Historical research guide · 0612 · 2004
Implosion dynamics of wire array Z-pinches: experiments at Imperial College
S. V. Lebedev · D. J. Ampleford · S. N. Bland · S. C. Bott · J. P. Chittenden · C. Jennings · M. G. Haines · J. B. A. Palmer · J. Rapley
In one minute
A Z-pinch is the simplest fusion machine there is: run a colossal current down a column of matter and its own magnetic field crushes it inward. At Imperial College the column is a cage of fine wires, and Lebedev and his colleagues report what the MAGPIE generator — about a million amps in a quarter of a microsecond — actually shows happening inside one. The wires do not simply collapse like a falling shell. Each wire stays put as a solid core wrapped in plasma, and that plasma streams inward ahead of everything else, quietly redistributing the array’s mass before the implosion proper begins. How fast a wire gives up its plasma depends on the collective magnetic field of the whole array and rises with it. Because the streaming is uneven along each wire’s length, the imploding sheath leaves material behind it, and that trailing mass carries some of the current on a path of its own — which is exactly the force the pinch loses when it finally stagnates.
Connection
Chapter 9 is about plasma that organises its own structure, and a wire array is the cleanest laboratory case there is: the cores, the coronas and the inward streams arrange themselves, and the machine’s output is decided by that self-organisation rather than by the drive alone. Chapter 12 gets the practical lesson — the current that leaks into trailing mass is the compression a fusion pinch does not get, which is why array geometry, not just stored energy, sets what these machines can do.
From the source
The implosion of a wire array is not a simple snowplough. A gradual redistribution of the array mass by the precursor plasma flow from the wire cores plays a very important role in the implosion dynamics — the wires form a core and corona structure, and plasma streams inward from stationary cores well before the sheath itself moves.Abstract
The rate of wire ablation depends on the magnitude of the global, collective magnetic field of the array, and increases as that field increases — so the array as a whole, not each wire on its own, sets how quickly mass is delivered to the axis.Abstract
The ablation rate is modulated along the length of each wire, and that modulation leaves a trailing mass behind the imploding current sheath rather than carrying all of the array material inward with it.Abstract
The trailing mass provides an alternative path for the current, and that shunted current reduces the force available for compression of the pinch at stagnation — the loss is in the current distribution, not in the drive.Abstract
The experiments are performed on the MAGPIE generator at Imperial College London, which delivers of order one mega-amp in about 250 nanoseconds, and the programme exists to feed measured implosion physics into the design of the much larger pulsed-power machines used for radiation and fusion work.Abstract; title and affiliation
The open question the paper hands forward is how much of the lost compression can be recovered by engineering the array — wire number, wire material and array geometry — so that ablation is uniform along the wires and little or no trailing mass is left for the current to find.Abstract, closing statement on stagnation
The way in
https://doi.org/10.1088/0029-5515/44/12/s12How to cite it
S. V. Lebedev, D. J. Ampleford, S. N. Bland, S. C. Bott, J. P. Chittenden, C. Jennings, M. G. Haines, J. B. A. Palmer, J. Rapley (2004) Implosion dynamics of wire array Z-pinches: experiments at Imperial College doi:10.1088/0029-5515/44/12/s12