Basic science with pulsed power & some off-the-wall ideas Page: 4 of 15
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about 50 eV. Expansion compresses the Lucite and drives shock into the Molybdenum at a
pressure of 20 Mb. Light pipes at various depths detect shock arrival and thereby determine
shock velocity. Moderated neutrons from the Lucite are detected at the end of a 20-m LOS
tube, and the resonance dips impressed on the neutron flux by the molybdenum and by
the gold and tungsten foils are resolved by time-of-flight techniques. The displacement or
Doppler shift of a dip from its normal position indicates the molybdenum particle velocity.
Having once established a standard, in this case molybdenum, EOS points for other ma-
terial can be obtained by impedance matching, which was done in several nuclear tests3'.
In the late 70s, we developed a technique for measuring EOS parameters using high in-
tensity lasers and the impedance matching technique5s,,7,a. Figure 2 shows how the laser
was used to generate a high-pressure shock wave by heating the back surface of a standard
material. Shock velocity could then be measured in both the standard and the test mate-
rial by watching the luminosity as the shock emerged at the back side. Figure 3 shows a
typical streak camera image of the emergent shock. In this case, aluminum was the stan-
dard and gold was the test material. The techniques became quite sophisticated9,1011,
but the data were never as good as those obtained from nuclear tests. Current plans call
for even further refining these technique with the enormous increase in power and total
energy available from the NIF. Perhaps they will surpass nuclear explosion experiments in
resolution of the data.
Figure 4 shows my version of an ATLAS-driven impedance-match experiment. The com-
posite flyer plate consists of a standard material (like molybdenum) and a driver material.
Of the readily available materials for the driver, aluminum has the best combined density,
electrical, and boiling-point properties This quality factor'2 is described
T ' CP(T)dT,
J o 77
where 77 is the resistivity, C, is the specific heat, To is the ambient temperature, and Tb is
the boiling point. Using the Pegasus experience12 with a flyer liner at 4 cm and a target
liner at 1 cm, and impact velocity of 1.7 cm - s- This could produce a shock pressure of
15 Mb. Scaled to ATLAS12, this would be on the order of 30 Mb. Figure 4 shows fiber
optics imbedded in the test material to measure the shock velocity. The velocity of the
standard would be obtained from x-ray or laser snap shots. Given the shock velocities of
the standard and test material, we can obtain a point on the Hugoniot for the test material
as we did in the laser-driven experiments. The geometry is different in two ways. First,
the shock is created by impact rather than passing through the standard and then through
the test material - a difference of no consequence. Second, the ATLAS experiment is
in a cylindrically converging geometry - introducing a minor correction to the planar
geometry.
A fundamental difficulty is the quality of the standard. The last nuclear tests to calibrate
a standard were performed over 15 years ago. An important challenge would be to find a
way to measure the particle velocity as well as the shock velocity in the test material. One
thinks of an experiment like the one shown in fig. 4 with a Doppler probe of some sort.2
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Solem, J.C. Basic science with pulsed power & some off-the-wall ideas, article, April 1, 1995; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc683253/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.