Ion Temperature and Flow Velocity Measurements on SSX-FRC
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2005
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Swarthmore College. Dept. of Physics & Astronomy
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Thesis (B.A.)
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Abstract
An Ion Doppler Spectroscopy (IDS) diagnostic was used to measure the flow velocity
and temperature of a plasma created by SSX-FRC. The diagnostic was based on the
principles of Doppler spectroscopy, namely, that the wavelength of a moving light
source is shifted proportional to its velocity and the width of an emission line varies
with temperature. The emission line at 229.7 nm of Carbon III, an impurity ion in
the hydrogen plasma, was imaged and its location and width measured. The IDS
system being a work in progress, the minimum resolvable linewidth is higher than the
linewidth we expect to see based on previous experiments and so detailed temperature
and velocity measurements could not be carried out. SSX's PMT's allow detailed
time resolution and the plot of temperature variation with time shows that the width
of the line peaks between 30 and 50 J1S before it drops to the minimum resolvable
width. Considering the basic plasma physics of the system, it is thought that the
wide line is due to velocity shear: oppositely directed jets resulting from magnetic
reconnect ion create both a red- and a blue-shifted emission line, which overlap and
are imaged as a single, very wide, line. A simple analytical model of a fluid system
with velocity shear was created to investigate whether or not shear could cause the
widening. The lineshapes this model returned were wide and double-peaked due to
overlap, supporting the shear hypothesis.