The flood guns are located just outside the horizontal deflection plates. A cloud of electrons is emitted by each flood-gun cathode. These clouds are combined, shaped, and accelerated by the two control grids, as well as the collimator. The collimator consists of a coating on the inside of the tube. The positive voltage on the collimator is adjusted so the flood-gun electron cloud only fills the CRT viewing screen. The cloud is further accelerated towards the storage mesh and viewing screen by the collector mesh. After passing through the collector mesh, the flood electrons are further controlled by the potentials of the storage mesh and storage layer. The cathode side of the storage mesh is coated with the nonconductive storage material, which is where the pattern to be displayed is stored. Because of the nonconductive property, only a capacitive coupling exists between the storage layer and the storage mesh. This capacitive coupling is required for the storage and erase functions. The rest potential of the storage mesh is approximately + 1 V with respect to the flood-gun cathodes. In the write and erase routines, the potential of the storage layer varies from O V to negative. This is accomplished through the storage mesh and the capacitive coupling.
When the storage-layer surface is at O V, most of the flood electrons pass through the holes of the mesh and reach the viewing screen. The remaining electrons are repelled by the storage-layer surface and picked up by the collector mesh. The postaccelerator voltage (approximately 6 kV with respect to the storage mesh) is linked to the transparent aluminizing layer of the phosphor viewing screen. As soon as flood electrons can pass the storage mesh, they are accelerated by the high potential and strike the phosphor, causing it to emit light. When the storage layer is made negative, the number of electrons passing the storage mesh is reduced greatly. At a specific value, (the cutoff level) no electrons are passed.
Operating the Variable-Persistence Tube
Write and Store Modes
Assume that the storage-layer surface has been prepared by an erase routine so the layer is below the cutoff level and flood electrons cannot reach the screen. The writing cathode is activated and the beam writes in one sweep the pattern at the storage layer. The writing cathode is at about – 1500-V potential with respect to the storage layer, which is adequate to cause the secondary emission. During the writing procedure, the highest potential of the storage layer that can be reached is 0 V (flood-gun cathode potential). Any higher potential would attract flood-gun electrons, thereby decreasing this potential. After writing, with the storage layer at 0 V, most of the flood-gun electrons can pass through the mesh. Therefore, the pattern of charge on the storage layer will become visible on the screen.
The brightness of the displayed pattern, as it is stored on the storage layer, can be changed by applying a pulse-shaped waveform to the accelerator grid. The amplitudes of the pulses are +50 V with respect to the flood-gun cathode and the storage brightness has a linear relationship with the pulse width. For full brightness, the pulse width is at maximum. When the pulse width is at minimum, the brightness is also at minimum, but now the storage time is at maximum.
At this point, it must be understood that the structure of the storage mesh is extremely fine, about 240 lines/cm, to ensure a bright and crisp display. As the focused writing beam has a spot diameter of about 0.3 mm, one trace covers about 7 to 8 mesh lines.