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Time Resolved Visible Spectroscopy of Surface Plasma Radiation
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Imploding a solid metal liner onto a central, current carrying conductor shows some promise as a means of inertial and magnetic confinement of deuterium plasma for a controlled fusion reaction. Thick wire explosions provide a means to study some of the physics that arises during the liner implosion. Both the liner and thick wire form metallic plasmas near the surface, which have resistivity of the same order, and both have radius, or thickness in the case of the liner, larger than skin depth. Thick wire explosion experiments were performed using the Zebra 1MA current generator, with rise time of approximately 70ns, at various wire radii. To confirm temperature estimates made with other diagnostics (see Tom Awe Dissertation, December 2009, UNR), a diagnostic was designed to examine the visible part of the spectrum emitted during the wire explosions. The spectral diagnostic, which consists of a spectrograph that has a photodiode array positioned at the focal plane, has temporal resolution of 0.5ns and spectral resolution of roughly 30nm. At each time step eleven brightness data points, at wavelengths ranging from 391.9nm to 687.3nm in the visible part of the spectrum, were collected for thick wire explosions of radii 0.5mm, 0.8mm and 1mm. Because of the work of colleagues, dating back to 1960, it was believed that the emitted spectrum would be blackbody. The spectrum emitted from each radius of wire was compared to the blackbody radiation spectrum at several time steps. At early times, the spectra peak in the visible, at roughly 450.5nm, and do not have a shape that conforms well to the blackbody spectrum at a temperature that would cause a peak at roughly 450nm. When the spectra are plotted at later time, the peak seems to move further into the blue, and eventually into the ultraviolet. When the spectral peak is in the ultraviolet, long wavelength values correspond fairly well to the blackbody spectrum, but there is some departure from blackbody in the blue. Based on these results, it seems blackbody radiation is too simple a model to accurately describe the emission of shorter-wavelength visible radiation from the thick wire explosions. However, these results need to be adjusted for the spectral transmission of the vacuum window and debris shield inside the zebra chamber viewing port.