High Time Resolution Spectroscopic Measurements of Electron Temperature in the SSX plasma
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2007
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Swarthmore College. Dept. of Physics & Astronomy
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Thesis (B.A.)
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Abstract
We present 1 μs time resolution calculations of the electron temperature
of the Swarthmore Spheromak Experiment (SSX) plasma during
magnetic reconnection. The non-LTE excitation kinematics code PrismSPECT
is used to simulate emission spectra for a variety of plasma conditions.
These model spectra are compared to experimental data from
two main diagnostics: a vacuum ultraviolet (VUV) monochromator and
a low-resolution soft x-ray detector (SXR). Analysis of simulation results
reveals that the plasma quickly (< 10 μs) approaches equilibrium conditions
in the density regime of interest; as a result we can safely use
steady-state simulations for comparisons with the data. Measured UV
line strength ratios depend primarily on the electron temperature in the
plasma, so we are able to use measurements of carbon impurity emission
lines in conjunction with SXR measurements as a temperature diagnostic.
In particular, the C III 97.7 nm / C IV 155 nm line intensity ratio proves
to be extremely useful, while the C III 229.7 nm line appears anomalously
strong in experimental measurements. Measurements of oxygen and nitrogen
lines allow us to conclude that the carbon/ oxygen number ratio in
SSX is approximately 1000/ 1, while nitrogen concentrations are negligible.
Temperatures derived from the 97.7 nm / 155 nm line ratio average
20 eV for single spheromak shots and 20 eV early in counter-helicity shots,
increasing to 35 eV after the two spheromaks merge. SXR measurements
suggest a mean electron temperature of 30-35 eV for single spheromak
shots and 40 eV for counter-helicity merging. The counter-helicity temperature
profile shows a distinct peak at t ~40 μs, the time at which
reconnect ion is believed to occur; however, the timing of this peak is not
in precise agreement with the peak in the average temperature profile
derived from carbon line ratios.