Ion trap#Cathode ray tubes

An ion spot is a nearly circular shading in the middle of a historic cathode ray tube screen. It was created by heavy metal ions, which start next to the desired electrons from the cathode. These ions damage in time the photosensitive phosphor layer of the screen.

Such ions can escape from the one part of the cathode surface (in the form of metal ions ), but also occur by ionization of residual gas atoms in the cathode ray tube, since it can never have an absolute vacuum. Because of their mass ions heavier than electrons can be deflected to be guided through the image deflection over the entire phosphor screen. So there is a clearly defined spot in the middle of the screen.

Ion spots spoiled in the early days of television, the visual impression, especially in the pre-war but also in the early postwar period, so that the - at that time expensive - picture tubes were soon useless despite good cathode emission values ​​.

As a remedy, the ion spot was used from about 1950 ion trap suppression: the diagonally installed in the tube neck -ray generation system ensures that the ions move in the axial direction on the metal wall of the tube anode. The magnetic ion trap, a weak permanent magnet, is mounted externally on the tube neck, and adjusted so that the electron beam is deflected again into the direction of the tube axis, and thus the light-emitting layer. The high-mass ions are hardly affected by the weak magnetic field and meet to be on the anode of the electron gun, where they do no relevant damage.

In the late 1950s, the technology was so far that before the luminescent layer (from the perspective of the electron gun ), an aluminum film with a thickness of about 0.2 microns could be applied. The ions remain in the layer stuck, the much smaller electrons flying almost unhindered to the light emitting layer. Simultaneously, the light emission is suppressed by the back of the piston tube, which increases the brightness of the luminous spot (reflection of the aluminum layer ) and the contrast ( no whitening of dark areas of the image by scattered light from the tube bulb ).