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Experimental Exploration of High-Speed Non-Equilibrium Dynamics

Author: Paul M. Bellan
Requested Type: Consider for Invited
Submitted: 2012-12-04 11:28:33

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Caltech
1200 E. California Blvd
Pasadena, CA   91125
USA

Abstract Text:
Traditional analysis of plasma behavior first postulates an equilibrium exists and then either considers perturbations to this equilibrium or that the equilibrium slowly evolves through a sequence of neighboring states. An important example is the Taylor relaxation hypothesis wherein it is assumed that a magnetized plasma evolves (relaxes) towards a unique minimum-energy state while conserving magnetic helicity. Because the final state is independent of the initial state, the plasma is said to self-organize. Taylor relaxation has applicability to magnetic fusion confinement (reverse field pinches, spheromaks, and tokamak coaxial helicity injection), the solar corona, and certain astrophysical plasmas. While Taylor theory predicts the minimum-energy end state reasonably well, it fails to describe what actually happens, i.e., it does not predict the underlying dynamics. Furthermore, Taylor theory assumes zero beta whereas actual plasmas have finite beta. The actual dynamics underlying self-organization of a magnetized plasma is being investigated using magnetized plasma guns that inject magnetic helicity. Plasma dynamics is tracked using high speed imaging (movies) that resolve sub-Alfven time scales. These movies reveal unexpected, new phenomena. In particular, in contrast to the Taylor point of view where flows and pressure gradients are neglected, the movies show that a highly collimated MHD-driven plasma flow is a critical feature of the dynamics. This collimated flow can be considered to be a jet, but the jet has both axial and azimuthal magnetic fields and so can also be considered to be a plasma-confining flux tube with embedded helical magnetic field (flux rope). The jet velocity is in good agreement with an MHD acceleration model [1]. Axial stagnation of the jet compresses embedded azimuthal magnetic flux and so results in jet self-collimation. Depending on how the flux tube radius varies with axial position, flux tubes can have jets flowing into the flux tube from both ends [2] or from just one end [1]. Jets kink when they breach the Kruskal-Shafranov stability limit. The acceleration of a sufficiently strong kink can provide an effective gravity that provides the environment for a spontaneously developing fine-scale, extremely fast Rayleigh-Taylor instability that erodes the current channel to be smaller than the ion skin depth [3]. This cascade from the ideal MHD scale of the kink to the non-MHD ion skin depth scale can result in a fast magnetic reconnection whereby the jet breaks off from its source electrode [3]. Supported by USDOE, NSF, and AFSOR.

[1] D. Kumar and P. M. Bellan, Phys. Rev. Letters 103, Article Number 105003 (2009)
[2] E. V. Stenson and P. M. Bellan, Phys. Rev. Letter 109, Article Number 075001 (2012)
[3] A. L. Moser and P. M. Bellan, Nature 482, p. 379-381 (2012)

Characterization: 3.0,4.0

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