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140°
150°^
100° 90° 80° 70°
TYPE 7NP4
ANODE VOLTS = 80000
MEASURED AT DISTANCE OF 15 INCHES FROM CENTER OF FACE
10 20 30 4O 5O 6O 7O
X-RAY INTENSITY — ROENTGENS/HOUR/ MA
Fig. 6. Polar distribution of X-Ray radiation from 7NP4.
with an oxide-coated cathode. An oxide-coated cathode is desirable because its high emission permits formation of a high-current-density beam and thus helps provide sharp focus. Furthermore its low work function greatly reduces the control voltage required. Reliable operation of an oxidecoated cathode at very high voltage is made possible by means of excellent exhaust and getter systems which produce and maintain a very low gas pressure and limit ion bombardment of the cathode surface. The getter is flashed over the entire inside surface of the cone, providing a relatively large active area close to the main sources of ionized gas molecules. A getter flash over the inside of the bulb is possible because the aluminum screen backing protects the luminescent screen and because a deposit of getter on the screen does not appreciably reduce the energy of the high-voltage beam.
Deflection Considerations
Magnetic, rather than electrostatic,
deflection is always used for high-voltage cathode-ray tubes to avoid insulation and distortion problems inherent with the latter.
A smaller deflection angle than that used in kinescopes for home-television receivers was chosen for the 7NP4 in order to reduce deflection-power requirements and to minimize loss of edge resolution. The larger-diameter neck required for the double-neck insulation makes deflection more difficult because the length of the deflecting yoke field is determined by the clearance of the inside neck. The deflecting power needed is of reasonable proportion, however, being not much greater than that required for wide deflection-angle, home-television receivers. A good margin of deflection power for reliability and good linearity is, therefore, readily provided. A narrow deflection angle also requires an increase in the distance between the electron gun and the screen. The increase in distance increases spot magnification, but this effect is compensated for by making the electron gun longer.
Swedlund and Thierf elder: Projection Kinescope
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