US2225917A - Electron discharge device - Google Patents

Description

p38, 24, H MAHL 2,225,917

ELEL'IRON DISCHARGE DEVICE Filed may 23. 193B A TTORNEV Patented Dec. 24, 1940 PATENT OFFICE n ELECTRON DISCHARGE DEVICE Hans Mahl, Berlin, Germany, assigner to General Electric Company, a corporation of New York Application May 28, 1938, Serial No. 210,612 In Germany June 2, 1937 Claims.

The invention relates to electron discharge devices, and more particularly, to electron lens systems in cathode ray tubes wherein mesh electrodes are provide.

5 In the past, it was proposed to use mesh electrodes in connection'with electron-optical systems. The data of earlier times have been collected in the book by Brche and Scherzer, Geometrische Elektronenoptik (Berlin 1934).

But as explained there, (compare page 44, note 4) not only the author of the book, but also other experts have been convinced of the uselessness of mesh electrodes in electron-optical ray-passages. Hence, every kind of mesh electrode, whether twofold meshes disposed one behind the other for attaining a double layer, (Fig. 31 of the boo-k cited), or single mesh, for attaining a focal spot (Brennfleck)-(Fg. 82 of this book), or simple acceleration mesh electrodes (compare the article by Henneberg and Recknagel in the Zeitschrift fur Technische Physik, vol. 16, p. 232, note 15, 1935), has been avoided in electron-optical systems, and in view of those cited and other objections to the use of mesh electrodes, such as have repeatedly been made of late years by experts, the state of the art can be assumed to be such, that mesh electrodes do not come into the question has electron lens elements in electron-optical systems.

In connection with the experiments, on which the present invention is based, this generally existing prejudice has been ignored and the surprising result was attained, that the introduction of a mesh into the electron-optical ray-passage exercised no unfavorable influence on the reproduction, if only the mesh electron be made ne enough. Since these experiments have once been carried through, the explanation for this result appears simple. Whilst indeed, hitherto several mesh electrodes disposed closely one behind the other have been used for attaining double layers for electron-optical purposes and, from the unfavorable results attained therewith, the general uselessness of mesh electrodes was a foregone conclusion, according to the invention only one element of the electron lens is developed as mesh electrode in the electron-optical ray passage. In this case, the path of the rays shows, in fact, only those disturbances which occur at single mesh electrode, and which need not lead to any considerable disturbance of the picture. On the other hand, the case is dilerent, when several mesh electrodes are provided in the ray passage one behind the other. Since the meshes of the different mesh electrodes do not lie on correspond- (Cl. Z50-162) ing points of the ray passage, mutual inliuencing of (interference with) the mesh electrodes occurs, which renders the picture useless.

The employment of one mesh electrode, as lenselectrode in electron optical devices, has various -5 advantages. It is, as a matter of fact, diii'icult to make the electron optical copy of great cathode surfaces, particularly of fiat cathodes. The reason for this is that the electron optical aberrations increase sharply, as the distance from the 10 optical axis increases. On the other hand, owing to the nature of the potential field, either the electron exit is restricted to a very small scope or, if electrons can indeed pass out of a considerable scope, then generally the electrons passing 15 out at a greater distance from the axis will fall on the focusing electrodes, which operate as apertured diaphragms o-r stops. These disadvantages can be avoided by the employment of suitably shaped, ne mesh electrodes, or of very thin 20 foil electrodes which are equally pervious to the electrons.

The invention will be more particularly described with reference to the drawing, in which Fig. 1 shows diagrammatically one simple form 25 of the invention, wherein the focusing mesh electrode has substantially the same curvature as the focusing electrostatic eld, while Figs. 2 through 6 show varying modifications of the electron lens system shown in Fig. l. 30

In Fig. l, K indicates the electron emitting, large-surface cathode from which the electrons are focused, A is a mesh electrode which serves as an anode, and L the luminescent screen. The potentials can be selected so that for instance, as 35 compared with K, A has a potential of 5000 volts and L has the same or a higher potential. In the figure there are drawn in further the equi-potential lines and the passage of the ray, starting from a point of the cathode. 'Ihe lens effect of the 40 device can easily be seen from the potential field and the passage of the ray.

While it was assumed, in connection with Fig. 1 that the mesh electrode was developed as lens electrode, and represents, apart from the cathode 45 and luminescent screen, the only lens electrode, in Fig. 2 the case has been illustrated, in which the mesh electrode serves solely for the attainment of a high electron emission of the cathode. The mesh electrode N is here developed as a Ilat 50 one and disposed in front of the cathode K. For focusing, the three cylindrical electrodes Z1, Z2 and Z3 have progressively increasing potentials. While these three electrodes may have potentials, for instance, of 300, 900 and 5000 volts as com- 55 pared with the cathode, the mesh electrode N has advantageously a higher voltage as compared with the cathode than the first cylinder, for instance 2000.

In most cases, however, the mesh electrode will be employed for the improvement or for the support of the lens field generated by the remaining electrodes. It can iind employment in the same way, in combination with electric as also with magnetic lenses. In Figs. 3 and 4 the employment of a mesh electrode N is provided for, in combination with a further cylindrical electrode A. The potential `field, developing between the cathode K and the anode A is iniiuenced by the curve of the mesh electrode. In the case illustrated in Fig. 4, in which, owing to the effect of the mesh electrode in front of the cathode K, a collecting lens effect is attained, it may in some circumstances be desirable to impart to the mesh electrode a concave curve with regard to the cathode, so that first a dispersing eifect takes place. The collective effect, necessary for the focusing, can be generated through the eld between the mesh electrode and the cylindrical anode A.

The employment of a mesh electrode for the production of corrected electron lenses, offers special advantages. In this respect, the correction may refer to the geometrical faults (spherical aberration and the like) or to the elimination of the chromatic fault. For the elimination of the chromatic fault, it is of importance that with mesh electrodes dispersing systems can be produced, while lenses working without mesh electrodes, as is known, always represent collecting lenses, inasfar as they are in i'leld free space.

The possibility of the correction of a lens field, by imparting a suitable form, may be seen from Figs. 5 and 6. In Fig. 5 the uncorrected lens device is represented, in which the mesh electrode has a hollow spherical form. Fig. 6 shows the same system, but corrected by a special forma.- tion of the mesh electrode.

It is, of course, self evident that the correction of a system can only take place if, with otherwise unchanged potentials and with unchanged form and distance apart or the electrodes, there is imparted to the mesh electrode a certain form, a certain position with regard to the other electrodes and a certain potential as compared with the remaining potentials. Thle focusing system in accordance with the invention can be employed everywhere, where the systems hitherto known have been employed, thusl more particularly, with cathode ray tubes, electron microscopes, in electron valves and the like.

As will be clear from the previous explanation, the devices, working with mesh electrodes in accordance with the invention, and cited as embodiments as examples, have yet ai further property, which distinguishes them. from the devices of the known kind, provided with mesh electrode lenses. Since between the mesh electrode and the next electrode (cathode or anode) there is a considerable space, the effect of the mesh electrode on the potential iield can be divided into two constituent parts, into one undesired distortion, effective in the direct proximity of the mesh electrode, owing to the field penetrating through the meshes and into an influencing of the potential. In the examples given, the second eifect preponderates, owing to the high speed of the electrons at the mesh electrode and owing to the great intermediate space between the mesh electrode and the remaining electrodes. It is possible, that this fact alone, or in combination with the above mentioned avoidance of interferences, with the employment of one mesh electrode only, guarantees the usefulness of the devices described. In the first case, the employment of several mesh electrodes at a sufficient distance from each other might be possible. Since with foils no penetration of the eld takes place, all the devices hitherto proposed with mesh electrodes can have thin foil electrodes substituted for the mesh electrodes.

Having described my invention, what I claim is:

1. An electron focusing system comprising a planar cathode, a cylindrical electrode positioned normal to said cathode, one end of said cylindrical electrode surrounding said cathode, and a curved mesh electrode at least partially surrounded by said cylindrical electrode and normal to the axis of said electrode.

2. An electron focusing system comprising a planar cathode, a cylindrical electrode positioned normal to said cathode, one end of said cylindrical electrode surrounding said cathode, and a curved mesh electrode normal to the axis of said cylindrical electrode and partially surrounded by said cylindrical electrode.

3. An electron focusing system comprising a planar cathode, a cylindrical electrode positioned normal to said cathode, one end of said cylindrical electrode surrounding said cathode, and a curved mesh electrode normal to the axis of said cylindrical electrode and completely surrounded by said cylindrical electrode.

4. A cathode ray tube comprising a symmetrical planar cathode, a cylindrical anode in register with said planar cathode, said cylindrical anode having its axis coinciding with the axis of symmetry of said cathode, said anode being adapted to be supplied with positive potential relative to said cathode to produce an electrostatic field having axially symmetrical uni-potential surfaces, and a curved mesh focusing electrode in register with said cathode and at least partially surrounded by said cylindrical anode, said mesh electrode having a curvature substantially identical with one of said unipotential surfaces.

5. A cathode ray tube comprising a planar cathode, a target electrode, said target electrode and said cathode being symmetrical about a common axis, cylindrical anode means positioned intermediate said cathode and said target electrode to produce an electrostatic iield having uni-potential surfaces of predetermined curvature between said cathode and said target electrode, and a curved mesh electrode normal to said common axis and positioned intermediate said cathode and said target, said mesh electrode having a curvature substantially the same as one of said uni-potential surfaces.

HANS MAHL.

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