A review of the dense Z-pinch

Plasma Phys. Control. Fusion 53 (2011) 093001
Topical Review
Figure 99. The evolution of magnetic tower jets is shown (a) in synthetic x-ray emission images
from 3D simulations at 210, 220, 230 and 240 ns (from left to right, the intensity being on a
logarithmic scale spanning 10 3 ) and (b) experimental, time-resolved extreme UV images at 268,
278, 288 and 298 ns. Reprinted with permission from [706, figure 5]. Copyright 2007, American
Institute of Physics.
the axle of which is the cathode. This and the resulting large J r B ϑ upward force, strongest
near the cathode, is illustrated in figure 98. In figure 98(a) the scheme of the experiment is
shown, the MAGPIE generator being able to drive a 1 MA current, rising over 240 ns. The
wire ablation phase is illustrated in figure 98(b) in which an almost current-free axially moving
background plasma is formed. Then because the mass ablation rate increases with the global
field [65] which here varies as 1/r, gaps appear at 220 ns in the wire cores near the cathode and
a high R m plasma carrying the current is driven up axially as shown in figure 98(c). A jet-like
column develops on the axis in figure 98(d). A time sequence of soft x-ray images is shown in
figure 99(b) showing the magnetic tower together with the central current carrying jet which
undergoes m = 0 and m = 1 instabilities as it moves with Mach ∼10 in the z-direction. The
magnetic cavity grows axially at 200 km s −1 and radially at 50 km s −1 . The reason is the large
J r B ϑ force at the tip of the toroidal cavity. The instability evolves on the Alfvén transit time
scale but the jet is not destroyed and persists for ∼20 growth times. Later the magnetic tower
and plasma jet get detached from the source. Ciardi et al [732] have undertaken a 3D MHD
simulation and reproduce in synthetic x-ray emission images in figure 99(a) the main features
of the experiment. The presence of the initial background plasma is found to be essential for the
evolution of the magnetic tower. It supplies some of the mass of the jet through the occurrence
of a bow shock ahead of the tower. In this experiment and simulation the ion mean-free path
in low density regions however is long ∼200 mm, but the ions are strongly magnetized with a
Larmor radius of 0.3 mm. This localizes the ion behaviour and permits the use of MHD fluid
equations. When a thin foil replaces the radial wires a second magnetic tower is generated
through a re-strike of current in the residual mass [733]. This episodic behaviour, similar to
that found in astrophysics, was found experimentally [734] and in simulations [735].
In conclusion, the Z-pinch permits scaled astrophysical experiments [736] both in
magnetic-free and magnetically dominated phenomena; particularly shocks, jets, magnetic
acceleration and magnetic towers.
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