US2862140A - Signal storage discharge tubes - Google Patents

Description

Nov. 25, 1958 G. B. BANKS SIGNAL STORAGE DISCHARGE TUBES Filed Dec. 23, 1955 E w H m m i0 m v 6 w 1 S g z w m z w, -l/l w EEMZ WE/T/A/G BEAM 2 7400- 5l4M F0502 SIGNAL STORAGE DISCHARGE TUBES George Baldwin Banks, Rawreth, England, assignor to English Electric Valve Company Limited, London, England, a British company Application December 23, 1955, Serial No. 555,141 Claims priority, application Great Britain March 17, 1955 2 Claims. (Cl. 315-12) This invention relates to signal storage discharge tubes, and more particularly to signal storing cathode ray tubes of the kind in which a cathode ray, modulated in accordance with signals to be stored, is caused to scan a target structure to store the signals in the form of charges thereon, the said target structure being subsequently scanned by a cathode ray to read-oil the stored charges. A typical storage tube of this kind has two electron guns-a writing gun and a reading gun-directed towards a target structure which consists of a plate of glass or other suitable dielectric material sandwiched between a continuous conductive signal plate on the side remote from the two guns and a large number of discrete conductive electrodes on the side facing the said guns. When the discrete conductive electrodes are scanned by a signal modulated cathode ray from the writing gun, electrical charges corresponding to the signals are set up in the minute'condensers which are in effect constituted by the target structure. quently, the discrete electrodes are scanned by the reading gun the stored signals are read-oil). the stored charges being of course thus wiped o The invention is illustrated in and further explained in connection with the accompanying drawings in which Fig. l is an explanatory graphical figure;

Fig. 2 is a schematic representation of one form of tube embodying the invention; and

Fig. 3 is a view of part of the target structure of the tube of Fig. 2.

It will readily be appreciated that in any particular known cathode ray tube as above described there are limits to the permissible Voltages which can be allowed to be set up either across the glass plate in the target structure, or between the individual discrete electrodes on one side of that plate. Limitation of voltage across the glass plate, i. e. between the discrete electrodes on one sided the plate, and the signal plate on the other, presents no difficulties for the only requirement is that this voltage shall not exceed a value suited to the dielectric strength of the glass. In a typical case the permissible strength might be 30 kv. per mm., so that 6 kv. would be the maximum permissible voltage across a typical glass plate 0.2 mm. thick. The limitations of potential as between adjacent discrete electrodes are, however, much more severe. Consider the case of a glass plate able to withstand 30 kv. per mm. with adjacent discrete metallic electrodes on one side thereof spaced apart by 0.005 mm. Then the maximum permissible potential between adjacent discrete electrodes is only 150 volts if one considers merely the absolute spacing, but when it is remembered that the discrete electrodes present very sharp edges towards one another it will be seen that a voltage difference of this value could excite field emission at the edges and thus involve arisk of break down of glass at the electrode with consequent tracking. In practice, therefore, the voltage between adjacent discrete electrodes cannot safely be allowed to exceed about two thirds of the voltage calculated on the basis of the strength If, subse" United States Patent I 7 2,862,140 Patented Nov. 25, 1958 of the glass, i. e. for the typical example just given, a voltage of is about as much as is safely allowable.

Further, as is well known, it is necessary, in the interests of eificiency, so to operate the writing gun of a known tube as above described, that the electrons therefrom reach the target with a velocity corresponding to a voltage above the point at which the curve connecting ratio of emitted secondary electrons to incident primary electrons crosses the unity ratio line for the second time. In other words the writing gun has to be operated under conditions such that the number of secondaries released is smaller than the number of incident primaries, so that the negative charges are stored. On the other hand the reading gun must be so operated that the electrons arrive at the target with a voltage above that corresponding to the first crossing of the unity ratio line, i, e. under conditions such that the number of secondary electrons released at the target exceeds the number of incident primaries so that the minute condensers of the target structure are discharged and the discrete electrodes restored to a suitable datum potential determined by the potential of the normally provided internal conductive coating on the tube envelope. This will be clearer from Fig. 1 of the drawings which shows a typical characteristic curve connecting the ratio d of released secondary electrons to incident primary electrons (ordinates) with velocity of incident primaries (abscissae) in terms of voltage V. The horizontal line X is the line for unity ratio and it will be seen that the curve crosses this line in two places, Y and Z, the former corresponding, in a typical case, to about 20 volts and the latter to about 5000 volts. Although the reading beam ray is operated at a voltage between Y and Z, the -writing ray must, in practice, be operated at a point well beyond the point Z owing to the small slope of the curve in the neighborhood of Z. There is therefore a real danger that, if the reading beam becomes for any reason inadequate or momentarily interrupted, some of the discrete electrodes will become charged to a voltage near that of the point Zof Fig. l, and inter-electrode breakdown will occur. There seems to be no doubt that the spurious signals frequently experienced when reading-01f known tubes as above described are due very largely to this cause. In addition such known tubes are very inefiicient, particularly as regards the writing process for which operation takes place with a secondary-primary electron ratio which is commonly not less than 0.9. There' is considerable liability, moreover, for secondary electrons emitted at the target during both reading and writing to fall back on the target and thus produce spurious shading signals like those commonly experienced in television pick-up tubes of the so-c-alled Iconoscope type.

The main defects of a known cathode ray storage tube as above described may be therefore summarised as 'follows (1) They are liable to permanent break down at the target due to over-charging of the individual discrete electrodes.

'(2) They are liable to produce spurious signals due to permanent or temporary break down of insulation between one discrete electrode and its neighbors.

(3) They are ineflicient both for writing and reading but especially the former, and

(4) They are liable to produce spurious shading signals due to the return of secondary electrons to the target structure.

The present invention seeks to avoid the above mentioned defects. 1

According to this invention a cathode ray storage tube comprises a storage target structure composed of a plurality of minute condensers with a common electrode on one side of a dielectric plate and a plurality of separate for scanning said separate electrodes with a signal modulated writing cathode" ray to charge the said condensers, means for scanning said separate electrodes with a freadingYcathode ray to collect the charges, and interposedmea'ns closelysadjacentisaid separate electrodes for: setting up a potential surface of one pre-determined'value through which the 'writing ray reaches said electrodes and a potential surface of a widely different value through which'the fre'ading ray reaches said electrodes.

In a preferred'embodiment there are two guns arranged to scan the targetstruct-ure along-adjacent separate paths and there is provided between each gun and the part of the target structure it scans -a.-separate gr-id orsimilar electrode. Thus the ray fromleachgun can reach the target structure only. thi'cughitsappropriate interposed grid, and by applying different voltagesto the two grids, the voltage excursion of the .discrete electrodeduring Writing and during reading can be positively. limited as required. To quote practical figures the voltage of the grid across the path of the writing my (hereinafter called the suppressor-grid) might be 100 volts below earth, while the voltage on the other grid (hereinafter termed the collector grid) might be zero, i. e. earth potential in a case in which the signal plate was returned, through the usual output resistance, to earth and the normally provided metallicdeposit on the inside of the tube envelope was connected to earth.

Fig. 2 of the accompanying drawings shows schematically one form-of tube in' accordance with the invention, and

Fig. 3 is abroken away face View of the target structure of the tube of Fig. 2.

Referring to Fig. 2, the tube therein shown has an evacuated envelope 1 with two necks 2, 3, at the end of each of which is an electron gun 4 or 5 of known construction and arrangement. In the main bulb 1 of the tube is a target structure consisting of a. glass plate .6, for example, 0.2 mm. thick, with a continuous 'metal layer 7 constituting the signal plate on the rear side thereof and a large number of a discrete rectangular metallic electrodes .3 on the front or gun side thereof. These elements 6, 7 and 8 form a plurality of minute condensers. beabout 8 mm. long ('thevertical. dimension in Fig. 3.) and about :06 mm. wide, and each might be separated from its neighbors -by=about 0.005 mm. It is of course impracticableto show dimensions such asthese in a drawing, and no :attempt haslaccordingly been made to indicatedimensions and proportions correctly in Figs. 2 and 3, which are, -as stated, purely schematic. Each gun-4 or is arranged toscan:across the target in its own line, the two lines-being spaced apart by a few-mm. The scanning-rays areindicated conventionally in the bulb 1 by arrow headed chain lines in Fig. 2. Thus for the sake of clarity in'i'drawing, the two scanningrays are indicated asfurth'er 'apart in FigrZ and respectively-nearer the tops and bottoms' ofthear'eas 8 than they wouldbe in practice.

which the usual output resistance 11 from the signalpla te 7'-is returned to earth and the usual--deposit-l2 on the inside of the bulb 1 is also earth'ed, the'suppressor grid 9 might beoperated at minus 100 volts, andthe other grid 10 (-thecollector grid) maintainedat-or about earth potential. Thesepotentials' are indi'catedin -Fig. 2, but are of'course given 'by'way of practicalexample only. Each grid extendsright across the target structure front-left to right in Fig. 3, but only half way down so that one scanningline is under one grid, and the other under the other. In order not to increase the signal plate capacitance unduly, the height of this plate, i. e. the dimension whichzappears vertical in Figs. 2 and 3, is made lessthan 'be"offinemesh' and are closeto the targetsurface, for

example, 4 mils therefrom! With this arrangement the writing beam can be operated at or below the voltage of the point Z of Fig. 1,never above itwhile the reading beam can be operated very close to the condition which gives the maximum secondary-primary electron ratio. Secondaryelectrons released by the writing beam are unable to escape'from thertarget because of the suppressor;grid..9. and the (discrete electrodes..8 .thu's acquire negative charges. This process can continue until the charge on said electrodes reduce to the potential of the suppressor grid after which further electrons can escape. Moreover, theisuppressor grid .is so. close to the target that suppressed secondariescannot spread and cause loss of resolution but are returned substantially to their points of origin at the target. Since, as stated, the writing gunoperatesbeloyv ,t-he pointZ where the secondary-primary electronsratiojs unity-or above, all the ray current is utilized-and-there is high Writing etficiency. As will be -.a pparent, the voltage excursion of the discreterelectrodes fi can be restricted to any desired range byadjustment of the suppressor and collector grid potentials and cannot exceed thedilference between these two voltages. There is a great increase in ray efficiency for both-writi11g.and reading andflsince much lower primaryray currents are therefore required, the raycross-sections can 4 be :reduced withconsequent greatly improved resolution.

, cathode ray. to charge the said condensers, means includ- Each metallic electrode-might, for example, I

ing a second electron gun for scanning said separate electrodes-with a reading cathode ray to collect the charges, both said first electron gun and said second electron gun being disposed to the same side of-said target structure as saidseparate electrodes, two separate fine meshlgrid electrodes adjacent the target-structure, each extending fully. across. said target structurein the direction of scanning and substantially half-way across in a direction perpendicular thereto .in the'iplane of said target structure, one ofsaid. grid electrodes being interposed across the electron paths fromsaid'first electron gun to said target structure and the other of said grid electrodes being interposed across the electron paths between'said second electron gun. and said target structure, said two grid clec trodes' being. closely. adjacent"said separate electrodes, means for applying potentials to said-grid electrodes for setting up two different potential surfaces, the potential surface through which the writing ray reaches said electrodesihaving' such predetermined value that secondary electrons'zfrom the-target are suppressed and the potential surface, of a widely'ditferent value, through which the readingf ray reaches said electrodeshaving such predetermined value thatsecondary-emission from the target islassisted'.

2. A'tube as claimed in claim 1 wherein the grid electrodes are spaced from the adjacent target structure by'a-distance of the order of 4mils.

' References Citedin the n1; ofjthis'patent UNITED STATEStPATENTS 2,612,634 Mesner Sept. 30, '1952 2,618,762 Snyder 'Nov. 18,1952 2,674,704 Weimer Apr. 6,1954 1,687,492

. -Szegho-et al. *Aug. 24, '1954

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