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Coherent Diffraction Method for Imaging Dusty Plasma Ice Grains

Presentation #147.09 in the session Laboratory Astrophysics (LAD) Division Meeting: iPoster Session.

Published onJun 29, 2022
Coherent Diffraction Method for Imaging Dusty Plasma Ice Grains

Water ice grains spontaneously form in the Caltech ice dusty plasma experiment [1] which involves cryogenic (~190 K) neutrals, water vapor, and weak (~10-6) ionization. The ice grains have been imaged until now using a long-distance microscope lens—with a 3μm resolution limit—mounted on a camera. Reference [1] showed that ice grain size and ellipticity could be ascertained from diffraction patterns of a HeNe laser beam. Re-visiting diffraction methods, we aim to use the diffraction pattern to obtain a complete detailed image of the ice grains. This pattern on a screen is the absolute value of a 2D Fourier transform (FT) of the ice grain shape so phase information is lost. We will use the Fienup phase restoration method [2] where iterative numerical guesses restore the missing phase using the physical constraint that the image intensity is non-negative. By taking absolute values of FT’s, we have demonstrated this method on synthetic diffraction patterns, first removing then recovering phase. Theoretical considerations indicate that this diffraction/Fienup restoration method should improve on the microscope lens resolution by an order of magnitude.

1. K. B. Chai and P. M. Bellan, Geophys. Res. Lett. 40, 6258 (2013); 2. J. R. Fienup. Opt. Lett. 3, 27 (1978)

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