Historical research guide · 0507 · 2001
Effect of discrete wires on the implosion dynamics of wire array Z pinches
S. V. Lebedev · F. N. Beg · S. N. Bland · J. P. Chittenden · A. E. Dangor · M. G. Haines · K. H. Kwek · S. A. Pikuz · T. A. Shelkovenko
In one minute
A Z-pinch is the most direct fusion machine there is: send an enormous current down a column of matter and the column’s own magnetic field crushes it inward. At Imperial College the column is a cage of hair-fine wires, and Sergey Lebedev and his colleagues use the MAGPIE generator — about a million amps in a quarter of a microsecond — to establish what actually happens inside one. Their answer replaces the picture of a shell collapsing all at once. For roughly the first eighty per cent of the implosion the wires do not move: each stays put as a solid core, boiling off plasma that jets continuously inward and fills the interior of the array with a precursor cloud. Only when the boiling opens gaps through the cores does the implosion proper start, and it then sweeps up that pre-filled cloud in a rapid snowplough. Lebedev argues the cloud’s density, peaked on the axis, is a key reason these implosions stay stable, and sets out how to scale the picture to larger machines.
Connection
Chapter 9 is about plasma that builds its own structure, and this is the cleanest laboratory case of it: cores, coronas and inward jets arrange themselves, and the machine’s output follows from that arrangement rather than from the drive alone. Chapter 12 takes the practical lesson — what a pinch does at stagnation is decided by how the mass got there, which is why array geometry is a design variable on the same footing as stored energy.
From the source
For about the first eighty per cent of the implosion the wire cores remain stationary in their initial positions, while coronal plasma jets continuously from those cores to the axis of the array. The array is redistributing its own mass long before anything that looks like an implosion begins.Abstract
That stationary phase ends with the formation of gaps in the wire cores, and the gaps open because the ablation rate is not uniform along the length of each wire.Abstract
The final phase, starting at that moment, is a rapid snowplough-like implosion of the radially distributed precursor plasma that was injected into the interior of the array during the stationary phase — so the imploding sheath runs into material that is already there rather than into vacuum.Abstract
The density distribution of that precursor plasma is peaked on the array axis, and the authors propose it as a key factor providing the stability of wire array implosions operating in the regime of discrete wires — stability supplied by the plasma’s own arrangement rather than imposed from outside.Abstract
The model is phenomenological and built directly on measurement: it is derived from experimental data taken on the MAGPIE generator, and its purpose is to supply modified initial conditions for one-dimensional and two-dimensional radiation-magnetohydrodynamic simulations in the r and z plane, which had been started from the wrong picture of the early implosion.Abstract; title and affiliation
The paper closes on a scaling question it hands to the larger machines: whether this discrete-wire behaviour, and the axially peaked precursor it produces, carries over to a substantially larger drive current.Abstract, closing sentence on scaling to a larger drive current
The way in
https://doi.org/10.1063/1.1385373How to cite it
S. V. Lebedev, F. N. Beg, S. N. Bland, J. P. Chittenden, A. E. Dangor, M. G. Haines, K. H. Kwek, S. A. Pikuz, T. A. Shelkovenko (2001) Effect of discrete wires on the implosion dynamics of wire array Z pinches doi:10.1063/1.1385373