A review of the dense Z-pinch

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Plasma Phys. Control. Fusion 53 (2011) 093001 Topical Review Figure 42. A 3D simulation of a 16 mm diameter array of 16 × 15 µm Al wires driven by the MAGPIE generator. The (red) surface of constant density equal to 0.1 kg m −3 is representative of the dense plasma region, while the (green) surface at 0.003 kg m −3 is representative of the coronal plasma. On the right are contours of ln ρ in a slice through the x–z plane [324]. shown in figure 42 have demonstrated much agreement with this interpretation of the data. It should be noted that the snowplough implosion is far less susceptible to RT instabilities. The main x-ray pulse occurs at stagnation of the snowplough implosion compressing the precursor plasma. Cuneo et al [323] have shown in detail that high current implosions on Z also agree 71
Plasma Phys. Control. Fusion 53 (2011) 093001 Topical Review with the snowplough interpretation. Figure 43 shows x-ray images of the pinching plasma on the Z-accelerator. It is to be noted that at stagnation and during the radiation pulse the pinched plasma retains gross cylindrical symmetry, has structures, but certainly is not disrupted. As a means of reducing the amplitude of the RT instability, Davis et al [60] proposed employing a second wire array of smaller diameter nesting coaxially inside the first array. It was conceived as a transparent array to which the current would switch. Experimentally Lebedev et al [325] showed that this would be the case, later confirmed by Cuneo et al [326] on Z. Chittenden et al [327] in simulations showed that there were three modes possible distinguished by the amount of momentum transfer and magnetic flux compression during collision. At this collision a radiation pulse occurs, and, if it can be enhanced in a controlled way could be the driver of one of the early shocks in a hohlraum (see section 6). At the final stagnation the rate of conversion of kinetic and magnetic energy to radiation in high current (>10 MA) Z-pinches is generally far greater than resistive MHD models predict, by factors of up to 4, this conversion taking place in about an Alfvén transit time a/c A .An analytic theory based on ion-viscous heating through the fastest growing, short wavelength m = 0 MHD instabilities gives such a heating rate and predicts, for a light stainless-steel array, record ion temperatures of 200–300 keV which have been measured by Doppler broadening of lines [130]. Earlier theories were based on larger scale m = 0 toroidal cavities [328] closing off and carrying magnetic flux tubes to the axis with drag heating (viscosity) and anomalous resistivity or turbulence [329–332]. At this time many bright, hot spots are observed on the axis [323] and are an important contribution to the x-ray emission. A 3D simulation of MAGPIE experiments at a current ∼1 MA with no viscosity and a relatively coarse mesh showed the onset of m = 1 instabilities leading to helical current paths increasing the Ohmic dissipation [324] and so the radiated energy. However large amplitude perturbations to the wire ablation rate had to be introduced to achieve these results. Further discussion is deferred to section 5.7. In this brief overview it could be asked, why not use a foil instead of wires? The typical mass of a wire load on Z is 300–6000 µgcm −1 . A solid metallic shell would be 40–800 nm thick, i.e. as thin as a few hundred atomic spacings. Despite the difficulty in mounting such a shell, Nash et al [58] successfully drove an implosion but with lower performance than the equivalent wire array. A more detailed review now follows, dividing up the processes in time as in [280, 323]. 5.2. Melting and vaporizing wire cores; plasma formation It seems crucial to understand the early phases of the evolution of the current-carrying wires in expanding liquid/vapour cores surrounded by a plasma which then carries most of the current. X-ray backlighter images of single wires of 7.5–25 µm diameter metal wires carrying currents of 2–5 kA per wire in 350 ns show that the cores are expanding and have a heterogeneous structure [333]. These novel results were obtained using an X-pinch (see section 7.6) inone of the return conductors as a hard x-ray backlighter, which yields 2.5–10 keV x-rays (when filtered by a 12.5 µm Ti foil) by a time
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A review of the dense Z-pinch

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