A review of the dense Z-pinch

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Plasma Phys. Control. Fusion 53 (2011) 093001 Topical Review Figure 92. (a) Schematic of a cylindrical Hall thruster. (b) Typical externally applied magnetic field distribution which leads to azimuthal Hall currents. Reprinted with permission from [670, figure 1]. Copyright 2003, American Institute of Physics. and extra Joule heating of the electrons by the Hall current was found. A central core of the plasma appeared well defined, stable and detached from the walls. But the next stage of this programme was not pursued as interest switched to pulsed-power driven Z-pinches. However a UCLA team under Dawson [68, 69] reinvented the scheme, but they too was unable to take it further. A related experiment in Sweden was the EXTRAP concept [672]. But here the external multipole fields (n = 4 or 8) were added to a straight Z-pinch, and carried external currents alternately parallel and anti-parallel to the J z current of the plasma. The objective of the multipole fields was to provide positive curvature to the fields near the plasma surface, and hence provide a stable equilibrium. A toroidal version was also experimentally studied, but the temperature was limited by radial losses through the gaps in the multipolar fields [673], as here, in contrast to the Polytron, there was no induced transverse ion c.m.-motion to reduce particle and thermal losses. This programme evolved into a reversed field pinch. 141
Plasma Phys. Control. Fusion 53 (2011) 093001 Topical Review Figure 93. Schematic drawing of the Polytron apparatus; a toroidal Z-pinch threading through 36 cusp-shaped magnetic fields which enable both Hall acceleration leading to ion currents and stabilization of MHD modes [66, figure 1]. 8.6. Multi-species ion separation If a dynamic Z-pinch is formed with several ion species it is possible during the implosion phase to separate them because the J × B force is exerted on the electrons. A slight charge separation creates a radial electric field which accelerates the ions inwards radially with a value ZeE r /Am p . Thus ions with different Z/A will be accelerated by different amounts unless collisions between the species dominate. In a gas-puff mixture Rahman et al [674] showed that the plasma shell split into two distinct annuli which imploded concentrically. This separation of species is however a little impaired by the onset of the RT instability. Perhaps a series of compressions with axial flow would be more effective. A related paper by Chakrabarti et al [675] employed two ion species. This has the geometry more of a plasma opening switch, and cases could be found where the ions acquired a velocity component perpendicular to the J × B force. In a plasma opening switch geometry Osin et al [676] with 150 kA of 400 ns duration showed a marked difference of behaviour of the two ion species; rapid magnetic field penetration into the heavy-ion plasma and specular reflection of the light-ion plasma, leading to ion-species separation. 8.7. Compression of axial magnetic field; the Z– pinch Two purposes exist for adding an axial magnetic field to the azimuthal field of the Z-pinch. One is to enhance the stability of the plasma configuration, especially for fusion; the other to compress the axial magnetic field to very high values, possibly for application in understanding the physics of dense matter in a uniform intense magnetic field. One of the earliest experiments of an ‘ultra-fast pinch’, using a Blumlein circuit involving charged coaxial cables, was undertaken in 1963 by Adlam and Holmes [677]. An axial magnetic field of up to 1 T was first established in a Helmholtz coil and a fast Z-pinch of 30 kA rising in 20 ns (or millimicroseconds as it was then termed) applied in a silica discharge 142
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A review of the dense Z-pinch

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