Historical research guide · 0915 · 1999
Z-pinch discharges in aluminum and tungsten wires
J. Ruiz-Camacho · F. N. Beg · A. E. Dangor · M. G. Haines · E. L. Clark · I. Ross
In one minute
A Z-pinch turns a hair-thin wire into plasma by pushing an enormous current through it, and the current’s own magnetic field then crushes that plasma inward. What the wire is made of turns out to matter a great deal. Ruiz-Camacho, Beg, Dangor, Haines, Clark and Ross at Imperial College fire 160,000 amps into single wires, rising to peak in 65 billionths of a second, and compare tungsten wires of several thicknesses against a fine aluminium wire. Tungsten’s vapour cloud expands at about 9,400 metres a second; aluminium’s goes at least twice as fast. Aluminium also loses its shape sooner — the sausage-like necking of the column starts at about 8 nanoseconds against 20 for tungsten — and the bright spots, the intensely radiating knots strung along the pinch, appear earlier and, on the thicker tungsten wires, last far longer. Only tungsten radiates hard X-rays above 6 kiloelectronvolts from the plasma itself, while both metals drive an electron beam that lights up the anode.
Connection
Chapter 9 is about plasma that organises itself into structures which hold together and carry current, and this paper is a direct measurement of how much that self-organisation depends on the material it is made from: the same current, the same machine, and two metals give different expansion speeds, different instability clocks, different bright spots and different radiation. Anyone designing a pinch — as a radiation source or as a fusion driver — is choosing that behaviour when they choose the wire.
From the source
The same driver gives materially different pinches. Driven by a pulsed-power generator delivering 160 kiloamps with a 10 to 90 percent rise time of 65 nanoseconds, tungsten wires of various diameters and a 15 micrometre aluminium wire behave differently enough that wire material and wire diameter are treated as the experiment’s variables.Abstract, opening sentences
Schlieren imaging measures the expansion of the coronal plasma around the wire: for tungsten wires of various diameters it is 9.4 plus or minus 1.0 times 10 to the 3 metres per second, and the aluminium pinch expands at least a factor of 2 faster.Abstract
The instability clock runs on the material. The m equals zero sausage perturbations appear at about 8 nanoseconds for aluminium against about 20 nanoseconds for the tungsten pinch, and for both wires the wavelength and the diameter of the perturbations grow with time, relatively faster for aluminium.Abstract
Wire thickness controls how long fine structure survives: short wavelength perturbations of about 200 micrometres persist for a longer time on larger diameter tungsten wires.Abstract
Bright spots — the intensely radiating knots that form along the column — appear after 60 nanoseconds from the start of the current for tungsten and after 40 nanoseconds for aluminium, and they decay in 40 nanoseconds on the smallest diameter tungsten wire against only a few nanoseconds on the larger diameters.Abstract
Hard X-ray emission above 6 kiloelectronvolts is observed from tungsten wire pinches but from neither the bright spots nor the plasma column of the aluminium pinch; separately, hard X-ray emission from the anode, produced by an electron beam, is observed for wires of both materials.Abstract, closing sentences
The way in
https://doi.org/10.1063/1.873529How to cite it
J. Ruiz-Camacho, F. N. Beg, A. E. Dangor, M. G. Haines, E. L. Clark, I. Ross (1999) Z-pinch discharges in aluminum and tungsten wires doi:10.1063/1.873529