Observation of dust acoustic waves with high phase velocity in a cryogenic dusty plasma experiment

A capacitively-coupled plasma source equipped with liquid nitrogen cooling system has been developed to study water-ice dust grains in a plasma environment at the Korea Atomic Energy Research Institute [1]. It was found that elongated, submillimeter dust grains (100-250 um in length) are typically formed in the experiment [2]. Recently, self-excited dust acoustic waves (DAWs) were observed in the experiment and the phase velocities were measured by Fast Fourier transform of the sequential images of DAWs [2]. The measured phase velocities ranged from 8 to 15 cm/s, more than 10 times faster than the phase velocity predicted by the conventional linear theory. In order to explain the observed high phase velocities, we consider the effect of the dust thermal speed. The horizontal motion of the dust grains was analyzed to estimate the random thermal velocity of the dust grains, which ranged from 1.5 to 2.8 cm/s. Using the obtained thermal velocity, it is found that the calculated phase velocities with dust thermal speed term are on the same order of magnitude as the observed high phase velocity. In addition, the discrepancy between the calculated and observed phase velocity can be further decreased by including the rotational speed of the nonspherical dust grains.