US2425682A

US2425682A - Cathode-ray tube with shielded deflecting plates

Info

Publication number: US2425682A
Application number: US551863A
Authority: US
Inventors: Liebmann Gerhard
Original assignee: Cathodeon Ltd
Current assignee: Heraeus Noblelight Analytics Ltd
Priority date: 1943-08-19
Filing date: 1944-08-30
Publication date: 1947-08-12
Anticipated expiration: 1964-08-12
Also published as: GB570672A

Description

Aug. 12, 1947. G. LIEBMANN 2,425,682

CATHODE RAY TUBE WITH SHIELDED DEFLECTING PLATES v Filed Aug. 50, 1944 Patented Aug. 12, 1947 CATHODE-RAY TUBE WITH SHIELDED DEFLECTING PLATES Gerhard Liebmann, Cambridge, England, assignor to Cathodeon Limited, Cambridge, England,

a British company Application August 30, 1944, Serial'No. 551,863 In Great Britain August 19, 1943 17 Claims. (Cl. Z50-'458) The invention relates to cathode ray tubes, and more particularly to cathode ray oscillograph tubes. In this kind of cathode ray tube an electron beam is formed which throws a small spot on to a suitable screen, which may be a uorescent screen or a photographic plate or any other means to register or record the position, or the position and intensity, of the electron spot. The beam forming this spot is deflected by deflecting plates on which suitable potentials are impressed. Usually two sets vof deecting plates are employed, and in many applications of this kind of tube the potential to be investigated is brought to one set oi plates and an alternating potential of a suitable and known Wave form is applied to the other set of deflecting plates.

If the alternating potentials applied to one or both sets of deflecting plates are of a fairly high frequency, of the order of a megacycle/sec. or more, a serious interaction of the two deilecting potentials becomes apparent owing tothe capacitative coupling between the two sets of deilecting plates. An object of the present invention is to eliminate this fault.

In a cathode ray tube, and particularly a cathode ray oscillograph tube, according to the present invention two or more sets of deflecting plates are entirely enclosed in a shielding metal cylinder of circular or other suitable cross-section which is adapted to be grounded when the tube is in use, this cylinder being partitioned by metal screening into compartments entirely shielded from each other, each compartment containing one set of deiiecting plates. Openings of suitable shape and size have, of course, to be provided in the partitioning screens to let the electron beam pass through, taking into account the diameter of the lbeam and any deection it may have suffered before passing through the opening. Further openings are provided in the mantle of the shielding cylinder to allow lead-in wires to the deilecting plates to pass through. These lead-in wires are sealed through the 'wall of the glass envelope near the position of the respective deflecting plates.

Preferably, the diameter of the shielding cylinder is only slightly smaller than the internal diameter of the glass envelope. The envelope is surrounded near the sealing through position of the deecting plate leads by a metallic screen adapted to be grounded, which, preferably, has a screening flange extending perpendicularly to the axis of the tube and interposed between the leads from each set of deiiecting plates. The bottom end of the internal shielding cylinder, facing the cathode end of the cathode ray tube, is

constituted lby a metal discor `metal cup, or a combination of both, and may be shaped and arranged in such a way that it forms the nal accelerating anode of the cathode ray tube gun` structure, and thus cci-operatesV with the other electrodes in the formation of the electron beam.

The internal shielding of the cathode ray tube isI completed in a known way by a metal or graphite or other conductive coating on the inner wall of the tube envelope connected to the nal anode. This conductive coating may extend from the bottom end of the internal shielding cylinder to the registering or recording screen.

The described internal shielding cylinder may:

be used at the same' time as a means for lining up correctly the various sets of deilecting plates, by assembling the sets of deilecting plates, conveniently with the help of jigs, on suitable insulating annular discs, for instance mica discs, which are such as1to engageV the internal wall of the shielding cylinder when the assembly is placed therein, thereby correctly locating the sets of deflecting plates.

In order that the invention may be more clearly understood, reference will `now be made, by way Way of example, to the accompanying drawings, in which v Fig. 1 shows a section through a cathode ray oscillograph tube embodying one form of the invention.

Fig. 2 shows a in Fig. 1.

Referring to the drawings, l represents the neck portion of the envelope of the cathode ray tube which encloses the cathode ray gun and de-- ilecting plates and 2 the bulb portion of thetwbe, the front wall 3 of which is coated with fluorescent material 4' to form the viewing screen. Y, The neck portion I accommodates the cathode ray.

gun structure 5 and two pairs of deilecting plates 6 and 1 for 'deecting the beam in the X andY tedwith the cap Scarrying contact pinsin'knownl manner. N y A ,Y

According to the `present invention,` the two p Fig. 3 is an external view of slightly modifiedv tube.

screens II, I2 and I3 to allow the electron beam.

from the cathode gun to pass through. The apertures in the partitioning' screens increase progressively in size from the screen II to the screen I3 to allow for the increased transverse: deflection of .the beam as it approaches the fluorescent screen. Thus, the aperture at the centre of the screen II may comprise a small circular hole I4 since there is no deflection ofthe. beam at this point, and conveniently this hole may be formed in the cup-shaped metallic member I5 which is welded to a larger aperture in the centre of the screen I Ix as shown in Fig... 1which member I5 constitutes they final accelerating anode.

The aperture I6 in the screen I2 may likewise be formed.. in. a cup-shaped member I1 welded in a` larger aperturey inthe centre of the screen I2T the aperture lr6 having a width in the direction perpendicular .to the plane of the drawing which is slightly greater than the distance between` the ends. of the pair of deecting plates `6` which lie adjacent tothe aperture I6A so as to allow for the maximumdeection4 oi the beam effected. by this pair of plates The width of the aperture in the direction of the plane of the drawing may be smaller since the defiecting plates 6`- do not deflect. the beam in this plane, this dimension being preferably slightly smaller thanI the distancel between the endsof the pair of deflecting. plates 1 which are adjacent to. this aperture.v The-aperture I8 in the screen I3 is made larger stillrk being. of a size in the. direction. of the plane of the. drawing slightly larger than. the distance between the ends of the pair of deflecting plates 1 which areadjacent the screen I3 and in. the direction perpendicular to the plane ofv the drawing of still larger size. toallow forthe deection` which the. beam may have suffered by the action of the plates. 6. Fig. 2 shows the arrangement of the aperturesin the screens. A

The outer peripheries of the screens I I, I2 and- I3 are provided with flanges so that they may be welded to the cylindrical shielding metal cyl.- inder I0 within. which they are arranged in closely fitting contact. The leads from the pairs of deecting plates, shownat I9 and 20 in the caseof the plates (1,. pass through. insulating sleeves 2| and 22 to the external contactsf23, 24,

apertures

25, 26 being provided in the metal shielding cylinder I'to aIl-owthe leads and insulating sleeves to passtherethrough. 'Ihe leads (not shown) from the deflecting plates 6 are similarlyconnected through .insulating sleeves and appropriately positioned apertures inthe shielding"cylind`er Ill to their corresponding

external contacts

21, 28. y

Also passing through the screening assembly are fourv main stay-wires 29- which are welded to-portions of the screens II, l2y and I3 struck up from apertures 3U therein-through which the wires pass. These wires terminate Vat the screen I3 but extend beyond the screenv I I, their ends being welded .to eyelets 3l secured in apertures in a -mica disc 32 which supports onelendy of the cathode ray gun structure.V The other epd of the cathode gun structure is similarly supported by a mica disc 33 having apertures therein corresponding to the positions of the main stay wires 29, which apertures are fitted with eyelets 34 which are also welded to the stay wiresr In this way, the. cathode gun structure may-Y be easily assembled in correct alignment with the apertures in the screens I I, I2 and I3.

The deflecting plates 6 and 1 are also supported by and located relative to the main stay wires- 29 by being insulatingly supported therefrom by means of mica discs at opposite ends of. eachy compartment, the mica discs having apertures which rit-closely to the main stay wires and; between which are supported further wires which carry ,the deecting plates. In the case of: the' deflecting plates 6,

mica discs

35, 36 are arranged' at opposite ends of the compartment enclosing these deecting plates which discs have apertures through which the stay wires 29 pass and; prevent rotation of the discs relative to the stay' Wires. Mounted betweenand` welded to the eyeletsy 3-1, 38 secured in the

discs

35 and 36. respectively are twoy pairsl of stout wires- 39, each pair of which are bridged by a further pair of stout wires 4Ilto each of which are attached opposite, ends of one ofthe deflecting plates Similarly, inthe case of the deflectingr plates- 1, these are supported by two mica discs 4I, 42

arranged at. opposite ends of the compartment.

enclosing thev plates 1 and apertured to fity the:

stay wires-29,. the plates 1 being supported-by two, pairs of longitudinal stout wires- 43 welded. to

eyelets44-,- 45 secured respectively to the mica discs 4I and 42., each-pair of longitudinal wires 43 being bridged by a pairof transverse wires 46 to which opposite ends` ofY the deflecting plates 1. are welded.l

It will be appreciated. that, with the method of construction described, the cathode gun, the apertures in. the. partitioning screens and the deflecting plates are all located relative tothe main stay wires 29 and. can. be easily assembled outside the tube envelope in correct align-ment and spacial. relationship..

In order to4 ensure insulation between the .ends of-the

eyelets

31, 38, 44 and 45 and the adjacent portions of the partitioning screens I I, IZ and I3, the screens may be formed with recesses 41 opposite the ends of the eyelets so as to provide. additional clearance between the eyelets and the screens. Such a recess is shown at 4.1 in screen I2.. Similar recesses may .be provided as shown at.41' and. 4-1 in theV case of screens I I and I3 but asan alternative or additional measure, the drawing shows a further mica disc y48 disposed between the screen II and the ends of the eyelets 31 to ensure this insulation.y This mica disc has no apertures which willY allow the eyelets 31 to contact the screen II. The drawing shows similar insulating mica discs at 49 and 50. for respectively insulating the ends ofthe eyelets 44 and 45 from the screens I2 and I3, A11 these additional insulating. mica discs are perforated to register with the main stay wires 29.

'The metal shielding Cylinder I0 preferably' closely lits the intern-al surface of the neck. portion I ofthe tube envelope so that it serves accurately to locate the entire assembly within the neck portion of the tube. The remaining part of the tube. between the neck and extending almost up to the fluorescent screen may be screened in known manner by coating the internal surface thereof with graphite 50 which may be electrically connected to the screening assembly I0,

Il, l2 and I3 by means of the contact spring 5|.

The lead 52 to the final anode and the screening assembly may be taken out through the stem of the tube.

Figs. 1 and 2 show an arrangement in which the two pairs of deflecting plates are respectively connected to external contacts arranged at right angles to each other. Fig. 3 shows a slightly modified arrangement in which the external contacts to both sets of plates are brought out in the same plane. IIhis facilitates mounting the tube on the chassis on which the circuit components are assembled and permits shorter leads from the output valves connected to the deflecting plate leads. Fig. 3 also shows an additional external contact 53 which provides an additional short connection to the shielding cylinder I to provide an effective earth connection to the internal shielding arrangement. A lead to the shielding arrangement, which also constitutes the linal anode, is also brought out through the tube stem asA shown in Fig. 1 in order to facilitate wiring connection of the static tube circuits.

Owing to its high deflection sensitivity the tube should be carefully shielded by earthed mu-metal shields. The screening between the two deecting plate systems can be further increased by providing a well fitting earthed externalscreen 54 between the leads to the two sets of deiiecting plates.V

An alternative method of assembly which has been found useful in many cases is to form the sets of deflecting plates into packets, each packet being constituted by a pair of deflecting plates with ilexible lead wires attached to them, and mounted on a pair of suitably shaped mica discs, the planes of which extend perpendicularly to the axis of the tube. One of the shielding partitions may be Vfixed to one of the mica discs. The packets may then be combined into` a single long packet by joining the individual packets together, for instance by eyeleting the top mica disc of one packet to the shielding partition xed to the bottom mica disc of the succeeding packetj The complete long packet, having the deflecting plates and shielding partitions in their proper relative positions, is then inserted into the shielding cylinder, care being taken to thread the flexible plate leads through the appropriate holes in the mantle of the shielding cylinder, and is nally secured by welding the bottom shield, forming the final anode, to the shielding cylinder, the internal partitions making contact with the shielding cylinder. The complete assembly is then lxed to the other parts of the electron gun structure by means of insulated wires, clamps or beads in a known way.

The described methods of assembly, which form part of the invention, combine the advantages of the low capacity of deflecting plates having leads sealed through the sides of the glass envelope with a precise jig assembly, which was hitherto only possible if the entire electrode structure were mounted on a common stem.

I claim: y a

1. In a cathode ray tube, an electrostatically shielded beam deiiecting structure comprising a plurality of deeoting plate units assembled in end to end relation, each unit including spaced insulating supports disposedv transversely of the unit at opposite ends thereof, a pair of `dei'lecting plates extending longitudinally of the unit between said supports, and means for supporting said plates from said supports, a metallic shielding tube embracing said assembly to enclose said units therewithin, and transverse metallic shielding means partitioning said tube into compartments each containing one of said units, whereby the pairs of deflecting plates ofsaid units are electrostatically shielded from each other, said partitioning screening means and said supports having aligned apertures therein to permit an electron beam to pass therethrough.

2. In a cathode ray tube, an electrostatically shielded beam defiecting structure comprising a plurality of deliecting plate units assembled in end to end relation, each unit including spaced insulatingv supports disposed `transversely of the unit at opposite ends thereof, a pair of deflecting plates extending longitudinally of the unit betweensaid supports, and means for supporting said plates from said supports, a metallic shielding tube embracing said assembly to enclose said units' therewithin, and a plurality of transverse metallic screensincluding a screen between each of said units and a screen at each'end of said assembly to partition said' tube into metallic Walled compartments each containing one of said units, whereby the pairs of deflecting plates comprised in said units are electrostatically shielded from one another, said partitioning screens and said supports having aligned apertures therein to allow an electron lbeam to pass therethrough,

3. A cathode ray tube comprising a vessel having an electron responsive surface at one end thereof, an electron gun structure at the other end thereof forprojecting a beam of electrons towards said surface, and a beam deecting structure between said gun structure and said surface,`

i said beam deilecting structure comprising a plu` rality of deflecting plate units assembled in end to end relation, each unit including spaced insulating supports disposed transversely of the unit at opposite ends thereof, a pair of deflecting plates extending longitudinally of the unit between said supports, and means for supporting said plates from said supports, a metallic shielding tube embracingsaid assembly to enclose said units therewithin, and a plurality of transverse metallic screens including a screen between each of said units and a screen at the end of said assembly which faces said gun structure to partition said tube into metallic walled compartments each containing one of said units, wherebythe pairs of deflecting plates comprised in said units are electrostatically shielded from one another, said partitioning screens and said supports having aligned apertures therein to allow an electron beam produced by said gun structure to pass 1 therethrough, and said partitioning screen at the end 'of said assembly which faces said gun structure constituting a nal accelerating anode of said gun structure.

4. A cathode ray tube comprising a vessel having a neck portion connected to a bulb portion, an electron responsive surface in said bulb portion, an electron gun structure in said neck portion for projecting a beam of electrons towards said surface; and a beam deflectingstructure disposed longitudinally in said neck portion between said gun `structure and said surface, said beam deflectingstructure comprising a `plurality of defleeting plate units assembled in end to end relationlongitudinally in said neck portions, each unit including spaced insulating supports disposed transversely of the unit at opposite ends thereof, a pair of deflecting plates extending longirtudinally of the unit between said supports, and means for supporting said plates from said supports, a metallic shielding tube embracing said assembly to ,enclose said units therewithin, .and transverse metallic shielding means partitioning said tube into .compartments each containing one of said units, whereby the pairs of deflecting plates .comprised in said units are electrostatically shielded from one another, said partitioning screening means and saidsupports having aligned apertures therein to permit an electron beam produced -by said gun structure to pass therethrough, and said shielding tube engaging the Wall of said neck portion to thereby locate said beam `deflecting structure in said neck portion.

5. A unitary beam-deflecting electrode structure for a cathode ray tube, .comprising a plurali-ty of structurally independent deflecting plate units each of which includes a pair of deecting plates mounted between and supported from a pair of insulating plates disposed transversely at opposite ends of the unit, said units being assembled end to end Valternately with transversely disposed metallic screening plates, means xing said units and screening plates in assembled relation. and a tubular metallic shield fitted on said assembly and aiiixed thereto, said shield enclosing said units therewithin and said screening plates peripherally engaging said shield to partition said shield into compartments each containing one of said units, whereby to shield the pairs of deflecting plates comprised in said units from one another, said screening plates and said insulating plates having aligned apertures therein to permit an electron beam to pass therethrough.

6. A unitary beam-deilecting electrode structure for a cathode ray tube, comprising a plurality of structurally independent defiecting plate units each of which includes a pair of deilecting plates mounted between and supported from a pair of insulating plates disposed transversely at opposite ends of the unit, said units being assembled end to end alternately with transversely disposed metallic screening plates, a plurality of stay Wires extending through said insulating plates and said screening plates to support said units and screening plates in aligned assembly, means xing said assembly to said stay Wires, and a tubular metallic shield tted on said assembly and affixed thereto, said shield enclosing said units therewithin and said screening plates peripherally engaging said shield to partition said shield into compartments each containing one of said units, whereby to shield the pairs of deflecting plates comprised in said units from one another, said screening plates and said insulating plates khaving aligned apertures therein to permit an electron beam to pass therethrough.

'7. A unitary electrode structure as defined in claim 6, wherein said screening plates are conductively xed to said stay wires and said tubular shield is conductively fixed to said screening plates. Y l

8. A unitary beam-deecting electrode structure for a cathode ray tube, comprising a plurality of structurally independent deecting plate units each of which includes a pair of deiiecting plates mounted between and supported from a pair of insulating plates disposed transversely at opposite ends of the unit, said units being assembled end to end alternately with transversely disposed metallic screening plates including a plate at each end of the assembly, a plurality of parallel spaced stay wires extending through said insulating plates and said screening plates to support said units and screening plates in aligned assembly, said screening plates being conductively xed to said stay wires, and a tubular metallic `shield extending the .full length of said assembly being fitted therearound in engagement with the peripheries of said screening plates and conductively iixed thereto, whereby said screening plates partition said shield into shielded compartments containing said units individually, said screening plates and said insulating plates having aligned apertures therein to allow an electron beam to pass therethrough.

9. A unitary electrode assembly for a cathode ray tube, comprising an electron gun structure for producing a beam of electrons, and a unitary beam-deiiecting structure comprising a plurality of structurally independent deflecting plate units each of which includes a pair of dellecting plates mounted between and supported from a pair of insulating plates disposed transversely at opposite ends of the unit, said units being assembled end to end alternately with transversely disposed metallic screening plates, a plurality of stay wires extending through said insulating plates and said screening plates to support said units and screening plates in aligned assembly, means fixing said assembly to said stay Wires, and a tubular metallic shield tted on said assembly and afiixed thereto, said shield enclosing said units therewithin and said screening plates peripherally engaging said shield to partition said shield into compartments each containing one of said units, whereby to shield the pairs of deflecting plates comprised in said units from one another, said screening plates and said insulating plates having aligned apertures therein to permit an electron beam to pass therethrough, said stay wires having extensions projecting from the end of the beam deilecting structure which faces said gun structure, and means supporting said gun structure from said extensions in alignment with said apertures in said screening plates and insulating plates.

10. A unitary electrode assembly as deflned in claim 9, wherein said electron gun structure includes av pair of axially spaced insulating support plates through which said stay Wire extensions pass to support said gun structure therefrom, said support plates being fixed to said extensions.

11. A unitary electrode assembly for a cathode ray tube, comprising an electron gun structure including a pair of axially spaced insulating support discs disposed perpendicular to the axis of said structure; a plurality of beam deflecting units each comprising a pair of axially spaced insulating support discs at opposite ends of the unit, support members extending between and xed to said discs to hold said discs in spaced relation, and a pair of deflecting plates disposed between said discs and supported by said support members in insulated relation to each other; a plurality of peripherally flanged metallic shielding discs; said gun structure, units, and shielding discs being assembled in axially aligned relation with said gun structure and said units disposed end to end in succession and said shielding discs disposed one at each end of the succession of units and between the end insulating support discs of adjacent units, said shielding discs and said support discs of both said gun structure and said units all having locating holes similarly posiltioned therein, a plurality of stay wires extending parallel through said locating holes in said shielding and support discs to thereby support the assembly in axially aligned relation, means xing said support discs of said gun structure to said stay Wires, means conductively nxing said shielding discs to said stay Wires, and a metallic shielding cylinder fitted on saidassembly and extending the full length of said succession of units, said cylinderengaging the peripheral flanges of said shielding discs and being conductively fixed thereto, whereby said shielding discs partition said cylinder intoshielded compartments containing said units individually, said shielding discs and said support discs of said units having apertures thereingin` alignment withsaid gun structure to permit anelectronv beam produced by said gun to pass.` therethrough, Vsaidcylinder having apertures therein Opposite the respective compartments thereof, and lead-in conductors extending through said apertures and connected to the` pairs of deflecting plates in the respective compartments.

12. A cathode ray tube comprising an envelope enclosing an electrode assembly mounted therein, said assembly including a beam deflecting structure comprising a plurality of deiiecting plate units assembled in end to end relation, each unit including spaced insulating supports disposed transversely of the unit at opposite ends thereof, a pair of deflecting plates extending longitudinally of the unit between said supports, and means for supporting said plates from said supports, a metallic shielding tube embracing said assembly to enclose said units therewithin, and transverse metallic shielding means partitioning said tube into compartments each containing one of said units, whereby the pairs of deflecting plates of said units are electrostatically shielded from each other, said partitioning screening means and said supports having aligned apertures therein to permit an electron beam to pass therethrough, said shielding tube having apertures in the wall thereof opposite the respective compartments thereof, and lead-in conductors connected to the pairs of deflecting plates in the respective compartments extending respectively through said apertures in said tube and being sealed through said envelope at points adjacent the respective positions of s said apertures.

13. A cathode ray tube as defined in claim 12, and further comprising external metallic shielding means around said envelope and extending between the lead-in conductors of the respective pairs of deilecting plates.

14. A cathode ray tube as defined in claim 12, wherein said units include at least two pairs of deiiecting plates adapted for beam deflection in different planes, and the lead-in conductors connected to the plates of said repairs are sealed through said envelope at points'in substantially the same plane.

15. A cathode ray tube comprising a vessel containing an electrode assembly including an electron gun for producing a beam of electrons, and a beam deiiecting structure mounted in alignment with said gun, said deilecting structure comprising a plurality of structurally independent deflecting plate units each of which includes a pair of deflecting plates mounted between and supported from a pair of insulating plates disposed transversely at opposite ends of the unit, said units being assembled end to end alternately with transversely disposed metallic screening plates including a plate at each end of the assembly, a plurality of parallel spaced stay wires extending through said insulating plates and said screening plates to support said units and screening plates in aligned assembly, said screening plates being conductively iixed to said stay wires, and a tubular metallic shield extending the full lengthof said assembly being tted therearound in engagement with the peripheriesof said. screening plates andv conductively fixed. thereto. v,whereby said screening plates partition said shield into shielded compartments containing said units individually, said screening plates and said insulating plates having apertures thereinin alignment with said gun ,toallow an electron beam produced by said gun toV pass therethrough, and the said` one of said screening plates at the end. of said unit assembly Awhich facesV said gun constituting a xed anode of said gun.

161A cathode` ray `tube comprising a vessel having an electron responsive surface at one end thereof, an electron gun at the other end thereof for projecting a beam of electrons towards said surface, and a beam deflecting structure between said gun and said surface mounted in alignment with said gun, said deflecting structure comprising a plurality of structurally independent deiiecting plate units each of which includes a pair, of deflecting plates mounted between and supported from a pair of insulating plates disposed transversely at opposite ends of the unit, said units being assembled end to en d alternately with transversely disposed metallic screening plates including a plate at each end of the assembly, a plurality of parallel spaced stay wires extending through said insulating plates and said screening plates to support said units and screening plates in aligned assembly, said screening plates being conductively xed to said stay wires, and a tubular metallic shield extending the full length of said assembly being fitted therearound in engagement with the peripheries of said screening plates and conductively fixed thereto, whereby said screening plates partition said shield into shielded compartments containing said units individually, said screening plates and said insulating plates having apertures therein in alignment with said gun to allow an electron beam produced by said gun to pass therethrough, the said one of said screening plates at thel end of said unit assembly which faces said gun constituting a final anode of said gun, an internal conductive coating on said vessel extending from a point adacent said tubular shield to a point adjacent said electron responsive surface, and means carried by said deflecting structure electrically connecting said coating to said shield.

17. A cathode ray tube comprising an enclosing vessel containing an electrode assembly including an electron gun for producing a beam of electrons, and a beam deflecting structure mounted in alignment with said gun, said vessel terminating at one end in a base carrying terminal contacts for establishing electrical connection to electrodes of said assembly, said deflecting structure comprising a plurality of structurally independent deflecting plate units each of which includes a pair of deflecting plates mounted between and supported from a pair of insulating plates disposed transversely at opposite ends of the unit, said units being assembled end to end alternately with transversely disposed metallic screening plates including a plate at each end of the assembly, a plurality of parallel spaced stay `wires extending through said insulating plates and said screening plates to support said units and screening plates in aligned assembly, said screening plates being conductivelyliixed to said stay wires, and a tubular metallic shield extending the full length of said assembly being tted therearound in engagement with the pemames GERHARD Y LJEBMANN.-