Magnetic reconnection from a multiscale instability cascade |
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Paul Bellan Submitted: 2012-02-15 21:51
Magnetic reconnection, the process whereby magnetic field lines breakand then reconnect to form a different topology, underlies criticaldynamics of magnetically confined plasmas in both natura1, andthe laboratory. Magnetic reconnection involves localizeddiffusion of the magnetic field across plasma, yet observedreconnection rates are typically much higher than can be accounted forusing classical electrical resistivity. It is generally proposedthat the field diffusion underlying fast reconnection results insteadfrom some combination of non-magnetohydrodynamic processes that becomeimportant on the 'microscopic' scale of the ion Larmor radius or theion skin depth. A recent laboratory experiment11 demonstrated atransition from slow to fast magnetic reconnection when a currentchannel narrowed to a microscopic scale, but did not address how amacroscopic magnetohydrodynamic system accesses the microscale. Recenttheoretical models and numerical simulations suggest that amacroscopic, two-dimensional magnetohydrodynamic current sheet mightdo this through a sequence of repetitive tearing and thinning intotwo-dimensional magnetized plasma structures having successively finerscales. Here we report observations demonstrating a cascade ofinstabilities from a distinct, macroscopic-scale magnetohydrodynamicinstability to a distinct, microscopic-scale (ion skin depth)instability associated with fast magnetic reconnection. Theseobservations resolve the full three-dimensional dynamics and giveinsight into the frequently impulsive nature of reconnection in spaceand laboratory plasmas.
Authors: Auna L. Moser & Paul M. Bellan
Projects: None
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Publication Status: published as a Nature Letter, February 16, 2012, online on Feb 15
Last Modified: 2012-02-17 08:42
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Laboratory simulations of astrophysical jets and solar coronal loops: new results |
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Paul Bellan Submitted: 2010-01-28 16:28
An experimental program underway at Caltech has produced plasmas where the shape is neither fixed by the vacuum chamber nor fixed by an external coil set, but instead is determined by self-organization. The plasma dynamics is highly reproducible and so can be studied in considerable detail even though the morphology of the plasma is both complex and time-dependent. A surprising result has been the observation that self-collimating MHD-driven plasma jets are ubiquitous and play a fundamental role in the self-organization. The jets can be considered lab-scale simulations of astrophysical jets and in addition are intimately related to solar coronal loops. The jets are driven by the combination of the axial component of the JxB force and the axial pressure gradient resulting from the non-uniform pinch force associated with the flared axial current density. Behavior is consistent with a model showing that collimation results from axial non-uniformity of the jet velocity. In particular, flow stagnation in the jet frame compresses frozen-in azimuthal magnetic flux, squeezes together toroidal magnetic field lines, thereby amplifying the embedded toroidal magnetic field, enhancing the pinch force, and hence causing collimation of the jet.
Authors: P. M. Bellan, D. Kumar, E. V. Stenson, S. K. P. Tripathi, G. S. Yun, and A. L. Moser
Projects: None
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Publication Status: AIP Conf. Proc. (submitted), paper presented at Como Italy Symposium 2009
Last Modified: 2010-01-29 09:59
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Dynamic and Stagnating Plasma Flow Leading to Magnetic-Flux-Tube Collimation |
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Paul Bellan Submitted: 2005-08-18 14:41
Highly collimated, plasma-filled magnetic-flux tubes are frequently observed on galactic, stellar, and laboratory scales. We propose that a single, universal magnetohydrodynamic pumping process explains why such collimated, plasma-filled magnetic-flux tubes are ubiquitous. Experimental evidence from carefully diagnosed laboratory simulations of astrophysical jets confirms this assertion and is reported here. The magnetohydrodynamic process pumps plasma into a magnetic-flux tube and the stagnation of the resulting flow causes this flux tube to become collimated.
Authors: S. You, G. S. Yun, and P. M. Bellan
Projects: None
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Publication Status: published in Physical Review Letters, Vol 95, art. 45002 (July 22, 2005)
Last Modified: 2005-08-18 14:41
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Co- and counter-helicity interaction between two adjacent laboratory prominences |
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Paul Bellan Submitted: 2004-06-08 15:24
The interaction between two side-by-side solar prominence-like plasmas has been studied using a four-electrode magnetized plasma source that can impose a wide variety of surface boundary conditions. When the source is arranged to create two prominences with the same helicity (co-helicity), it is observed that helicity transfer from one prominence to the other causes the receiving prominence to erupt sooner and faster than the transmitting prominence. When the source is arranged to create two prominences with opposite helicity (counter-helicity), it is observed that upon merging, prominences wrap around each other to form closely spaced, writhing turns of plasma. This is followed by appearance of a distinct bright region in the middle and order of magnitude higher emission of soft x rays. The four-electrode device has also been used to change the angle of the neutral line and so form more pronounced S-shapes.
Authors: Hansen, J.F., Tripathi, S. K. P. and Bellan, P. M.
Projects: None
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Publication Status: appeared in June 2004 Physics of Plasmas, Vol. 11, pp. 3177-3185
Last Modified: 2004-06-08 15:24
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Why current-carrying magnetic flux tubes gobble up plasma and become thin as a result |
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Paul Bellan Submitted: 2003-01-23 09:09
It is shown that if a current-carrying magnetic flux tube is bulged at its axial midpoint z=0 and constricted at its axial endpoints z=+h,-h, then plasma will be accelerated from z=+h,-h towards z=0 resulting in a situation similar to two water jets pointed at each other. The ingested plasma convects embedded, frozen-in toroidal magnetic flux from z=+h,-h to z=0. The counter-directed flows collide and stagnate at z=0 and in so doing (i) convert their translational kinetic energy into heat, (ii) increase the plasma density at z~0, and (iii) increase the embedded toroidal flux density at z~0. The increase in toroidal flux density at z~0 increases the toroidal field Bphi and hence increases the magnetic pinch force at z~0 and so causes a reduction of the flux tube radius at z~0. Thus, the flux tube develops an axially uniform cross-section, a decreased volume, an increased density, and an increased temperature. This model is proposed as a likely hypothesis for the long-standing mystery of why solar coronal loops are observed to be axially uniform, hot, and bright.
(This is a short ASCII version of abstract - no LaTex)
Authors: P. M. Bellan
Projects:
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Publication Status: Physics of Plasmas (in press, to appear May 2003)
Last Modified: 2003-01-23 09:09
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