Ultra-high-density plasma experiments: MHD simulations Page: 4 of 6
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model includes thermal conduction, resistive diffus,-n, radiation,
dissoc ation, and ionization in addition to the Lorentz JxB force in a
fluid description of the plasma. The charged-particle transport
coefficients are "classical" and the atomic processes are based on
local thermodynamic equilibrium(LTE). An extensive post-processor
computes spectra from infra-red to x-ray frequencies, temperatures
based on x-ray absorption by two different thicknesses of aluminum
foil, and Schlieren shadowgrams, all of which can be compared directly
to experimental data.
The plasma is formed in a chamber containing 3at.m of H2 between
electrodes spaced 10cm apart. A current channel is formed from the
pulse of a 5 J neodymium-glass laser which is focussed along the
symmetry axis and which is fired near the peek of the voltage across
the electrodes(ft400kV). The current rises to 250kA in 200ns.
According to our computations the laser initiation is very
non-uniform and hence the initiation process is a complex
two-dimensional one involving virtual electrode formation and streamer
propagation. However, our two-dimensional alculations indicate that
the plasma column in remarkably one-dimensionii for greate: tnan 75ns.
Typical radial profiles are shown in rig. 1; the mass density profile
shows a shock wave prcpagating into the neutral embedding gas. Later
in time an ui0 instability occurs in the computations when the central
column expansion is stopped by the magnetic force. Even after th.
occurrence o; the m-0 instability the computations reproduce the
experimentally observed Schlieren shadowgrams(Fig. 2), "two-foil
x-ray" tmperatures(Fig. 3), and visible light measurements where the
computed non-uniform source rate and corresponding plasma reabsorption
are taken into account(Fig. 4). The computed shedowgram diameter of
Vig. 2 corresponds to the front of the outward moving shock wave shown
An Fig. 1; the actual current carrying channel is significantly
smaller in diameter than the shadowgram and has a diameter of less
than 1.Bmm at 200ns. After an initial rapid rise the temperature
reaches a plateau (Fig. 3; even though the curr-nt continues to rise.
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Brownell, J. H.; Lindemuth, I. R.; Oliphant, T. A. & Weiss, D. L. Ultra-high-density plasma experiments: MHD simulations, article, August 1, 1981; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1107312/m1/4/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.