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Quantification of MHD flows into arched open magnetic field structures

Author: Eve V. Stenson
Requested Type: Poster Only
Submitted: 2011-06-10 19:06:30

Co-authors: P. M. Bellan

Contact Info:
Caltech MC 128-95, 1200 E Cali
Pasadena, CA   91125

Abstract Text:
Open magnetic field lines are fundamental to spheromak formation, error fields in confinement devices, solar coronal structures, and astrophysical jets.

At Caltech, we use a magnetized plasma gun to create open magnetic field line structures in the form of arched flux tubes coupled at each end to the gun electrodes. An individual flux tube can be described in terms of its axis and its minor radius. Shortly after breakdown, the axis is nearly semicircular, and in the minor radial direction there is a force balance between the pressure gradient and JxB forces. Plasma is thus confined inside the arched tube, forming a bright "loop." Over the course of the following microseconds, the axis undergoes a rapid evolution, extending in length as much as sevenfold and becoming kink unstable. Equilibrium in the minor radial direction is maintained during this process, and the minor radius remains relatively constant. The total volume of the tube therefore increases substantially. Since the density does not decrease substantially, additional plasma must be entering the tube. We have shown this happens by means of flows from both footpoints (i.e., where the tube intersects the electrodes). Experiments with dual-gas plasmas -- in which a different gas species is supplied to each of the two footpoints -- demonstrate that these flows are independent of one another; each flow velocity depends only on the density of the plasma originating from the respective footpoint and on the electrical current flowing along the loop.

Measurements of flux tube length, magnetic field, and electrical current produce quantitative agreement with two related MHD models: the increase in major radius is explained by the hoop force, and the inflows of plasma are explained by the gobble model [1]. The gobble model predicts MHD-driven bulk flows along the axis from areas of stronger axial current density (i.e., the footpoints) into areas of weaker axial current density (i.e., the apex of the arch); if the axis were not lengthening these flows would only persist until the arched, semicircular tube achieved uniform density. The observed lengthening is consistent with the hoop force. Initially, the electric current increases approximately linearly, in which case the flux tube length is predicted to be a quadratic function of time -- which is indeed observed. The hoop force acting alone, however, would result in the plasma becoming progressively less dense. The gobble and hoop force dynamics in tandem explain the observed structure: a rapidly lengthening loop that maintains collimation and minor radial force balance as a result of inflowing plasma from the footpoints.

[1] P. M. Bellan, "Why current-carrying magnetic flux tubes gobble up plasma and become thin as a result," Phys. Plasmas 10 Pt 2, 1999 (2003).

Supported by: DOE, NSF, AFOSR

Characterization: D1,D5

Please place with other Caltech posters. Thank you very much!

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