o f applied voltages, the screen (target) operates in the con
ventional mode, similar to a conventional cathode ray tube.
Circuit Description—434
The storage screens contain a special coated surface
which continues to em it light when bombarded by the
flood gun electrons, provided the surface has been w ritten
by the w ritin g gun beam and shifted to the stored state.
The tw o targets are electrically isolated from each other,
which allows simultaneous presentations of stored inform a
tion on one half and non-store (conventional) inform ation
on the other half of the viewing area.
Fig. 3-17 illustrates the basic construction of the 434
storage tube. The flood guns are low-energy electron guns
which direct a large area flow , or cones, of electrons toward
the entire screen. The coliim ation electrodes shape the
floo d spray fo r uniform coverage of the storage targets. The
operating level of the tube is the potential difference
between the target backplates and the flood gun cathodes.
The coliim ation electrodes have no effect on the bom
barding energy o f the flood gun electrons.
In the store mode ready-to-w rite state, the insulator sur
face of the target tends to charge down to a potential lower
than the backplate potential, and toward the potential of
the flood gun cathode. This is due to flood gun current
from the insulator surface. The potential to which the
target charges is called its rest potential. This potential is
such that the floo d gun electron landing energy is not
enough to illum inate the phosphor in the target. The target
is now ready to w rite. See Fig. 3-18.
In the w riting process, the target is scanned by the
w riting gun electrons. These high energy electrons increase
the target secondary emission over the area they scan, so
tha t the ratio of secondary current to prim ary current
becomes greater than one. (This is shown in Fig. 3-18 as the
first crossover point.) When this ratio exceeds one, that part
of the bombarded surface shifts to a new stable state.
W riting has been accomplished and this segment of the
target is now stored.
In the w ritte n state, the potential difference between the
flood gun cathode and target becomes greater and the flood
gun electrons now have a landing energy th at is sufficient to
provide a visual display. This visual display w ill continue as
long as the flood gun beam covers the target.
A t high sweep rates, the w riting beam current is not
adequate to bring the portion of the target scanned above
the crossover po in t; therefore, the flood gun electrons when
landing on the bombarded area w ill remove the charge
developed by the w riting gun electrons, and the target w ill
discharge to its initial ready-to-write state w ith o u t being
3-26
Fig. 3-17. P ictoria l diagram o f storage tu b e C R T.
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