IEEE Spectrum December, 2015 - 51

laboratory device into something more practical. Milestones cathode device. Testing showed that the cathode was much
to date include the first operation of a 100-W cold-cathode more resistant to, and tolerant of, individual emitter failures.
One challenge we're still addressing is the traveling-wave
TWT at a frequency of 4 to 6 GHz and the first 100-milliampere
electron beam generated by a cold cathode. Whaley and I tube's high operating voltage. While the field-emitter cathode
also demonstrated a TWT with current densities from the can emit electrons at around 100 V, you need a much higher
cold cathode exceeding 15 amperes per square centimeter, voltage to accelerate and focus the electron beam coming from
which far exceeds the typical 2 A/cm2 from a thermionic cath- the cathode to interact with the helix circuit. But you also need
ode. The current density from our cathode was so high, in to avoid any high-voltage arcs between, for example, the anode
fact, that to get the beam to travel through the helix, we had and the field-emitter cathode. We're also working on ways to
prevent stray ions from traveling back upstream and striking
to expand the beam first rather than compress it.
the cathode's gate and field emitter tips. In our
The cathode worked well for more than
TWT, we've created an electrostatic ion barrier to
150 hours before breaking down electrically.
post your CommEnts at
prevent such ions from making it to the cathode.
Breakdown is a major challenge for these field http://spectrum.ieee.org/
While much work remains on the cold-cathode
emitters because of the high fields within the travelingwavetube1215
TWT, I'm confident we will have a practical
structure and the thin-film gate electrode. An
device within five years. Advances like this one
electrical short between the gate and any one of
the thousands of emitter cones will burn out the entire array underscore the ongoing relevance and vitality of vacuum
electronics, even after more than a century of development.
of emitters and render it unusable.
Recently, our colleagues at SRI have developed a feature Maybe that's why the Defense Advanced Research Projects
to mitigate the damaging effects of such shorts. They inter- Agency announced in August a pair of new programs aimed
rupt the electrical breakdown path between the base of the at pushing the field even further. Vacuum technology concones and the gate by adding a dielectric layer between them- tinues to smash through performance barriers and open up
a shield, if you will. My company, L-3 Electron Devices, tested new areas of application. It refuses to fade into obscurity. n
one of these SRI cathodes in a traveling-wave tube that generated up to 10 W at 18 GHz-a record-high frequency for a cold- NOTE: The views are those of the author.
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Table of Contents for the Digital Edition of IEEE Spectrum December, 2015