Defense Intelligence Reference Document Pulsed High-Power Microwave Source Technology

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appears to be hydrogen ions, which, because of their low mass, contribute to the
problem of gap closure. The gap closure velocity using carbon cathodes quoted by most
reports researched is 2-2.5 cm/μsec, and diode gaps from the same reports were 1-4
centimeters. Carbon cathodes also have much longer lifetime than velvet. However, one
of the greatest advantages of carbon is the ever-expanding ability to form both macro-
and nanostructures using it as a base or substrate. Structures formed using carbon
include pyramids, fibers, microfibers, nanotubes, and tufts. Many possible carbon
structures still have yet to be formed and tested. Thus far, structures with the most
surface area appear to perform best.
CERAMICS
Ceramics, much like carbon, can be formed into at least microstructures and have some
features that have attracted interest in them for a couple of decades. These include
virtually unlimited lifetimes and extremely low outgassing. A problem with ceramics,
however, is that very high threshold fields are required for diode operation. Coatings to
improve the performance of ceramic cathode structures may exist, and research is
continuing in this area.
CESIUM IODIDE COATED
One of the most recent and impressive materials to be used in cathodes for HPM tubes
is cesium iodide. Cesium is a pure metal that has a work function of only 1.9 eV and a
melting temperature of 28 °Celsius; thus, it is liquid at only slightly above room
temperature. Cesium ions are quite heavy, and that is why this coating was used
initially. It was believed that the gap closure rate would be slowed since the heavy
cesium ions would progress much more slowly across the anode-cathode gap than
would other ion species, given the same electric field. This has proved to be the case,
and closure velocities that are about one-fourth those for velvet or bare carbon (0.4
cm/psec) have been attained. As a result, the HPM emission times have been extended.
Typically, the cesium salt is dissolved in water as a saturated solution and then the
carbon cathodes are dipped several times. Subsequently, the cathodes must be baked
under vacuum for several hours to remove the water from the surface and leave the
hardened cesium salt. Once completed, the cathodes have a very long lifetime unless
contaminated by back splatter of material from the anode. At present, HPM programs
investigating the performance of cathodes having some form of cesium coating over
carbon nanostructures show the most potential for progress in the state of the art. The
goal of these programs is hundreds of kiloamps for tens of microseconds, resulting in
gigawatt narrow-band HPM sources running at repetition rates of possibly 100 hertz and
thus capable of 100-megajoule energy output per burst. Also of great interest at
present are cathodes termed "hybrids," which utilize multiple emission mechanisms in
beam generation. The cathodes developed by these programs are to be used with the
magnetically insulated line oscillator (Sandia National Laboratories [SNL]), the
relativistic klystron oscillator (Kyle Hendricks, Air Force Research Laboratory [AFRL]),
the reltron (Bruce Miller, SNL), and the super reltron, among others.
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