Scanning Section

Introduction to Scanning Section:

The electron gun structure generates a beam of electrons which is accelerated in the direction of the target. As pointed out in the diagram, Positive accelerating potentials of 80 to 330 volts that are applied to grid 2, grid 3, and grid 4 that is linked internally to the metalized conductive coating on the inner wall of the tube. At the target the electron beam is focused through magnetic field of the external focus coil and through voltage supplied to grid 4. The alignment coil gives magnetic field which can be changed to adjust the scanning beams position if essential, for correct location. electron beams' deflection to scan the whole target plate is done through magnetic fields of vertical and horizontal deflecting coils that mounted on yoke external to the tube. These types of coils are feed from two oscillators, one working at 15625Hz, for horizontal deflection and the another operating at 50 Hz for vertical deflection.

The target plate is very close to zero potential and hence electrons in the scanning beam can be ready to stop their forward motion at surface of it and then return in the direction of the gun structure. The grid 4 voltage is adjusted to generate consistent deceleration of electrons for the complete target area. The result, electrons that are in the scanning beam are slowed down near the target. This removes any probability of secondary emission from this side of the target plate. If a specific element area on the target plate arrives at potential of, say 2 volts throughout the storage time, then as a result of thinness of it the scanning beam sees the charge deposited on it, part of which gets diffused to the scanned side and deposits an equivalent number of negative charges on the differing side. So out of the total electrons in the beam, a number of get deposited on the target plate, when the left over stop at its surface and turn back to go in the direction of the first electrode of the electron multiplier. Due to low resistivity across the two sides of the target, the deposited negative charge neutralizes the available positive charge in less than a frame time. The target can once more become charged like a result of the incident picture information to be scanned throughout the successive frames. Like the target is scanned one element then other, if there are no positive charges at specific points, all the electrons in the beam return in the direction of the electron gun and none gets deposited on the target plate. The quantity of electrons, that are leaving cathode of the gun, is specifically constant that travel backward give signal current which varies in amplitude in accordance with the picture information. Clearly then, the signal current is highest for black areas on the picture, since nonexistence of light from black areas on the picture does not effect in any type of emission on the photocathode, and there is no secondary emission at the subsequent points on the target and no electrons are required from the beam to neutralize them. On the different for high light areas, on the picture, there is highest loss of electrons from the target plate, because of secondary emission, and these effects in large deposits of electrons from the beam and this decreases the amplitude of the returning beam current. The resultant beam current which turns away from the target is so highest for black areas and I smallest for bright areas on the picture. High intensity light creates large charge imbalance on the glass target plate. The scanning beam is not capable to entirely neutralize it in one scan. So, earlier impression persists for various scans.

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