The Three-Dimensional Structure of Magnetic Reconnection in SSX
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2003
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Swarthmore College. Dept. of Physics & Astronomy
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Thesis (B.A.)
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Abstract
In plasma physics it is a commonly cited and often valid approximation that magnetic fieldlines are "frozen" to the
plasma fluid. However, in certain situations in solar and astrophysics, as well as in fusion engineering, this approximation
of "ideal magnetohydrodynamics" leaves out important physical processes. Large gradients in the field can
arise, causing effects that were previously negligible to become significant, and allowing the fieldlines to slip through
the plasma. Mysteriously, this "magnetic reconnection" effect appears to be much more significant than a back-of-theenvelope
calculation would suggest. Reconnection is investigated in the Swarthmore Spheromak Experiment (SSX),
a laboratory plasma device. The reconnecting field configuration in SSX is both largely reproducible and also more
three-dimensionally complicated than in previous experiments elsewhere. The principle diagnostic discussed herein
utilizes an array of small pick-up loops to measure the magnetic field. Use is made of a novel inexpensive highspeed
multiplexing data acquisition system, which for the first time permits observations of a plasma's complicated
dynamics in all three dimensions and their variation from shot to shot. Custom visualization software has been developed
to explore the complicated four-dimensional datasets. The magnetic probe exhibits an uncertainty of 2%, and
it correctly reproduces the theoretical expected fields for simple known current geometries. The probe provides evidence
that both a driven and a slower steady-state variety of reconnect ion do in fact occur in SSX, and several unique
three-dimensional properties distinguish the plasma configuration from previous "two-dimensional" experiments. An
unfortunate inherent curvature in the geometry prevents conclusive testing of a magnetic quadrupole phenomenon
reported in recent computational studies of collisionless reconnection. Various higher-order terms in the generalized
plasma Ohm's law can be computed, suggesting that the J x B Hall force and the pressure gradient force may be the
key phenomena in SSX among those neglected in ideal magnetohydrodynamics. Reconnection is correlated with the
detection of high-energy ions emanating from the site of spheromak collision. Recent modifications to the SSX device
to permit "full-merging" of the plasma have produced images of a more tempestuous and less controlled reconnection
geometry, although the physical processes involved seem to be the same as those in the original configuration.