Imaging Refractometry Technique Development*

The imaging refractometry technique is a valuable diagnostic tool for studying high energy density plasmas. Density fluctuations in these plasmas are challenging to measure, particularly in gas-puff Z-pinch implosions where turbulence is thought to be present [1,2,3]. After its introduction [4], the technique has been further developed at Cornell’s Laboratory of Plasma Studies for diagnosing imploding gas-puff Z-pinch plasmas using a 40 mJ, 150 ps, frequency doubled Nd:YAG laser pulse at 1064/532 nm. To obtain time-resolved wavenumber spectra a visible light streak camera is being used along with a ~150 mJ, ~12 ns, frequency doubled Nd-YLF laser pulse at 532 nm.

The imaging refractometry technique measures the angular deflection of a collimated laser beam as it passes through a plasma to generate, on a detector screen, an image where the y-axis is the deflection angle and the x-axis is the spatial direction axis. The spectrum of angular deflections can be linked to the wavenumber spectrum by measuring a stationary target with an analytically known Fourier transform [5].

The Beam Propagation Method (BPM) simulation [6] was developed to provide predictive ability for the Imaging Refractometer in specific plasmas and synthetic spectra for analysis of observed spectra. The PERSEUS 3D Extended MHD code [7] is being employed to obtain a 3D density pattern of a plasma column at a given time to provide the specific plasma for the BPM code. PERSEUS and the BPM simulation are run in combination using the Bridges-2 system [8] to obtain high spatial resolution.

Recent results, challenges, and future plans will be reported.