GB741946A - Improvements relating to ion suppression in cathode ray apparatus - Google Patents

Abstract

741,946. Ion-trapping in cathode-ray tubes. FERRANTI, Ltd. Jan. 7, 1954 [Jan. 8, 1953], No. 567/53. Class 39 (1). In a cathode-ray tube with magnetic scanning and in which the beam is almost or wholly extinguished during the fly-back periods, a pair of plate electrodes 2 are provided for deflecting ions from the beam path during and only during these periods; for this purpose they are energized by pulses derived from the magnetic scanning means. As particularly described, the plate electrodes 2 are energized during the fly-back periods by positive or negative going pulses 8 having a magnitude of about 1000 volts, the pulses being obtained from the anode of the output valve of the line timebase. The ions may be deflected sufficiently by a single pulse but preferably are deflected two or more times (in successive fly-back periods). This is possible due to the slow speed of the ions (relative to the electrons) which remain between the ion deflecting plates for two or more fly-back periods. There may be a plurality of pairs of plates 2. During the scanning periods plates 2 are at the same potential, equal to the potential of the first anode 5, i.e. 300 volts. The plates 2 need not, however, be associated with the first anode or even with the gun. The ions are collected by the final anode 10 at 10 kv., by the tube wall or by a " special electrode." The tube is preferably but not necessarily a television receiver; since the electrons are undeflected by the ion trap the picture is not distorted. The scanning means is preferably electromagnetic. The Provisional Specification describes a similar arrangement but in which the plates 2 are disposed between the first and final anodes. The ion deflecting electrodes may be energized by a " special " separate valve stage.