3.6M north10.pdf - Dean-O's Toy Box
3.6M north10.pdf - Dean-O's Toy Box
3.6M north10.pdf - Dean-O's Toy Box
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164 High-Power Microwave-Tube Transmitters<br />
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Figure 1O-10. Typical directly heated thoriated-tungsten wire-type cathode for power-gti tube.<br />
might expect for a tube with a wire grid and thoriated-tungsten (ThW) cathode.<br />
Region C defines the performance to be expected if the cathode is oxide-coated<br />
rather than thoriated tungsten. Oxide cathodes are, quite literally flakier than<br />
ones made of thoriated tungsten, and the products of surface deterioration can<br />
contaminate the grid wire, producing localized electric field enhancement. Tubes<br />
with higher p factors have more grid wires that are closer to the cathode so that<br />
the wire mesh functions more like an equipotential surface rather than individual<br />
wires with respect to the electric-field-strength enhancement produced by conductors<br />
of small radius-of-curvature. Smoothness of the wire is also important,<br />
but once an arc of sufficient action has terminated on a grid (or screen) wire, that<br />
wire, even if it is still there, will no longer be smooth.<br />
If oxide-coated cathodes perform less well with respect to high-voltage holdoff,<br />
why are they used at all? The reason is that thoriated-tungsten cathodes are<br />
not as efficient emitters with respect to heater power. For example, on a thoriatedtungsten<br />
cathode, one ampere of peak emission requires more than 15 W of<br />
heater power— and this is if a highly regulated filament voltage is used to precludetemperature-limited<br />
operation. (Pure tungsten filaments, on the other hand,<br />
are often deliberately operated in a temperatme-limited fashion in current-control<br />
diodes, where current is determined by filament power rather than anodecathode<br />
voltage). There is no pulse-duration limitation for such cathodes.<br />
A comparable oxide-coated cathode may require less than 1/3 as much heater<br />
power for the same emission current, but pulse durations are limited to tens of