Streaking or framing image tube with plural grid control
FIELD OF THE INVENTION 
     This invention relates to electron optical image tubes of the type known as streaking or framing image tubes, and more particularly to the gating of such image tubes. 
     A streaking or framing image tube normally includes an electrode assembly for forming a focussed image on the screen of the tube, the electrode assembly normally includes one or more focus electrodes which by application of the appropriate potential(s), shapes the electric field between photocathode and anode in such a way so as to produce a correctly focussed image. 
     The invention is particularly although not exclusively intended for streaking or framing image tubes in which the image on the screen is inverted relative to the optical image applied to the photocathode. 
     BACKGROUND TO THE INVENTION 
     British Patent Specification No. 1 458 399 (English Electric Valve Company Limited) describes a streaking or framing image tube in which two mesh electrodes are interposed between the photocathode and the focussing electrode. 
     In British Patent Specification No. 2 171 553 A (Hadland Photonics Limited, inventor Alexander E. Huston) a method of gating such a two mesh streaking or framing image tubes is described. According to Huston the method comprises raising and lowering the potential of the first grid, which is closest to the photocathode, over a short interval of time, to gate the electron beam on and off over the said interval, the second grid being held at a constant potential chosen so that the focussing conditions for the image tube are satisfied. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a method as claimed in each of claims 1 to 6, a streaking or framing image tube as claimed in each of claims 7 to 9 and the combination of the streaking or framing image tube with means as claimed in each of claims 10 to 13. 
     It is believed possible by means of the invention to gate an electron beam in such an image tube on and then off extremely quickly, so that the beam is of extremely short duration. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a streaking or framing image tube and some associated circuitry embodying the invention; 
     FIG. 2 is an enlargement of part of the tube of FIG. 1; and 
     FIG. 3 is a voltage pulse/time chart illustrating operation of the embodiment of FIGS. 1 and 2. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     (1) General Description Of Image Tube and Circuitry 
     The streaking or framing image tube is constructed so that it has three &#34;grid&#34; electrodes disposed between the photocathode and the focus electrode assembly as shown in FIG. 1. 
     The tube is regarded as operated in the &#34;normal gated off&#34; or &#34;standby&#34; mode when the potentials are as indicated in FIG. 2. In this situation electrons which are emitted from the photocathode 4 and pass through &#34;grid&#34; electrode 1 are returned to &#34;grid&#34; electrode 1 by the influence of the electric field between &#34;grid&#34; electrodes 1 and 2. 
     The image tube is &#34;gated on&#34; by the application of a voltage pulse to &#34;grid&#34; electrode 2 such that during the duration of the pulse the potential of &#34;grid&#34; electrode 2 is sufficiently greater than that of photocathode 4 such that electrons emitted from photocathode 4 which pass through &#34;grid&#34; electrode 1 can now pass through &#34;grid&#34; electrodes 2 and 3 to reach the screen 12. 
     The image tube is &#34;gated off&#34; by applying to the photocathode 4 an identical or at least very similar voltage pulse to that used to &#34;gate&#34; on the tube so that during the duration of the pulse the potential of the photocathode 4 is held at a value about that of &#34;grid&#34; electrode 1 such that electrons emitted from the photocathode 4 are returned to photocathode 4 by the action of the retarding field between photocathode 4 and &#34;grid&#34; electrode 1. 
     The &#34;grid&#34; electrode 1 is held at a constant potential during all the modes of tube operation. Likewise &#34;grid&#34; electrode 3 is held at a constant potential during all the modes of tube operation, the potential being that required for optimum focus of the electron image on the screen. 
     The action of the voltage pulses in gating the image tube on and off can be seen by reference to FIG. 3. In FIG. 3 typical operating potentials are indicated including a 50 V amplitude for the applied voltage pulses. 
     From FIG. 3 it is apparent that repetitive gating on and off of the photocathode emission current can be achieved by two trains of overlapping voltage pulses, one applied to grid electrode 2 and the other to the photocathode 4. It is also apparent from the arrangement described in FIG. 3 that the gate &#34;on&#34; (or frame exposure) time is equivalent to the time delay between the pulses applied to grid electrode 2 and the photocathode 4. 
     It will also be apparent to those skilled in the art that if grid electrode 2 and the photocathode 4 are connected by a delay circuit (e.g. transmission line) then the application of one pulse train to the &#34;grid&#34; electrode 2 can effect the arrangement as shown in FIG. 3. 
     The advantages of the method described here are: 
     1. An accurately specified gating pulse corresponding to the exposure duration is no longer required as the delay between the gating pulses applied to &#34;grid&#34; electrode 2 and the photocathode 4 determine the gate &#34;on&#34; period. The gating pulse is of course required to have a fast rise time but the full time and exact pulse shape is not critical. 
     2. The gate &#34;on&#34; period is independant of the period of the gating pulse and, for very short gate &#34;on&#34; periods, e.g. 1 n sec (one nano-second) or less, the gate period is readily controlled by a length of transmission line e.g. 1 n sec corresponds approximately to use of 10 cm length of transmission line (assuming group velocity of 10 8  metres/second). 
     3. Because of 2, there is no difference in the gate &#34;on&#34; period between different frames in a sequence. 
     (2) Detailed Description of Image Tube 
     FIG. 1 illustrates the streaking or framing image tube diagrammatically. All potentials are specified relative to photocathode 4. 
     The image tube components are incorporated into a vacuum envelope 9. The photocathode 4 is formed on the inside surface of the glass faceplate 11 whilst a semi-transparent conducting electrode 10 is deposited on the outer surface of the faceplate 11 and connected electrically to photocathode 4 to ensure that capacitative charging effects within the glass faceplate are avoided during gating of the photocathode 4. First, second and third &#34;grid&#34; electrodes, 1, 2 and 3, preferably in the form of meshes, are arranged so that electrons from the photocathode must transverse all three electrodes on their passage down the tube. Included in the tube is a focus electrode 5 and anode 6. The image tube would normallY include deflector plates 7 and 8 as shown. The tube also includes a phosphor screen 12. The tube may be of the construction as described in GB-PS-1458399. 
     Potentials shown in FIG. 1 are typical for the correct focussing of the tube although exact values will depend on the particular design of tube used. 
     During operation of the image tube it is desirable that the potentials applied to grid electrodes 1 and 2 be kept to a minimum to ease the requirements of the gating circuitry. Also, to ensure that disturbance to the focus conditions is minimised the grid electrodes 1 and 2 should be operated at the equipotential values corresponding to the field between the photocathode 4 and grid electrode 3. Both above requirements militate for grid electrodes 1 and 2 being close spaced to the photocathode 4 with their construction being of fine mesh wire with a high transmission for electrons. The grids may be geometrically arranged to form a transmission line structure to ease high speed electronic operation. 
     The prefered situation is as below: 
     spacing-photocathode to grid electrode 3--3 mm 
     spacing-photocathode to grid electrode 1--0.5 mm 
     spacing-photocathode to grid electrode 2--1.0 mm 
     With a typical focus potential of 120 V on grid electrode 3 corresponding potentials for grid electrodes 1 and 2 of +20 V and +40 V respectively are adopted. 
     The mesh of the grid electrodes 1, 2 and 3 are of 1000 mesh/inch with a transmission for electrons of greater than 70%.