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Plasma Phys. Control. Fusion 53 (2011) 093001<br />
Topical Review<br />
Figure 78. Imperial College gas-embedded <strong>pinch</strong>; number density pr<strong>of</strong>ile <strong>of</strong> electrons and neutrals<br />
during preheating phase at a filling pressure <strong>of</strong> 0.3 bar [182, figure 3]. Reprinted with permission<br />
from <strong>the</strong> IAEA.<br />
In <strong>the</strong>ir conceptual fusion reactor based on <strong>the</strong> high plasma density Z-<strong>pinch</strong> Hartman<br />
et al [552] considered that one means <strong>of</strong> isolating <strong>the</strong> Z-<strong>pinch</strong> is by a <strong>dense</strong> blanket <strong>of</strong> cold<br />
neutral gas. Two <strong>pinch</strong> cycles were considered, one where <strong>the</strong> <strong>pinch</strong> was sufficiently long to<br />
minimize plasma flow out <strong>of</strong> <strong>the</strong> ends and <strong>the</strong> o<strong>the</strong>r where refuelling occurred by plasma flow,<br />
e.g. using axial flow as proposed by Morozov [553] with FLR stabilization This has developed<br />
fur<strong>the</strong>r to include sheared axial flow, considered in sections 3.5 and 7.7.<br />
Jones et al’s [554] laser-initiated, gas-embedded Z-<strong>pinch</strong> experiment with a 2D MHD<br />
model indicated a 200 eV electron temperature at a density <strong>of</strong> 10 20 cm −3 . The <strong>pinch</strong> was<br />
powered by a 600 kV Marx generator and (water) transmission line into 3 atm H 2 gas with<br />
260 kA, rising in 200 ns.<br />
In his <strong>the</strong>sis [555] Shlachter presented schlieren images showing <strong>the</strong> growth <strong>of</strong> helical<br />
structures inside <strong>the</strong> cylindrical, expanding envelope <strong>of</strong> <strong>the</strong> current-carrying plasma. This<br />
illustrated <strong>the</strong> dominance <strong>of</strong> m = 1inaZ-<strong>pinch</strong> with a centrally peaked current density (see<br />
section 3.2). Dangor et al [556] found that <strong>the</strong>ir laser initiated, gas-embedded <strong>pinch</strong> with<br />
a preheat current to raise <strong>the</strong> initial temperature and lower <strong>the</strong> density, did not expand but<br />
maintained a constant radius during <strong>the</strong> time <strong>of</strong> <strong>the</strong> main current rise. This is because <strong>the</strong><br />
current rise approximately satisfied <strong>the</strong> Haines–Hammel [103, 104] curve. After this time <strong>the</strong><br />
<strong>pinch</strong> transformed into an expanding helical structure similar to that reported by Smårs [548].<br />
No disruptive break-up occurred.<br />
Scudder [109] undertook a more complete 1D steady-state calculation <strong>of</strong> radial heat loss<br />
and bremsstrahlung losses. Problems arise when <strong>the</strong> Pease–Braginskii current is approached.<br />
Indeed at 1.8 MA both <strong>the</strong> pressure and radial heat flux go to zero at <strong>the</strong> plasma boundary.<br />
(However we should note that radiative collapse is probably precluded by ion-viscous heating<br />
associated with short wavelength m = 0 MHD instabilities, section 5.8.) The Imperial<br />
College gas-embedded <strong>pinch</strong> [53] was upgraded in current to 150 kA and in <strong>the</strong> use <strong>of</strong> laser<br />
interferometry to show that three phases could be distinguished. In <strong>the</strong> preheating phase<br />
<strong>the</strong> plasma column expanded from an initial radius <strong>of</strong> 100 µm with a radial velocity <strong>of</strong><br />
1.7×10 4 ms −1 driving a shock wave in <strong>the</strong> neutral gas. The Abel inversion <strong>of</strong> an interferogram<br />
gives <strong>the</strong> electron and neutral densities in figure 78. The equilibrium phase followed when<br />
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