Historical research guide · 0904 · 2012
Magnetically Driven Flows in Arched Plasma Structures
E. V. Stenson · P. M. Bellan
In one minute
Eve Stenson and Paul Bellan built an arched magnetic flux tube in a Caltech vacuum chamber — a horseshoe-shaped field with a gas nozzle and an electrode at each foot — and photographed what happens when current runs along it. By feeding a different gas into each footpoint and shooting through filters tuned to each gas, they colour-coded the plasma and could see exactly where the material came from. Both feet launch a fast flow up into the arch. The loop stretches to as much as ten times its starting length and kinks, yet it keeps the same width and the same brightness: it is being refilled from below as fast as it grows, and it stays collimated instead of fraying. The lengthening grows as time squared, and its acceleration tracks how fast the current rises — the signature of the hoop force on a current-carrying loop — while the inflow matches Bellan’s earlier ’gobble’ model. Ordinary magnetohydrodynamic forces are building and feeding a self-collimating plasma structure on a laboratory bench.
Connection
Chapter 9 asks how a plasma can hold a shape and keep itself fed, and this is a clean laboratory measurement of both at once, with the forces named and the numbers published — the same architecture the site reads in solar loops, astrophysical jets and plasmoid observations.
From the source
Feeding a different gas into each footpoint and imaging through species-matched optical filters shows two separate high-speed flows entering the arched flux tube, one from each foot, and each section lengthens at a rate set by its own gas rather than by the other section.p. 075001-2, Figs. 2 and 3 and accompanying text
The flux tube lengthens to as much as ten times its initial length and undergoes a kink instability while its minor radius and its brightness stay approximately constant, which requires that new material is being transported into the structure the whole time.p. 075001-2, Fig. 1(b) and the density argument that follows
Loop length grows in proportion to time squared at every charging voltage, and the constant acceleration of each loop is directly proportional to the initial rate of rise of the electrical current — the behaviour predicted by the hoop force on a current-carrying loop.p. 075001-3, Fig. 4 and Eqs. (1) to (4)
The apparatus is a pulsed magnetized plasma gun on a 1.6-metre by 1.4-metre vacuum chamber, with an arched vacuum field falling from 0.35 tesla at the footpoints to 0.01 tesla at the apex, a 59-microfarad capacitor charged to 3 to 6 kilovolts, peak output current 25 to 65 kiloamperes, about one tenth of that current flowing in the flux tube, and plasma densities of 10 to the 20th through 10 to the 21st per cubic metre.p. 075001-1, experimental setup; p. 075001-2, first column
Fitting the hoop-force model to the data returns a number density per unit length of 1.3 times 10 to the 17th per metre for hydrogen and about ten times less for nitrogen and argon, which for a one-centimetre minor radius gives densities consistent with the imaging.p. 075001-4, Fig. 5 and the paragraph fitting the parameter
Because magnetohydrodynamics has no intrinsic length scale, the authors expect the same footpoint-driven flows in other arched, boundary-intercepting flux structures — solar chromospheric upflows and jets are the named cases to look for them in.p. 075001-4, closing paragraph
The way in
https://doi.org/10.1103/PhysRevLett.109.075001How to cite it
E. V. Stenson, P. M. Bellan (2012) Magnetically Driven Flows in Arched Plasma Structures doi:10.1103/PhysRevLett.109.075001