AAWSAP DIRD, Pulsed High-Power Microwave Source Technology, January 2010

Text

This text came with the file. It may have mistakes. When the text and the image disagree, the image is right.

UNCLASSIFIED//FOR 8ffl@IJllt l:191!! l>flt I
VELVET
Velvet has been an explosive electron emission standard for more than 20 years. This
material works by means of the dielectric-metal surface flashover mechanism
mentioned earlier.
Five steps are involved in the explosive emission process for dielectric fibers:
•
Surface flashover generates a cold, dense plasma/gas column.
•
The applied electric field extracts a space-charge-limited current flow.
•
The flow of current resistively heats the gas column.
•
The gas columns expand at a rate determined by the gas temperature.
•
The gas continues to expand into the anode-cathode gap.
Velvet has several desirable properties that have endeared it to the pulsed power
community and kept it a useful material for all this time. First, it emits at relatively low
field strengths (~10 kV/cm), allowing a wider range of use than do many other
materials. Second, it has a fast turn-on time. Third, the insulating nature of the velvet
fibers provides a sort of built-in ballast during operation. Velvet also has a wide range
of vacuum compatibility (pressures from 10-3 to 10-s Torr), easing the expense of
vacuum hardware. Finally, velvet is inexpensive and readily available. All these factors
combined have made velvet cathodes common for the past two decades.
However, velvet cathodes also have drawbacks. First, velvet outgases heavily,
especially during and after explosive emission. Significant amounts of material are
released from the velvet during this process. The increased pressure inside the HPM
device then leads to gap closure (conductive bridging of the anode-cathode region) and
early termination of the RF output from the device. The closure rate can be estimated
from:
Velocity of closure (m/s) = 100 (d* /d) 213 Vd 1/ 2
where: d is the diode gap, d* is the velvet tuft density, and Vd is the diode voltage.
Second, velvet has a very limited lifetime, partly owing to the material lost during each
shot. Some material lasts for only about 100 shots in single-shot mode. Third, because
of the increase in pressure after each shot, the repetition rate is very limited. Finally,
lack of control over the manufacturing process resu lts in a wide variation in
performance. Results are not reproducible, even between one roll and the next from the
same manufacturer.
CARBON
Carbon cathodes have been used in diodes for more than three decades and have some
appealing characteristics. The outgassing characteristics of carbon cathodes are much
better than those of velvet, although the threshold voltage for emission is generally
much higher. The primary material given off during outgassing from carbon cathodes
UNCLASSIFIED/,CfOlil OFFIQIAk Uili QPlk¥
9