, 29-35, Some furher remarks concerning he produc (BH )max, may be found in he aricle referred o in noe 2), on p. 116 and p fig. 11) hen he poles which produce he demagneizaion field come closer ogeher and he srengh of his increases. n he case ofticonal E, his field would soon reduce he magneizaion o only a fracion of he remanence: if wo fla, opposiely magneized rings of Ticonal E were assembled o form a lens, hey would desroy each ohers magneizaion almos compleely. Evidenly his maerial canno be applied in he case of fla rings. Wih Ferroxdure, on he oher hand, however hin he ring is made, he magneizaion (fig.12) never falls below 80% of he remanence 5). n he exreme case, when he hickness 5) Lenses consising of wo opposiely magneized rings can be consruced of "Ticonal", provided ha Ticonal K is used; for compleeness is demagneizaion curve is also given in fig. 12. The applicaion of Ferroxdure, however, leads o a more economic soluion.

6 JANUARY 1954 FOCUSNG WTH FERROXDURE <MAGNETS 219 Hz 4 1R s 3 :1 1, 1,0 2 0,5-0, r' : 1..--" L- r-, ~ - r-. V _,.- f-... H 'v O~ V ~ l' /'.1_ 1-- r-- r- o of he ring is made equal o zero, he working poin (i.e. he poin on he demagneizaion curve corresponding o he magneic condiion of he maerial] is found o be he inersecion of he line J = -!1oH wih he demagneizaion curve (#0 is he permeabiliy of free space). The disasrous resul for Ticonal E can be seen in fig. 12, where he magneizaion drops from a very high value for a 1,0 s 2,0 V 0, o cm _ a " 'p,, n 5! i", : '' [' 2xlO"A/J Hz " 0,. 1,o.,!,:: ".: 4--1 :' i ~ 1 2, J..11 ~ p n -- ~ ~j.:! ~~ -, {4-0 ~ ~ ~ 2 o Fig. 11. Cross-secions of ring-shaped magnes for elevision ube foeusing. a) For a lens of he.convenional ype using magne seel. b) For a lens of he new ype, using Ferroxdure. long magne, o he poin -~l for a magne of zero lengh; on he oher hand he change in he magneizaion of Ferroxdure (R - P2) is very small indeed.,',., ~, r -0, -1, -1, -2, ' J v o 2 6 8cm -z b Fig. 10. a) The separae axial fields of wo opposiely direced Ferroxdure rings are depiced here in he one figure. The curves are displaced wih respec o each oher by a disance corresponding o he hickness of one ring. For one of he rings (he righ-hand one) he form of H.2 is drawn in doed lines. b) Curve 1 shows he resulan field along he axis, of a lens wih he wo rings placed face o face; curve 2 shows he field when he rings are separaed by 10 mm. The doed curves 3 and 4 show he form of H.2 along he axis for hese wo cases H o xfo~m Fig. 12. Demagneizaion curves of Ferroxdure (eurve a), "Ticonal" E (eurve b) and."ticonal" K (curve c). Noe ha he magneizaion J is ploed along he verical axis, and no he magneic inducion. The poin R represens he remanen magneizaion of Ferroxdure. Wih a focussing uni as in fig. 9 he working poin of he Ferroxdure lies beween A 2 and A3' depending on he disance beween he wo rings. Al is he working poin for a single ring no influeuced by he field of anoher magne. P l and P2 are he working poins forvery hin fla discs of Ticonal E and Ferroxdure respecively.

7 220 PHLlPS TECHNCAL REVEW VOL. ]5, No. 7 Even wih Ferroxdure, however, ' he rings.canno be made hinner indefiniely. The opposie poles on he wo end faces will neuralize each oher's exernal fields more and more as he rings are made hinner. Accordingly, he field along he axis decreases quickly as L becomes' considerably smaller han h. For he acual rings used, L ~ h,. The ceramic Ferroxdure rings can more easily be made homogeneous han rings made from magne seel. Hence he magneizaion shows he same roaional symmery as he ring and no sof iron pole pieces are necessary o ensure a field wih roaional symmery. The srengh of he Ferroxdure lens is' varied by adjusing he disance beween he wo rings (fig. 9). As he disance is increased, here is less muual neuralizaion of he main: fields, and hus he lens becom~s sronger. The neuralizing of he sray field is hen less complee, bu i is sill adequae, even over he whole range of adjusmen of he lens srengh, which is a leas as grea as ha of a convenional permanen magne lens, and ofen greaer. ("Adequae" neuralizaion implies ha he operaien of he ion rap magne and.of he deflecion coils are only slighly influenced by he elecron lens. The adjusmen of hese componens can hen be made independenly). n a given case, i proves possible o obain good focusing of he elecron beam for any acceleraion poenial beween 10 and 20 kv (see Eq (1)), by varying he separaion of he wo rings beween 0 and 12 mm. The wo rings, magneized in opposie direcions and siuaed in each oher's field, exercise on each oher a demagneizing field. f he separaion of he rings is such ha hey exer no demagneizing field on each oher, he working poin lies approximaely'a he poin Ai (fig. 12). As he ri~gs are brough closer ogeher, he working poin moves owards A 2 Wih he normal spacing of he rings, he working poin lies beween A 2 and A3' The posiion of he working poin is imporan because, wih Ferroxdure, he magneizaion depends on he emperaure, and. he.emperaure coefficien varies wih he posiion of he working poin. Near he poin R he emperarure coefficien is abou -0.2% per oe, bu in he region beween A 2 and Aa i is only abou half his value. Wih some hough, elevision ses can be so designed ha he rise of emperaure of he magne rings remains so small ha he influence on he picure definiion is impercepible. n pracice he picure is never accuraely cenered on he screen of he ube wihou some adjusmen. n he Ferroxdure lens, a hin sof iron ring is placed agains he firs magne (fig.. 9). By moving his sof iron ring, a movemen of he picure across he screen can be achieved.' Summary, By he applicaion of he disincive properies of Ferroxdure, he new ceramic maerial for permanen magnes, i is possible o consruc a simplified permanen magne lens for he focusing of he elecron beam in elevision picureubes. The lens is characerized by a sray field along he axis which is considerably less han ha of permanen magne lenses of he eonvenional ype. The lens consiss of wo axially magneized rings of Ferroxdure which are mouned ogeher, like poles facing each oher. The sray fields of he wo rings hen largely neuralize each oher. The srengh of he lens is adjused by varying he muual separaion of he rings. Cenering he picure on he screen is accomplished simply by moving a sof iron eenering ring. Adjusmen of he lens srengh does no affec he adjusmen of he ion rap magne, and causes only a negligible roaion of he picure. The adjusmen of he ion rap magne, he lens iself and he deflecion coils is hereby simplified. As a resul of he simple consrucion of he lens and of he low price of Ferroxdure, he new elecron lens is also cheaper han exising lenses for his purpose. - d", ~. - ','.. 0 ~,~: Jo," ";.1,. ',;. -..r '::!.:.; 1;,- ;