A review of the dense Z-pinch

Plasma Phys. Control. Fusion 53 (2011) 093001
Topical Review
Figure 52. A radial streak picture of a typical wire array on MAGPIE. Reprinted with permission
from [65]. Copyright 2001, American Institute of Physics.
Figure 53. Gated soft x-ray images of the precursor column in carbon, aluminium and tungsten
taken on MAGPIE. Reprinted with permission from [65]. Copyright 2001, American Institute of
Physics.
the precursor streams impinging on it. Because of radiative cooling of the higher Z plasma
there is a radial contraction of the column in W; the diameters of the columns in MAGPIE
are 3 mm for C, 1.25 for Al and 0.2 mm for W. Benjamin et al [350] and Aivazov et al [319]
earlier had shown the formation of a precursor column.
In a hybrid r −θ Monte Carlo treatment of the Fokker–Planck equation Sherlock et al [351]
show that for tungsten the precursor flow is relatively collisionless, while for aluminium it
becomes collisional almost immediately a significant amount of plasma arrives on axis. The
precursor column forms after a critical density occurs on axis through convergence, allowing
a nonlinear compression by collisional interaction with the incoming streams. The W ions are
more collisionless than Al ions because the W ions have a higher kinetic energy (because of
their higher atomic mass), causing an increase in the mean-free path not compensated by the
higher Z value. Figure 54 shows the evolution of ion number density profile with time and
also the ion–ion mean-free path and ion temperature. In contrast as shown by experimental
end-on laser probing by Lebedev et al [352] in figure 55, the Al is more collisional and shock
interactions between individual streams can be seen.
The low magnetic Reynolds number in the ablation zone was first shown by Haines [230]
in a 3D analytic model of the ablation process in the presence of localized m = 0 necks in which
Joule heating was concentrated. This led to flux limited heat flow to the cores, this providing
the energy for ionization and ablation. (It should be noted that while radiation transport
is dominant in the outer regions, the final heat flow to the cold cores is thermal transport.)
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