A review of the dense Z-pinch

Plasma Phys. Control. Fusion 53 (2011) 093001
Topical Review
the aluminium or tungsten. The prepulse current supply can rise to 10 kA. By varying the time
of the main LTD 5 MA current an optimum value of ∼7 µs delay was found. Amazingly the
cathode bubble disappeared, and an almost 1D snowplough-like and reproducible implosion
with no zipper occurred. The x-ray pulse is reduced to 20 ns (a quarter of its previous value)
and the peak power was increased by a factor of six to 1.5 TW cm −1 , with 25% conversion
efficiency. Single arrays performed as well as nested arrays, thus simplifying future loads.
Finally the design of petawatt-class pulsed-power accelerators has been considered by
Welch et al [472] and Stygar et al [473]. In [473] a comparison between Marx/pulse-forming
line and LTD is made, favouring the latter. Both systems require a magnetically insulated
transfer line (MITL), and Stygar et al [474] have studied in a relativistic model the critical issue
of electron current shorting here. By varying the charging voltage of the module cavities and
the timing, Stygar et al [475] have shown how the output current pulse of a LTD can be shaped.
A LTD is compact and can be rep-rated. These features are discussed by Mazarakis et al [476].
Already small LTD experiments are being undertaken in universities: Bott et al [477] have made
ablation studies of low number wire arrays at currents of 200 kA employing laser imaging
techniques to measure the wavelength of the axial density variations and ablation velocities. A
larger LTD-driven Z-pinch is being commissioned at the University of Michigan [478], while
a load current multiplier is being tested at the University of Nevada [479].
7. Other experimental configurations of the Z-pinch
As earlier discussed and illustrated in figure 3, there are many configurations of Z-pinches. The
experimental results from these will be reviewed, and interpreted using the theory developed
in earlier sections.
7.1. Compressional Z-pinch
Most of the earliest Z-pinch experiments were compressional Z-pinches in which there is
initially a uniform fill of gas inside an insulating cylinder, at the ends of which are the two
electrodes. In section 1.2 and [29–36] we reported some of these early experiments. They
are characterized by the formation through the skin effect of a current sheet adjacent to the
insulating wall, followed by an implosion. Whilst experiments were often interpreted using the
snowplough model, section 4.1 (which requires radiated energy loss for energy conservation),
it was soon found experimentally that preceding the magnetic piston was a shock wave
[257, 258] which caused dissociation and ionization [260] as discussed in section 4.2. A direct
experimental verification of the presence of the shock was published by Folkierski and Latham
[300]. Earlier, radiating luminous fronts were detected by Heflinger and Leonard [480] using
a streak camera, and attributed to a shock. At this stage there is no sign of an instability. Only
after the first pinch and a subsequent bounce with inward acceleration caused by the J ×B force
is the magneto- MRT instability observed and growth rates measured [39]. These instabilities
in the form of m = 0 constrictions are found to occur first in the vicinity of the electrodes, and
lead to axially moving luminous fronts which could be shocks [481]. These could be similar
to the end effects found more recently in wire-array pinch experiments (see section 5.10).
A theory of ionizing strong shocks was developed by Gross [482] and compared with
experiments. Further experiments on radially converging cylindrical shocks in various gases
by Kogelschatz et al [483] and Kleist et al [484] confirm several aspects of Potter’s slug
model [261] (see section 4.3), namely very little current travels with the shock wave, and the
gas pressure between the shock and piston is approximately uniform. However at higher filling
pressures significant current has been found to flow even ahead of the shock [483].
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