Defense Intelligence Reference Document Pulsed High-Power Microwave Source Technology

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These mechanisms include:
• Thermionic emission (apply heat - 1,000 "Celsius)
• Secondary emission (electron bombardment; > 100 eV)
• Field emission (apply a very strong electric field; 107 V/cm)
• Explosive emission (form a plasma on the surface; c:,.,= 0)
The emission mechanisms of major importance for HPM at present are thermionic
emission and explosive electron emission; however, field emission shows some hope
with the advancements in nanostructures. Explosive emission, creating a dense plasma
at the cathode surface, is of primary importance at this point. A review of pure metals
reveals a direct correlation between the work function (r.,.,) and melting temperatures.
When cathodes are made from metals with low work functions, there are problems with
metal deposition onto other components. Most cathodes of use in HPM tubes depend on
a surface flashover at a dielectric-metal interface. The surface flashover generates
plasma, typically at tens-of-kilovolts-per-centimeter electric fields. The threshold and
nature of the plasma depend greatly on the cathode materials. Therefore, the choice of
cathode material is of critical importance in the design and operation of any HPM tube.
No discussion of HPM diodes could be complete without mentioning space charge
limited current flow. This stems from the fact that at some magnitude of current
density, the density of electrons in the anode-cathode gap begins to shield the cathode
from further emission owing to their cumulative effect on the electric field at the
cathode surface. The current density at which this happens is given by the Child-
Langmuir law:
lsc(kA/cm 2
) = 2.33 x 10- 5
(V(MV) 3
i 2
/d(cm) 2
)
and is dependent on the diode voltage and the anode-cathode spacing. So, if we could
have the ideal cathode material, what would its characteristics be? The response has
not changed much in more than 60 years, as can be seen in the following extraction
from a textbook on the subject.
Primary Characteristics of an Ideal Cathode (J. R. Pierce, 1946):
• Emits electrons freely, without any form of persuasion such as heating or
bombardment (electrons would leak off from it into vacuum as easily as they pass
from one metal to another).
• Emits copiously, supplying an unlimited current density.
• Lasts forever, its electron emission continuing unimpaired as long as it is needed.
• Emits electrons uniformly, traveling at practically zero velocity.
Efforts are still under way to increase the output power, pulsed emission duration,
repetition rate, and emission uniformity by investigating new and existing cathode
materials in an effort to draw closer to the ideal cathode. Some of the materials
currently being investigated are ceramic cloth and felt, carbon structures including
nanotubes and microfibers, and carbon structures coated with cesium iodide.
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