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Investigations into what an open-field line, current-carrying magnetized plasma really does

Author: Paul M. Bellan
Requested Type: Poster Only
Submitted: 2011-06-07 16:08:25

Co-authors: A. L. Moser, E. V. Stenson, V. H. Chaplin, M. K. Kendall

Contact Info:
Caltech
1200 E. California Blvd
Pasadena, CA   91125
USA

Abstract Text:
Although magnetohydrodynamics (MHD) is presumed to be a fairly mature subject, experimental investigations continue to reveal remarkable new insights into how MHD really works.
The Caltech experimental research program has demonstrated that certain widely used MHD paradigms are either inappropriate or somewhat misleading. These paradigms date from the early days of fusion research when it was assumed that (i) a plasma could be considered to be in a static MHD equilibrium and (ii) there could be stable or unstable Fourier mode perturbations about this equilibrium. Another paradigm has been to assume that if the plasma has low beta, its behavior can be reasonably described by ignoring pressure gradients.
What we have found is that when there is electric current flowing along open magnetic field lines, the presumed static equilibrium does not exist. Instead complex non-equilibrium dynamics occurs. The plasma progresses through a sequence of non-equilibrium configurations such that there are strong unbalanced forces resulting in high-speed plasma flows involving MHD acceleration of plasma injected where the open field line intercepts the wall. These flows convect frozen-in magnetic flux which provides pinching force resulting in substantial pressure gradients. The pinched flow is effectively a collimated MHD-driven plasma jet. The jet is neither static nor in equilibrium because it is lengthening. Despite not being in static equilibrium, the jet can become unstable and kink. Hence instability is not restricted to static equilibria but is also an important feature of dynamically changing, non-equilibrium plasmas.
Most recently, it has been observed that (i) the kink itself can become unstable (see presentation by A. L. Moser and P. M. Bellan) yielding a finer structure instability (instability of an instability) and (ii) when current flows along a magnetic flux tube that is axially non-uniform such that it is bulged at its axial midpoint, MHD forces drive axial plasma jets from both ends towards the middle (see presentation by E. V. Stenson and P. M. Bellan). These effects are observed using high-speed imaging systems that capture the plasma evolution on the Alfven time scale.
Experiments are being planned to (i) increase the jet speed by lowering the density (see presentation by V. H. Chaplin and P. M. Bellan) and (ii) inject Alfven waves (see M. K. Kendall and P. M. Bellan).
Supported by USDOE, NSF, and AFOSR

Characterization: D5,D6

Comments:
Place Caltech abstracts together as:
1. Bellan, Paul
2. Moser, Auna (if not a talk)
3. Shikama, Taiichi
4. Stenson, Eve
5. Chaplin, Vernon
6. Kendall, Mark

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011