Historical research guide · 1025 · 2008
Cross-point coronal plasma dynamics in two- and four-wire x-pinches
R. E. Madden · S. C. Bott · D. Haas · Y. Eshaq · U. Ueda · G. Collins · F. N. Beg
In one minute
An X-pinch is two or more fine wires crossed like the letter X and blown apart by an enormous current pulse. Where the wires touch, the magnetic squeeze is strongest, and for a few billionths of a second that crossing point becomes one of the smallest and brightest X-ray sources anyone can make on a laboratory bench — which is why X-pinches are used to photograph other experiments. Madden, Bott, Haas, Eshaq, Ueda, Collins and Beg, at the University of California San Diego, looked at what the plasma does after that flash. They drove two-wire and four-wire tungsten X-pinches with 80,000 amps rising in 50 billionths of a second, and read the result with two optical methods at once: interferometry, which measures how much the plasma delays a laser beam and so gives the density, and shadowgraphy, which gives the shape. The gap that opens at the cross point grows at about ten kilometres a second in both geometries, and the thin plasma left behind collapses a second time, producing a second X-ray flash.
Connection
Chapter 9 is about plasma that organises itself — that pinches, holds together, and radiates hard from a knot far smaller than the machine driving it — and the X-pinch is the sharpest laboratory example there is. What this paper adds is a measured density and a measured second collapse, which is what turns a bright flash into a mechanism other people can model and reproduce.
From the source
The experiment is a study of late-time diode gap formation in two-wire and four-wire tungsten X-pinches, driven by a current pulse of 80 kiloamps with a rise time of 50 nanoseconds.Abstract, first sentence
The density is measured, not inferred: quantitative measurements of the coronal plasma density are recovered using interferometry simultaneously with laser shadowgraphy, so the number and the picture come from the same shot rather than from separate campaigns.Abstract, second sentence
Axial expansion of the gap occurs at about 10⁶ centimetres per second — roughly ten kilometres a second — and it does so for the two-wire and the four-wire systems alike, so the expansion rate is not set by how many wires cross at the point.Abstract, third sentence
The mechanism the authors propose for that expansion is the plasma pushing itself apart: an axial force from the current crossed with its own magnetic field, arising from radial current flow in what they call the plasma minidiode electrodes — the two plasma stubs left on either side of the gap acting as the electrodes of a tiny diode.Abstract, third sentence, the stated interpretation
The pinch happens twice. Radial density profiles suggest that the low-density plasma repinches after the main pinch, and the consequence is measured directly: a secondary X-ray emission peak more than 10 nanoseconds after the first, recorded with a pair of pin diodes.Abstract, closing sentence
What to watch is whether that second collapse can be controlled rather than merely observed. The measurement that would settle it is a campaign that varies the wire number, the material and the current rise time and shows the delay and the brightness of the second X-ray peak moving with them — which would turn one X-pinch shot into a timed pair of X-ray flashes for radiography.Abstract, closing sentence, read against the two-wire and four-wire comparison of the first
The way in
https://doi.org/10.1063/1.3008046How to cite it
R. E. Madden, S. C. Bott, D. Haas, Y. Eshaq, U. Ueda, G. Collins, F. N. Beg (2008) Cross-point coronal plasma dynamics in two- and four-wire x-pinches doi:10.1063/1.3008046