Patent Number: 041692292
Section: description

DETAILED DESCRIPTION FIG. 1 represents a deflection installation according to the invention in a deflection system 3 in an electron beam installation 1. Reference numeral 11 identifies the cathode, 12 the Wehnelt electrode and 13 the anode designed as a perforate disk. A first apertured disk lens is identified by 14. It has an aperture of 100 microns in diameter, for example. The electron beam 15 extends from the cathode 11 through the opening in the anode 13 and in the apertured disk lens 14. It continues through the deflection system 3 and in case of no deflection, through a second apertured disk lens 16 having a bore of 300 microns in diameter for example. Then the electron beam 15 enters the interior of a vacuum tube 17 in which it is exposed for example for screen scanning (or raster deflection) to a further deflection possibly with the aid of magnetic deflection installations not shown (as known from television tubes). Parts 11 to 16 and the deflection system 3 are all in the interior of a vacuum-tight housing which for the sake of clarity is not represented in FIG. 1. The deflection installation 3 has two deflection parts 31 and 32 which are the carriers of the deflection plates 33 and 34 as such. For the adjustment of the deflection parts 31 and 32 and thus also of the plates 33 and 34, installations 41 and 42 are provided, with which parts 31 and 32 are connected via shafts 43 and 44. Sealing rings are identified by 431 and 441, which shows that the adjustment systems 41 and 42 are located outside the above mentioned vacuum-tight housing (not shown). As indicated by the double arrows 45 and 46, it is possible to effect with the individual components of the adjustment installations 41 and 42 lateral and angular displacements of the deflection parts 31 and 32 in relation to the beam axis 15 inside the vacuum housing. The parts 33 and 34 identified above as plates have, as can be seen better from FIG. 2 in a view upon such a plate 33, an interdigital structure 51. It comprises a meander-shaped conductor track 52 and a screening or shielding conductor track 53. As shown schematically, the structure 53 is grounded. The conductor track 52 has a schematically indicated input connection 521 and an output connection 522 likewise grounded via a terminal resistance 523. A high frequency signal entering via the input 521 the conductor track structure 52 moves there according to the meander-shaped path so as to progress in the direction shown by the arrow 15, delayed in accordance with the extension of the path. This manner of operation of an interdigital structure 51 is a matter with which a person skilled in the art is sufficiently familiar, and the arrow 15 in FIG. 2 is identical with the beam axis indicated in FIG. 1 and with the electron beam 15 plotted there. The structure identified by 53 is used with its fingers extending into the meanders of structure 52 to avoid dispersion properties, as this is known as a matter of principle. The dimensioning of an interdigital structure 51 is in a manner known per se from prior art a function of the speed of the beam, that is the acceleration voltage of the electron beam 51 in a manner likewise known from prior art. The interdigital structure 51 may be a conductor track structure designed in a cantilever-like manner or it may be applied to a plate of dielectric material. Both deflection parts 31 and 32 have identical interdigital structures according to the invention at the site of the mentioned plates 33 and 34 so that the respective meander conductive paths 52 are directly in alignment with each other at each point therealong, as indicated in FIG. 1. Although not shown in FIG. 1 for the sake of clarity, respective deflection signals are fed to the interdigital structures 51 of the respective plates 33 and 34 at the input connections 521 thereof, FIG. 2, said signals being of opposite polarity in the two opposite interdigital structures. This measure, according to the invention, causes that even in the presence of potentials different from zero in the opposite interdigital structures 51, the potential field at the beam axis 15 at least as produced by the signals supplied to the deflection system 3 will not expose the beam to a field component in the longitudinal direction. As a result of the signals on the opposite interdigital structures 51 only transverse fields act upon the electron beam 15, and in fact such signals operate at the same velocity as the velocity of the electron beam 15. The synchronism between electron beam and the deflection signals on the meander path of the conductor track structures 52 causes over the length of the plates 33 and 34 a permanently uniform deflection effect upon the electrons moving synchronously in the electron beam. After a certain deflection voltage between the opposite interdigital structures 52 the electrons of the electron beam 15 are exposed to such a high deflection that they no longer pass through the aperture of the apertured disk lens 16 and thus they are screened out of the electron beam. The invention assures that electrons which undergo a deflection by which they are not yet screened out at least will not undergo any longitudinal acceleration which could lead to a defocusing at the point of destination which has been determined as disturbing according to the state of the art. FIG. 3 schematically shows in a graph the two signal voltages 61 and 62 to be applied to the prevailing input connections such as 521 of the interdigital structures 51 of both deflection parts 33 and 34 for keying in the beam. Time is plotted on the abscissa of FIG. 3. On the ordinate the positive potential of one of the interdigital structures 51 is plotted in an upward direction while the negative potential of the opposite other interdigital structure 51 is plotted in a downward direction. Under the keying in of the electron beam contemplated here it is a normal case for the electron beam to be scanned that is deflected in such a manner that it cannot pass through the apertured disk lens 16. Only in keyed-in condition will the electron beam move on the beam axis 15 shown in FIG. 1 and thus through said apertured disk lens 16. While the electron beam is scanned, opposite DC voltage potentials identified with 101 and 102 are applied to the two opposite plates 33 and 34, that is at the two opposite interdigital structures 51, causing a transversely oriented electrical deflection field to act on the electron beam. Reference numerals 103 and 104 identify impulses of opposite polarity in the form of sinusoidal semi waves, which are the mentioned deflection or key-in signals with opposite signs. The broken lines 105 and 106 identify two potential values which indicate the threshold for the electron beam key-in operation. The broken vertical lines 107 and 108 include the time range of the electron beam key-in operation having for example a length of 350 psec (350.times.10.sup.-12 sec.). FIG. 3 shows that over considerable portions of time during the keying in of the electron beam a transverse field still acts on the electrons. However, according to the invention, during this time no longitudinal field acts which--as the inventor recognized led to the defects in corresponding devices known from the state of the art. For the sake of completion it should be pointed out that for the keying in of the electron beam an apertured disk lens identified by 16 is not necessary in all cases. The electron beam current present during the time of the electron beam also can be deflected on the plates of the deflection installation (out of the beam axis) instead of on a diaphragm 16, for, like in the corresponding devices according to prior art, the apparatus according to the invention likewise contemplates that the opposite plates 33 and 34 and/or the opposite interdigital structures 51 are spaced apart only a small distance, and that just enough interstice remains between them so that the electron beam can pass without impediment (under the keying-in of the electron beam). In addition to the solution of the problem as mentioned above it is possible with an apparatus according to the invention for the keying-in of electron beams to carry out an extremely short-timed beam impulse scanning with repeatably accurate start and finish of the impulses. According to an improvement of the invention, impulses with steep flanks but not necessarily short impulse width are applied to the two deflection parts of a deflection system and are arranged symmetrically to the beam axis. The impulse applied to the one deflection part, however, is so shifted in time relative to the other impulse applied to the opposite deflection part that only a slight overlapping in time of both impulses is present (as viewed in a time diagram such as that shown in FIG. 4, for example). FIGS. 4 and 5 show the above mentioned conditions in an easily understandable form. Generally the details of the presentation in FIG. 4 agree with those in FIG. 3. However, the impulses 203 and 204 applied to the deflection parts 33, 34 opposite each other in each case are, as can be seen, shifted in time. Only a relatively short interval in time as indicated at 210 exists, during which both impulses 203 and 204 have such a difference in potential with respect to each other that the beam 15 passes the installation without beam deflection, that is it passes the diaphragm 16 without defocusing and axially. During this interval in time the relatively short beam impulse 200 shown in FIG. 5 occurs. Its flanks are a function both of the flanks of the impulses 203 and 204 and the timing control sensitivity of the installation according to the invention. Advantageously no comparable short impulses are necessary for a short impulse width, but impulses of comparatively substantially greater width, as represented, suffice. Such impulses can be generated, however, in a simple manner. However, if the short impulses comparable with FIG. 3 are applied to the deflection parts, with a controlled time offset according to FIG. 4, a correspondingly extremely short impulse results. For further details of FIG. 4 reference is made to the description relative to FIG. 3. It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts and teachings of the present invention.