3.6M north10.pdf - Dean-O's Toy Box

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