Historical research guide · 1085 · 2017
Filamentation in the pinched column of the dense plasma focus
P. Kubeš · M. Paduch · J. Cikhardt · B. Cikhardtová · D. Klír · J. Kravárik · K. Řezáč · E. Zielińska · M. J. Sadowski · A. Szymaszek · K. Tomaszewski · D. Zaloga
In one minute
A dense plasma focus dumps a bank of capacitors into a gas, drives a sheet of current down a pair of coaxial electrodes and lets it collapse onto the axis into a pencil of plasma dense and hot enough to fuse deuterium. Kubeš and eleven colleagues in Prague and Warsaw photographed that pencil in high-energy ultraviolet light on a mega-ampere machine, and found it is not a smooth column at all. Thin filaments run inward from the surface of the pinch, and they last longer than fifty billionths of a second — for a plasma this hot, long enough to count as structures rather than flickers. The filaments end on small plasma balls about a millimetre across which sit still inside the column and live as long as the filaments do. Every deuterium shot produced fusion neutrons. The authors read the picture as the current itself being discrete: flowing in separate channels through and around the column, and carrying plasma inward from the outer layers.
Connection
Chapter 9 is about plasma that organises itself — the pinch, the plasmoid, the ball that holds together far longer than it has any right to. This is a photograph of exactly that, taken inside a fusion machine: millimetre-scale plasmoids standing still in the middle of a mega-ampere discharge, wired to the outside of the column by current-carrying filaments, with fusion neutrons coming off every shot.
From the source
The pinched column of a mega-ampere dense plasma focus is not smooth. A filamentary structure is observed in the high-energy ultraviolet radiation, in discharges filled with hydrogen or deuterium and puffed with hydrogen, deuterium or helium, and the lifetime of that structure exceeds 50 nanoseconds.Abstract, sentences 1 and 2
The filaments connect the surface of the pinched column with plasmoids formed inside it, across different combinations of filling and puffing gases, and they should transport some current and some plasma between the two.Abstract, sentence 3
Fusion-produced neutrons were recorded during all the investigated deuterium shots. Deuterons must therefore be present in the region where they are accelerated, and this holds independently of which gas was puffed.Abstract, sentences 4 and 5
Small plasma balls of millimetre dimensions were observed inside the dense plasma column at the same time as the filaments. They had a similar lifetime — longer than the relaxation time — and quasi-stationary positions in the discharge volume.Abstract, sentence 6
The authors’ reading of the images is that the filaments and the balls may be a manifestation of two things at once: a discrete spatial structure of the current flowing through and around the dense plasma column, and transport of plasma from the external layers into the central region. Discrete current channels rather than a smooth sheath is the load-bearing part of that.Abstract, sentence 7, points i and ii
The structures were made easier to form and easier to see by adding air to the puffed gas — a diagnostic and experimental handle on a phenomenon that is otherwise hard to catch.Abstract, final sentence
The way in
https://doi.org/10.1063/1.4978558How to cite it
P. Kubeš, M. Paduch, J. Cikhardt, B. Cikhardtová, D. Klír, J. Kravárik, K. Řezáč, E. Zielińska, M. J. Sadowski, A. Szymaszek, K. Tomaszewski, D. Zaloga (2017) Filamentation in the pinched column of the dense plasma focus doi:10.1063/1.4978558