In conventional modulating systems of electron guns, the intensity variation in the beam is brought about by varying the emitting area of the cathode through the effect of the potential on the modulator ; the latter acts like an Iris diaphragm, which, in addition, controls the focal properties of the cathode lens system and may affect the quality of the focussed image. All these parameters can be kept constant by using a gun with fixed potentials on its electrodes ; the modulation of the beam is then made to take place after it has passed the final electrode by allowing it to traverse the potential trough between two boundaries. Schematic designs of systems which achieve this are shown in Fig. 1, types A to H. The final type H evolved, consists of two apertured discs, A l and A 3 separated by the modulating electrode, A 2, in the form of an open cylinder; a “stopper” disc, A4, with an aperture larger than that in A 3 is biassed negatively with respect to A 3 to prevent secondary electrons from entering the beam. Unlike the earlier types, this design gave a small image free from halo with a steep sided electron distribution in it; the modulation range was some 10 V and slopes up to 60 ,uA/V were recorded.
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