US2831143A - Method of eliminating ion spots on the photo-cathode of electronic optical devices - Google Patents

Description

Aprll 15, 1958 w. HEIMANN 2,831,143

METHOD OF ELIMINATING- ION SPOTS ON THE PHOTO-CATHODE OF ELECTRONIC OPTICAL DEVICES Filed July 7, 1954 FIG. 2

Inmemon WALTER HEIMANN (Ittorneg METHUD 01F ELIMINATING ION SPOTS ON THE PHOTO-CATHGDE F ELECTRUNIC GPTEQAL DEVICES Walter Heimann, Wiesbaden-Dotzheim, Germany Application July 7, 1954, Serial No. 441,749

Claims. (Cl. 315-) This invention relates to an improvement in electronic optical devices, specifically in relation to television camera tubes.

Referring to the drawing, Fig. l is a partial sectional view of an electronic optical device of the prior art including a photo-cathode, and Fig. 2 is a similar view of an electronic optical device embodying the present invention.

The well-known electronic optical device of a television image-dissector tube consists of a flat photo-cathode and a cylinder arranged ahead of this cathode which serves as anode. Fig. 1 of the drawing illustrates schematically such a device. At the bottom of a glass cylinder 1 a disc shaped photo-cathode 2 is located. A cylindrical anode 3 is placed ahead of the photo-cathode within this glass cylinder. If voltage is applied, the potential gradients generate an accelerating field 4 which acts upon the electrons released by the cathode and accelerates them in the direction of the positive anode cylinder. An electrically energized coil which surrounds the cylindrical part of the anode tends to arrange these electrons in a circular bundle. The coil is not indicated on the drawing.

Exeprience indicates that in most cases a spot develops on the photo-cathode after a certain period of usage which gradually leads to the destruction of the photo-cathode. Investigations have indicated that this spot is caused by the continuous bombardment of ions upon the photocathode. Even if the tube has been fabricated with an almost perfect vacuum, infinitesimal quantities of gas will remain within it which will become ionized as soon as the normal working potential of 1000 volts is applied. These ions are positively charged and therefore are directed towards the negative cathode, i. e. in the opposite direction of the electrons. Fig. 1 indicates the theoretical path 5 of some of the ions. As a result of the field potential 4 these ion paths are concentrated and havea tendency to strike a comparatively. small area of the photo-cathode. This concentrated ion bambardment upon a small area. creates the above mentioned ion spot.

The object of the invention is to prevent the appearance of these spots. The basic idea of the invention consists in the control of the field of acceleration of electrons ahead of the photo-cathode by means of several different anode potentials which prevent concentration of positive ions and achieve, as far as possible, an even and wide distribution of ions which strike the entire surface of the photo-cathode. In this manner it is possible to reduce the specific wear of the photo-cathode caused by clashing of ions against it, to the extent that the undesirable effect of the ion spot on the image disappears with corresponding increase in life of the photo-cathode.

The invention will be more fully set forth in the following description referring tothe accompanying drawings, and the features of novelty will be pointed out with particularity in the claims annexed to and forming a part of this specification.

2,831,143 Patented Apr. 15, 1958 The invention solves the above described condition as schematically illustrated in Fig. 2. As indicated in this figure, the cylinder representing the anode is divided in cylinder sections 3, 6 and 7. The two outer cylinder sections 3 and 7 have a positive D. C. potential in relation to the photo-cathode 2 as described above. The center cylinder section 6, however, has a negative D. C. potential in relation to the two outer cylinders. In addition, the potential of the center cylinder 6 may be positive or also negative in relation to the potential of the photo-cathode, and furthermore, it is possible to change the shape and diameter of the individual anode sections. Through this arrangement of different potentials and/or diflerent shapes of the anode sections, it is possible to shape the various potentials of the accelerating field 4 sothat the paths 5 of the ions are no longer concentrated. On the contrary, they are now spread out in order to obtain, as far as possible, an even distribution of the ion charge over the total area of the photocathode.

There is also the possibility to arrange the system of acceleration in only two stages, i. e. by arranging two anode cylinders ahead of the cathode and by giving the cylinder nearer the cathode a considerably higher potential, for instance 1,500 volts as against 1,000 volts of the second cylinder.

Depending on the difference in potential between the two cylinders, the ions may be dispersed or considerably retarded or may even be reversed in their direction of travel so that they will no longer clash with the cathode.

It should be understood that this invention is not limited to specific details of construction and arrangement thereof herein illustrated, and that changes and modifications may occur to one skilled in the art without departing from the spirit of the invention.

What I claim is:

1. Electronic optical apparatus particularly for television camera tubes having a plane photo-cathode and a. cylindrical anode arranged ahead of said cathode for the purpose of precluding formation of ion spots on the photocathode, comprising an anode cylinder divided into at least three adjacent sections, means applying potentials of different magnitudes between adjacent sections for controlling the field of acceleration ahead of the photo-cathode and for controlling the spreadingand dispersal of bombarding positive ions over substantially the whole surface of the photo-cathode thereby to prevent bunching and concentration of said ions on a relatively small area of the cathode.

2. Electronic optical apparatus as specified in claim 1 wherein the anode consists of at least three cylindrical sections, of which an intermediate section has applied thereto substantially the same potential as the cathode.

3. Electronic optical apparatus as specified in claim 1 wherein the anode consists of at least three cylindrical sections, and a negative potential exists between an intermediate section and two adjacent sections.

4. Electronic optical device in accordance with claim 1, characterized in that a negative potential exists between a center anode cylinder and two adjacent cylinders, and that a positive potential exists between the center anode cylinder and the cathode.

5. Electronic optical device in accordance with claim 1 characterized in that a negative potential exists between a center anode cylinder and two adjacent cylinders as well as between the center anode cylinder and the cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,652,515 McGee Sept. 15, 1953

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