Dynamics of Water–Ice Grains Formed in a Plasma Where Gravitational Force is Compensated by Thermophoretic Force

A capacitively coupled discharge source equipped with liquid nitrogen cooling system has been developed to create water-ice grains in a plasma environment at the Korea Atomic Energy Research Institute. We found that the gravitational force exerted on 5-μm water-ice grains is nearly compensated by the thermophoretic force when the upper electrode is cooler than that of the bottom electrode by 10-15 K and we observed two distinct, axisymmetric vortex flows are formed in the upper and lower parts of the plasma. The numerical calculation solving a set of equations including the vorticity equation confirms that the observed axisymmetric vortices result from the nonconservative ion drag force.