Interfacing of PLIF Measurements with 2D-MHD Calculations

Summary form only given. We have recently employed data obtained from laser-induced fluorescence (PLIF) measurements to provide the initial density distribution for 2D-MHD calculations of argon Z-pinches. In previous studies those data were derived from hydrodynamics calculations. The measured data introduce a feature not present in the calculated initial conditions, namely, the presence of sub-millimeter density fluctuations. In view of the key role played by the Rayleigh-Taylor instability in these plasmas, particularly as shorter wavelengths grow more rapidly, it is necessary to understand the importance of this feature. To address the issue we have performed MACH2 calculations and compared results using the raw, measured data and compared them with cases where the short wavelength components were filtered out. It appears that longer wavelength excitations, produced by the macroscopic density variations (~10 mm) dominate much of the pinching process. With a uniform initial distribution perturbed at a given wavelength, the shorter wavelength behavior would be expected to grow most rapidly. With a non-uniform system where the swept-up material shows long wavelength dominance, the sub-millimeter variations do not grow to large magnitude although they do modify the long wavelength behavior and indirectly, to the compressions achieved near axis. While the short wavelengths do modify plasma compression, the effect on compression saturates after a small number of smoothing iterations