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Plasma Phys. Control. Fusion 53 (2011) 093001<br />
Topical Review<br />
with <strong>the</strong> snowplough interpretation. Figure 43 shows x-ray images <strong>of</strong> <strong>the</strong> <strong>pinch</strong>ing plasma on<br />
<strong>the</strong> Z-accelerator. It is to be noted that at stagnation and during <strong>the</strong> radiation pulse <strong>the</strong> <strong>pinch</strong>ed<br />
plasma retains gross cylindrical symmetry, has structures, but certainly is not disrupted.<br />
As a means <strong>of</strong> reducing <strong>the</strong> amplitude <strong>of</strong> <strong>the</strong> RT instability, Davis et al [60] proposed<br />
employing a second wire array <strong>of</strong> smaller diameter nesting coaxially inside <strong>the</strong> first array.<br />
It was conceived as a transparent array to which <strong>the</strong> current would switch. Experimentally<br />
Lebedev et al [325] showed that this would be <strong>the</strong> case, later confirmed by Cuneo et al [326]<br />
on Z. Chittenden et al [327] in simulations showed that <strong>the</strong>re were three modes possible<br />
distinguished by <strong>the</strong> amount <strong>of</strong> momentum transfer and magnetic flux compression during<br />
collision. At this collision a radiation pulse occurs, and, if it can be enhanced in a controlled<br />
way could be <strong>the</strong> driver <strong>of</strong> one <strong>of</strong> <strong>the</strong> early shocks in a hohlraum (see section 6).<br />
At <strong>the</strong> final stagnation <strong>the</strong> rate <strong>of</strong> conversion <strong>of</strong> kinetic and magnetic energy to radiation in<br />
high current (>10 MA) Z-<strong>pinch</strong>es is generally far greater than resistive MHD models predict,<br />
by factors <strong>of</strong> up to 4, this conversion taking place in about an Alfvén transit time a/c A .An<br />
analytic <strong>the</strong>ory based on ion-viscous heating through <strong>the</strong> fastest growing, short wavelength<br />
m = 0 MHD instabilities gives such a heating rate and predicts, for a light stainless-steel array,<br />
record ion temperatures <strong>of</strong> 200–300 keV which have been measured by Doppler broadening <strong>of</strong><br />
lines [130]. Earlier <strong>the</strong>ories were based on larger scale m = 0 toroidal cavities [328] closing<br />
<strong>of</strong>f and carrying magnetic flux tubes to <strong>the</strong> axis with drag heating (viscosity) and anomalous<br />
resistivity or turbulence [329–332]. At this time many bright, hot spots are observed on <strong>the</strong><br />
axis [323] and are an important contribution to <strong>the</strong> x-ray emission. A 3D simulation <strong>of</strong> MAGPIE<br />
experiments at a current ∼1 MA with no viscosity and a relatively coarse mesh showed <strong>the</strong> onset<br />
<strong>of</strong> m = 1 instabilities leading to helical current paths increasing <strong>the</strong> Ohmic dissipation [324]<br />
and so <strong>the</strong> radiated energy. However large amplitude perturbations to <strong>the</strong> wire ablation rate<br />
had to be introduced to achieve <strong>the</strong>se results. Fur<strong>the</strong>r discussion is deferred to section 5.7.<br />
In this brief overview it could be asked, why not use a foil instead <strong>of</strong> wires? The typical<br />
mass <strong>of</strong> a wire load on Z is 300–6000 µgcm −1 . A solid metallic shell would be 40–800 nm<br />
thick, i.e. as thin as a few hundred atomic spacings. Despite <strong>the</strong> difficulty in mounting such<br />
a shell, Nash et al [58] successfully drove an implosion but with lower performance than <strong>the</strong><br />
equivalent wire array.<br />
A more detailed <strong>review</strong> now follows, dividing up <strong>the</strong> processes in time as in [280, 323].<br />
5.2. Melting and vaporizing wire cores; plasma formation<br />
It seems crucial to understand <strong>the</strong> early phases <strong>of</strong> <strong>the</strong> evolution <strong>of</strong> <strong>the</strong> current-carrying wires in<br />
expanding liquid/vapour cores surrounded by a plasma which <strong>the</strong>n carries most <strong>of</strong> <strong>the</strong> current.<br />
X-ray backlighter images <strong>of</strong> single wires <strong>of</strong> 7.5–25 µm diameter metal wires carrying currents<br />
<strong>of</strong> 2–5 kA per wire in 350 ns show that <strong>the</strong> cores are expanding and have a heterogeneous<br />
structure [333]. These novel results were obtained using an X-<strong>pinch</strong> (see section 7.6) inone<br />
<strong>of</strong> <strong>the</strong> return conductors as a hard x-ray backlighter, which yields 2.5–10 keV x-rays (when<br />
filtered by a 12.5 µm Ti foil) by a time