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Abstract. Plasma formation and implosion dynamics of wire array z-pinches
have been studied experimentally using the MAGPIE generator (1.4MA, 240ns)
at Imperial College. Simulations and theory verify much of the data. Both
laser probing and x-ray radiography show after an initial volumetric heating
of the wires the presence of dense wire cores surrounded by low density
coronal plasma. Radiography shows development of perturbations on the dense
core of each wire, while laser probing shows inward jetting of the coronal
plasma caused by the global JxB force, and these plasma streams are axially
non-uniform on the same spatial scale as later seen in the wire cores. The
spatial scale of these perturbations (
0.5mm for Al, 0.25mm for
W) increases with the size of the wire cores ( 0.25mm for Al,
0.1mm for W). The inward flow of the coronal plasma is usually field
free and leads to formation on the array axis of a straight plasma column,
the dynamics of which is strongly affected by radiation cooling. Images
obtained by optical streak camera show that the wire cores start their
inward motion late and the implosion trajectory deviates significantly from
the expected from 0-D analysis. An increase of the number of wires (decrease
of inter-wire gap) resulted in a transition to 0-D trajectory for aluminium
wire arrays, but not for tungsten. In experiments with nested wire arrays
two modes of behaviour are observed; in the first the inner array is
transparent to the imploding outer array, but the current transfers to it,
leading to a fast implosion. The second mode occurs when a significant
fraction of current is flowing in the inner array and the two arrays
apparently implode simultaneously. In both modes the x-ray pulse is
significantly sharpened in comparison with that generated in implosion of a
single wire array.
IAEA 2001
