Measuring Ice Grain Growth Rate in Caltech Dusty Plasma from Size and Wavelength Dependent Laser Extinction

The Caltech Ice Dusty Plasma Experiment studies the dynamics of ice grains suspended in a weakly ionized plasma at T=80-100 K. In one to two minutes, water vapor forms elongated fractal-like ice grains that grow up to 1-750 microns. While there is substantial research on the ice grains once they stabilize in size, the growth rate is unexplored. One possibility for tracking this growth rate is by measuring the attenuation of light that passes through the dusty plasma. The degree and direction of scattering depend on the incident angle, particle size, grain number density, and wavelength. A laser beam emitting two wavelengths at 532 and 1064 nm passes through the plasma, and the exiting beam is split by a wavelength-dependent beamsplitter and measured using two photodiode detectors. When 2πr/λ is smaller than 1, Rayleigh scattering occurs. For 2πr/λ >1, geometric scattering occurs. As the grains grow, the crossover from Rayleigh to geometric scattering will occur first for the 532 nm and later for the 1064 nm. This time difference will give an indication of the growth rate.