Comparison and analysis of 2-D simulation results with two implosion radiation experiments on the Los Alamos Pegasus I and Pegasus II capacitor banks Page: 4 of 8
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t (u) time (j)
Fig. 1. (a) Experimental (solid) and 2-D simulation (dash-dot) currents and radiation powers (experimental
- dotted, simulation - dashed) for the PegII-25 experiment. The experimental and calculated powers have
both been scaled by a common factor to allow them to be displayed together with the currents. (b) PegI-41
experimental (solid) and 2-D simulation (dash-dot) currents with XRD data (two filterings, dotted and short
dash) and the simulation radiation power (long dash). The XRD data and calculated radiation power for
PegI-41 have been normalized to an arbitrary peak value of 2.0 .
simulations begin at a point after the plasma has expanded from the original thin solid foil. Perturbations
in the density of the plasma are imposed to mock-up variations which arise in the experiment. These
perturbations then seed the growth of magnetically driven Rayleigh-Taylor instabilities. The simulations
can then be compared with experiments where the effects of the instability development are seen not only in
the observed radiation pulse, but in current and voltage waveforms, in visible light framing camera photos
of the imploding plasma and in time-dependent spectroscopy measurements.
The PegII-25 experiment, conducted on the Pegasus II capacitor bank (an upgrade of the original
Pegasus I bank) imploded a 14.32 mg pure aluminum foil. This experiment gave the largest radiation yield
of any experiment in the Pegasus series, with a total of about 250 kJ as measured by bolometry. The peak
current obtained was 5.1 MA. In Fig. la, a comparison is shown between the measured and calculated load
currents, and the measured and calculated radiation powers for PegII-25. The peak powers have been scaled
by a common factor to allow them to be displayed in the same figure as the currents. The peak measured
radiation power is 0.83 TW and the calulated peak power is 0.76 TW. The total radiated energies are similar,
with a measured value of 250 kJ and a calculated value of about 280 kJ. It can be seen that the general
pulse shape and duration are similar for the measured and calculated radiation pulses.
The 2-D simulation which was used for the comparison in Fig. la began at t = 0.74 ps with a
random perturbation in the density of a 3 mm thick, 2 cm long Al plasma shell. The density perturbations
ranged between -15% and +15% of the average plasma density. The dynamics of the resulting magnetically
driven Rayleigh-Taylor instability growth shows an evolution from short to long wavelengths and an eventual
breakthrough of the plasma shell by "bubble" regions and finally a re-acceleration of "spike" material. In
Fig. 2, a comparison is shown of the implosion as seen by a visible light framing camera and the corresponding
development of instabilites in plasma density from the simulation. Examination of Fig. 2 reveals the classic
spike-and-bubble pattern of Rayleigh-Taylor instability growth arising from the acceleration of a heavy fluid
(the plasma cylinder) by a light fluid (the magnetic field). The horizontal striations which can be seen in the
first two photos (i = 1.32 ps and 1.48 ps) are indicative of the cylindrical nature of the instability growth
(the instabilities grow in the r-z plane and the density enhancements and depletions circle around the plasma
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Peterson, D.L.; Bowers, R.L.; Lebeda, C.F.; Matuska, W.; Benage, J.; Idzorek, G. et al. Comparison and analysis of 2-D simulation results with two implosion radiation experiments on the Los Alamos Pegasus I and Pegasus II capacitor banks, report, September 1, 1995; New Mexico. (digital.library.unt.edu/ark:/67531/metadc622344/m1/4/: accessed January 22, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.