A review of the dense Z-pinch

Plasma Phys. Control. Fusion 53 (2011) 093001
Topical Review
Figure 97. Spinning plasma jets; (a) schematic of twisted wire-array configuration illustrating
the introduction of angular momentum into converging plasma flow, (b) end-on XUV images of
untwisted and (c) twisted conical wire arrays. The jet in (c) is hollow due to rotation [715].
as the foil was placed closer to the jet. It is illustrated in figure 96. Various shock features can
be identified including a working surface as found in simulations by Ciardi et al [720]. Indeed
Hartigan [721] has expressed satisfaction that now laboratory experiments and simulations
with Z-pinches are able to reveal scaled astrophysical phenomena. In HH objects the eruption
of jets from the parent star occurs in pulses, which, as they move in the same direction but
at different speeds, different jets create working surfaces where streams of gas collide and
generate shock waves.
Astrophysical jets emanate from rotating accretion disks. It follows that the jets could
also have angular momentum [722]. In the laboratory by twisting the conical wire array, an
azimuthal component of the current is introduced causing axial and radial components of the
global magnetic field. In turn the (J z B r − J r B z ) force close to the wire causes rotation of the
incoming precursor plasma. The ablated plasma consists of a supersonic, rotating and radially
converging flow. Ampleford et al [723] have shown that this results in a hollow column on
axis. The incoming flow causes an equilibrium radius of the standing shock in which the
centrifugal force of the column is balanced by the ρv 2 ram pressure of the flow. End-on XUV
images showing the hollow precursor column are shown in figure 97. The axially, emerging,
rotating jet is observed to be hollow with a twisting filamentary structure.
The acceleration mechanism for jets in AGNs and YSOs is widely considered to be
magnetic [204]. Collimation over large distances is more problematic. Livio [724] wrote
that ‘there is no direct observation which confirms that jets are hydromagnetically driven.’
However in discussing HH flows, Reiparth and Bally [713] quote evidence for magnetic fields
from the circular polarization of the radio emission. Appl et al [726] have considered the
current-driven instabilities in astrophysical jets. Both axial and azimuthal components of
magnetic field are assumed to be present as in a tokamak or reversed field pinch (RFP). The
pitch of the magnetic field, rB z /B ϑ , on axis determines the growth rate of the fastest growing
kink instability. Unlike a tokamak it is likely that the two components of the magnetic field are
comparable. If the pitch is constant, the jet is unstable, while for variable pitch or magnetic
shear the work of Suydam applies [191] and instabilities occur at resonant surfaces where
k · B is zero, k being the perturbing wave number. Further away from the source of the jet the
azimuthal magnetic field will dominate, and the jet is more like a Z-pinch. In modelling the
effect of the instability Appl et al [726] assume that the jet is bounded by a rigid cylindrical
wall, representing the ambient medium, and the return axial current is a skin current in this
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