Measurements and Computations of the Visible Light Spectrum Emitted from the Ionized Surface of Ohmically Heated Aluminum Rods

Measurements have been reported of surface brightness temperature for green light in the UNR rod heating experiments (other papers these proceedings and Invited Presentation by T. J. Awe.). We also measured the entire visible spectrum to compare with a blackbody (T. S. Goodrich, M.S. Thesis, UNR, 2010). Results show a spectrum measurably different than a blackbody. Numerical modeling suggests the following interpretation. Bremsstrahlung is the primary emission and absorption process from aluminum with Z$\sim $3. Deviation from a blackbody occurs under some conditions (lower temperatures early in time) because the optical thickness is less than unity, and the spectrum tends towards that of bremsstrahlung. Under other conditions (higher temperatures later in time) the optical depth can be above unity, but a temperature gradient length comparable to the absorption length alters the emission spectrum. The comparison of experiment and modeling allows an estimate of plasma density, which is difficult to measure directly, and in general adds credence to numerical modeling.