Plasma–grain interaction in ultracold complex plasmas

The present particle-in-cell simulation for grain–plasma interaction at cryogenic temperatures springs from recent experimental realization of ultracold dusty plasmas with atoms and ions at superfluid helium temperatures. In this work, we discuss the results of particle-in-cell simulations (taking into account ion-atom collisions) for dust particle charge, ion drag force, and interaction between grains in ultracold dusty plasmas. The single grain as well as two grain system is investigated, considering both streaming ions and equilibrium ions. The dependence of plasma mediated inter-grain interactions on the streaming velocity of ions and inter-particle separation is delineated in detail. Additionally, ion density distribution around grains is discussed. The interactions at cryogenic temperatures are distinct from those at room temperature by their differences in magnitude and interaction mechanism details. Most remarkable features are (i) the inter-grain attraction in equilibrium plasmas due to plasma absorption on the surface of grains, (ii) the ion drag force directed against ion streaming velocity in the case of a single dust particle, and (iii) non-linear response of the plasma polarization around grains with an increase in the streaming velocity of ions.