US2264709A

US2264709A - Electron mirror

Info

Publication number: US2264709A
Application number: US181289A
Authority: US
Inventors: Nicoll Frederick Hermes
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1936-12-24
Filing date: 1937-12-23
Publication date: 1941-12-02
Anticipated expiration: 1958-12-02
Also published as: FR831215A; GB489428A

Description

ELECTRON MIRROR Filed Dec. 23. 195'? ATTORNEY.

Paienteel Der:u 2, l94l ELECTRON WRROR Frederick Hermes Nicoll, lickenham, England,

assigner to Electric d; Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application December 23, 1937, Serial No. 181,289

n Great Britain December 24, 1936 (Cl. 17g- 7.2)

Claims.

This invention relates to electron mirrors. Electron mirrors are known in which a beam of electron is reliected by the use of a plate maintained at a relatively low potential with respect to a grid electrode mounted in front of the plate the latter being maintained at a negative potential with respect to the source of electrons, the potential between the grid and plate serving to cause reflection of the electrons through the grid.

This form of electron mirror is disadvantageous owing to the use of a grid and further, other defects occur and in particular a diiculty arises in obtaining reiiection free from distortion.

In another kind of electron mirror refiection occurs between an apertured plate and -an electrode maintained at an appropriate negative potential. In this case the reflected electrons pass back through the aperture. Hence difficulties arise due to limitations of the size of the aperture.

It is the chief object of the present invention to provide an improved electron mirror in which the difficulties above mentioned are avoided or reduced.

According to the invention, an electron mirror is provided comprising a tubular and preferably cylindrical electrode, and a further electrode, the tubular electrode being adapted to be disposed nearer to the source of electrons, than the further electrode and being adapted to have applied to it high positive potential relative to that of the source of electrons, and the further electrode being adapted to be maintained at such a potential as to produce or constitute a zero equi-potential surface eiectively closing the end of the first electrode remote from said source so that electrons passing into said first mentioned electrode are reected back towards the source. The further electrode is also preferably of cylindrical form maintained at a potential substantially zero or negative with respect to the source of electrons. In this case, the positive electrode causes the electrons to be accelerated towards the negative electrode and due to the diiering potentials of the two electrodes a zero equi-potential surface is formed between the electrodes so that when the electrons approach this surface, they are decelerated and reected towards their origin. The electrodes are preferably of circular shape in cross section and it is found that by adjusting the negative potential applied to the electrode remote from the source that the mirror can be arranged to function as a convex, concave or plane mirror. It will be understood that between the two electrodes numerous equi-potential surfaces are formed all of which are of different shape or curvature and hence by selecting one of these surfaces to function as the reecting surface a. mirror of concave, plane, or convex form can be obtained. An electron mirror is therefore provided in which it is not necessary to employ a grid such as is described above, and hence a suitable screen can be disposed to receive the reflected electrons, the screen producing an unimpaired image of the electrons.

The electron mirror in accordance with the invention may be applied to such devices as light transformers, television transmitting tubes and measuring instruments as will hereinafter be more particularly described, but is also applicable for many other purposes.

Since the electrons are reflected adjacent to the zero equi-potential surface, it is possible in certain cases that this equi-potential surface can be replaced by a conductor in the form of a plate suitably shaped and maintained at a zero potential, thus rendering it unnecessary to provide an additional cylindrical electrode and an additional voltage source to cause reection of the electrons.

For the purpose of describing the invention and the method of carrying the same into effect more in detail, reference will now be made to the accompanying diagrammatic drawing, in which:

Fig. 1 illustrates an electron mirror constructed in accordance with the invention,

Fig. 2 illustrates the application of the invention to a light transformer,

Fig. 3 illustrates the application of the invention to a cathode ray television transmitting tube,

Fig. 4 illustrates a modification of Fig. 3 in which the electron mirror is also employed as a lens to focus a scanning beam of cathode rays, and

Fig. 5 illustrates the application of the invention to a cathode ray measuring instrument.

As shown in Fig. 1, two concentric cylindrical electrodes I and 2 are provided, electrons from a source not shown entering the electrode I as indicated by the arrow 3. The electrode I is maintained at a positive potential and the electrode 2 is maintained at a negative potential with respect to the electron source, and thus electrostatic eld indicated by the curved equipotential lines shown in Fig. 1 is generated between the two electrodes, and at a particular position a zero equi-potentlalsurface is formed. The electron beam is decelerated by the equipotential surfaces until it is nally reflected at the zero equi-potential surfacel whence it is accelerated back towards the source. By adjusting the potential applied tothe electrode 2, the position of the zero equi-potential surface can be adjusted at will, audit is therefore possible to provide a concave, plane or convex reflecting surface, whilst, in each casethe equipotential surfaces in front of the reflecting surface constitute a divergent lens. It is also found that the magnification and focal length of the reflecting surface is adjusted on variation of the negative potential applied to the electrode 2.

It is known that 'electron lenses give rise to types of distortion which impart to an image a barrel or pin-cushion shape. Itls found that with the present invention such distortion can be avoided by suitably adjusting the negative potential applied to the electrode 2, this negative potential being adjusted either by variation of the actual potential applied to the electrode 2 or by variation of the potential of the electron source, the image so produced being varied through a series of configurations from an image of a substantially barrel shape to an image of a pincushion configuration. Distortion. is therefore avoided by selecting a suitably shaped equipotential surface. The diameters of the electrodes I and 2 may be the same or they may be different, the ratio of the diameters in conjunction with the applied potentials being so chosen to provide an image free from aberration. Since reflection occurs at a zero equi-potential surface, and in as much as the electrons do not pass through such surface, it is possible to replace the zero equi-potential surface by a conductor maintained at a zero potential, thus avoiding the necessity of an additional voltage to cause reflection. In addition a magnetic iield or fields generated by a suitable coil or coils may be provided to assist in the production of the mirror.

The conductor maintained at zero potential may have a fiat or curved surface, and if desired it may be formed of such a-shape as to correct the electron optical system in respect of aberration set up due to the geometrical properties of the system not being ideal.

Fig. 2 of the drawing illustrates the application of the invention to a light transformer or to an electron telescope. In this case an optical image is projected on to a photo-electrically active screen 4 which may be transparent, the screen 4 being connected to a cylindrical electrode 5 maintained at the same potential as the screen l, for exampl'e at zero potential. A second electrode 6 maintained at a high positive potential as for example from 5,000 to 10,000 volts, is disposed caxially with the electrode 5, and a further electrode 'I also co-axially arranged is provided maintained at a negative potential with respect to the screen 4. At a suitable position in the electrode 6 there is arranged a fluorescent screen 8, the disposition of the screen being such that it does not affect the passage of electrons and is uninfluenced by the electrostatic iield existing between the several electrodes; that is to say it is disposed in a substantially field-free space. A magnetic focussing coil 9 may or may not be provided to assist in focus-sing the photo-electrons emanating from the screen d. In operation the photoelectrons liberated from the screen II are accelerated by the field between electrodes and B and are focussed by such field as indicated by the dotted lines, and are eventually decelerated by the field between the electrodes 6 and 'I which functions as the electron mirror, the electrons are reflected by the zero equi-potential surface and are projected on to the fluorescent screen 8. The image produced on the fluorescent screen may be viewed through the end of the electrode 1. 'I'he electrons when reflected by the zero equipotentlal surface are accelerated towards the screen 8 and are focussed by the dlverging field. Aberration and distortion of the image is corrected by choice of the potential applied to the electrode 1 which determines the curvature of the reflecting equi-potential surface. If it is desired to replace the reflecting surface by a conductor as suggested above, the conductor may be formed by providing a semi-transparent conducting layer on a transparent support.

Known forms of light transformers suffer from the disadvantage that the light amplification obtainable is limited partly by retroaction which occurs between the fluorescent screen and the photo-electric screen. With the arrangement shown in Fig. 2 it will be observed that no such retroaction can occur in as much as the fluorescent surface of the screen 8 is not opposite the photo-electric screen Il, and hence the fluorescent image cannot be received or projected on to the screen d.

It will of course be understood that in this example and in the examples hereinafter referred to, the various components are mounted in an evacuated envelope which is omitted from the drawing for the sake of clarity. In addition, it must be understood that the figures shown are purely diagrammatic.

Fig. 3 of the drawing illustrates the application of the invention to a cathode ray television transmitting tube. In this case a photo-electrically active screen li is maintained at the same potential as an electrode 5, electrodes 6 and I being provided similar to the arrangement shown in Fig. 2. In this case however, the uorescent screen 8 is replaced by a mosaic screen I0 associated with a signal plate II, and between the screen d and the signal plate Il a shield I2 is disposed. Photo-electrons reflected by the electron mirror are focussed on to the mosaic screen which is constructed in known manner, such photo-electrons causing electrostatic charges to be accumulated in the mosaic screen according to the intensity of the incident electrons. The screen is arranged to be scanned by a beam of cathode rays generated from a source I3 and focussed in known manner by an electrode system I4, the cathode ray beam so generated being moved in co-ordinate directions by suitable deilecting means either of the electrostatic or electromagnetic type not shown, the beam restoring the elements of the mosaic screen to an equilibrium potential, such restoration generating signals in the signal plate II which are fed to a suitable transmitter. In the arrangement shown in Fig.l 3, the screen I and electrode 5 may be maintained at a zero potential, the electrode 6 at about 400 volts, the electrode 1 at a suitable negative potential, the source I3 at a negative potential of about 1100 Volts and the electrode system I4 at a negative potential of about 800 volts. Any distortion of the image produced on the mosaic screen IU can be corrected by a suitable adjustment of the negative potential of the electrode l. The zero equi-potential surface may be replaced by a conductor as stated above lthe shape of the conductor being governed by the shape of the desired zero equi-potential surface. The shield I2 is of course necessary in order to prevent impingement of the photo-electrons on the signal plate II. Since the cathode ray beam scans the mosaic screen I obliquely giving rise to what is known as keystone distortion, it is of course necessary to correct for such distortion which is effected in known manner.

Fig. 4 illustrates a4 modification of the arrangement shown in Fig. 3 in which the cathode ray beam is arranged to scan the mosaic screen substantially normally and in which the electrostatic eld of the electron mirror is arranged to function as the focussing means for the scanning beam of electrons. The various elements of the apparatus shown in Fig. 4 are similar to those shown in Fig. 3 with the exception that the source of electrons I3, electrode system I4, are arranged co-axlally with the other elements as shown. The electrode 1 in this example may be maintained at a negative potential of about 600 volts, the other elements being maintained at the potentials referred to in connection with Fig. 3 or at other suitable potentials as may be found necessary. In this case so long as the electrostatic lens afforded by the field between the electrodes 6 and 1 is, of a suiciently large aperture, and is also substantially free from coma, then the lens serves to focus the electrons from the source I3 without substantial distortion.

Fig. of the drawing illustrates the application of the invention to a W voltage measuring instrument of the cathode ray type. It is found with existing types of similar instruments in which a cathode ray beam is deflected on to a fluorescent screen, the amount of deflection affording an indication for measuring purposes, that the brightness of the fluorescent screen is greater on the side bombarded by electrons, and that the secondary emission properties of the fluorescent screen determined the lowest voltage that can be used. In the latter case the conductivity of the screen is also a determining factor, and where large screens are employed the secondary emission properties are of prime importance. By employing an electron mirror in accordance with the invention, it is possible to employ a metal screen or support on which the fluorescent material is deposited and in this way it is possible to employ lower voltages to obtain greater brightness from the fluorescent screen. It is also possible to obtain a greater degree of sensitivity compared with the known type of cathode ray tube for the same overall length. As shown in Fig. 5 electrons emanating from a source I5 associated with a shield I6 maintained at the same potential as the electron source, are accelerated and partially focussed by a lens existing between electrodes I1 and I8 which may be maintained respectively at positive potentials of 50 and 100 volts. At a suitable position in the electrode I8 is a pair of defiecting plates I9 to which the potential to be measured is applied. The electron beam passing through the two plates is deflected to an extent determined by the amount of the applied potential. At the end of the electrode I8 a fluorescent screen 20 is provided having a central square shape aperture, the fluorescent screen being deposited on a metal support. Two further electrodes 2I and 22 are provided, the electrode 22 being maintained at a negative potential with respect to the source of electrons I5, such potential being approximately volts. The electrodes 2I and 22 generate an electron mirror as described above, and the electrons from the source I5 are thus reflected by the mirror on to the fluorescent screen 20. The distance of movement of the spot of light generated by the screen is twice as great compared with the case in which no electron mirror is employed. If desired, the zero equi-potential surface of the mirror may be replaced as described above by a conductor which is semi-transparent so that movement ol' the light spot can be viewed through the open end of the electrode 22.

An arrangement generally similar to that shown in Fig. 5 might be used in an electron microscope. In this case the electron beam from the source I5 would have to be made of the necessary cross-sectional area to cover an object or part of an object to be examined and deflecting plates I9 might be omitted. The object to be examined would be mounted on a suitable grid or slide which might be located within the cylindrical electrode I1 clear of the lens field at the end of that electrode.

Although in the above description certain specic applications of the invention have been described it is of course to be understood that the electron mirror in accordance with the invention may be employed for other purposes.

I claim:

l. An electron device comprising a source of electrons and an electron mirror comprising a tubular electrode and a further electrode, said tubular electrode being positioned in register with and nearer to the source of electrons than said further electrode, means to maintain said tubular electrode at a high positive potential relative to that of the source of electrons, said further electrode being positioned in register with said tubular electrode, means to produce a zero equi-potential surface effectively closing the end of the first electrode remote from said source by maintaining said further electrode at a potential negative with respect to said source and an electron utilizing surface enclosed by said tubular electrode, said utilizing surface facing said zero equi-potential surface.

2. An electron mirror as claimed in claim 1 and in which said further electrode is tubular and maintained at a potential lying between zero and a predetermined maximum negative value with respect to the source of electrons.

3. An electronic device comprising a source of electrons, a tubular electrode and a further electrode, said tubular and said further electrodes lying on an axis common to said source of electrons, means for maintaining said tubular electrode at a positive potential with respect to said source of electrons, an electron lens positioned intermediate the source of electrons and saidtubular electrode, a target electrode having an active surface positioned within said tubular electrode and means to produce a zero equi-potential surface intermediate said tubular electrode and said further electrode by maintaining said further electrode at a potential whose value lies between zero and a predetermined maximum negative value with respect to said source of electrons, said active surface of the target electrode facing said zero equi-potential surface.

4. An electronic device as claimed in claim 3 and wherein said further electrode has a tubular electrode of substantially the same diameter as said first-named tubular electrode.

5. An electron optical system including in combination a source of electrons adapted to emit a beam of electrons. a tubular electrode maintained at a high positive potential relative to said source, a further electrode remote from said source to produce Aa. zero equi-potential surface relative to said source between said tubular electrode and said further electrode whereby electrons from said source are reflected, and target electrode means positioned within said tubular electrode for utilising the electrons so reflected, said target electrode means having an active surface facing said further electrode on which said reiiected electrons are focussed.

6. An electron optical system including in combination a source of electrons adapted to emit a beam of electrons, a tubular electrode maintained at a high positive potential relative to said source, an electron lens arranged between said source of electrons and said tubular elec, trode, a further tubular electrode at the end of said first named tubular electrode remote from said source adapted to produce a zero equi-potential surface relative to said source intermediate said rst and said second named tubular electrodes the electrons being reected from said surface, and target electrode means positioned within said first named tubular electrode for receiving the electrons so reected, and cathode ray means for scanning said target electrode.

7. An electro-optical system as claimed in claim 6 wherein the source of electrons is a substantially planar photoelectric cathode for receiving an optical image.

8. An electro-optical system as claimed in claim 6 wherein said source of electrons is a photoelectric cathode and wherein said cathode ray means are positioned angularly with respect to the target electrode.

9. An electro-optical system as claimed in claim 6 wherein a source of electrons is a photoelectric cathode and said cathode ray means are positioned normal to said target electrode and facing said photoelectric cathode.

l0. An electron optical device comprising a planar photoelectric cathode, a tubular electrode l positioned in register with said photoelectric cathode, means to maintain said tubular electrode at a positive potential with respect to said cathode, an electron lens intermediate said cathode and said tubular electrode, a further elecl1. An electron optical device as claimed in claim 10, and wherein said target electrode has an active surface comprising luminescent material. l

12. An electron optical device as claimed in claim 10 wherein said target electrode comprises a. photosensitive mosaic.

13. An electron optical device comprising a planar photoeiectric cathode, a tubular electrode positioned in register with said photoelectric cathode, means to maintain said tubular electrode at a positive potential with respect to said cathode, an electron lens intermediate said cathode and said tubular electrode, a further electrode positioned in register with said tubular electrode, means to maintain said further electrode at a potential negative with respect to said tubular electrode whereby a zero equi-potential surface is formed intermediate said tubular electrode and said further electrode, a target electrode having an active surface positioned normal to the axis of the tubular electrode and wholly surrounded by said tubular electrode, said active surface being on the side opposite from the side of the target facing said cathode, and an electron gun positioned beyond said target electrode for scanning said target electrode.

la. An electron optical device comprising a planar photoelectric cathode, a tubular electrode positioned in register with said photoelectric cathode, means to maintain said tubular electrode at a positive potential with respect to said cathode, an electron lens intermediate said cathode and said tubular electrode, a further electrode positioned in register with said tubular electrode, means to maintain said further electrode at a potential negative with respect to said tubular electrode whereby a zero equipoten tial surface is formed intermediate said tubular electrode and said further electrode, a target electrode having an active surface positioned normal to the axis of the tubular electrode and wholly surrounded by said tubular electrode, said active surface being on the side opposite from the side of the target facing said cathode, and an electron gun positioned beyond and in angular relationship to said target electrode for scanning said target electrode.

l5. An electron optical device comprising a planar photoelectric cathode, a tubular electrode positioned in register with said photoelectric cathode, means to maintain said tubular electrode at a positive potential with respect to said cathode, an electron lens intermediate said cathode and said tubular electrode, a further electrode positioned in register with said tubular electrode, means to maintain said further electrode at a potential negative with respect to said tubular electrode whereby a Zero equi-potential surface is formed intermediate said tubulai electrode and said further electrode, a target electrode having an active surface positioned normal to the axis of the tubular electrode and wholly surrounded by said tubular electrode, said active surface being on the side opposite from the side of the target facing said cathode, and an electron gun positioned on the same side as the active surface of said target electrode and normal thereto for scanning said target electrode.

FREDERICK HERMES NICOLL.