Plasma Crystal: Coulomb Crystallization in a Dusty Plasma

A macroscopic Coulomb crystal of solid particles in a plasma has been observed. Images of a cloud of $7-\mu m$ "dust" particles, which are charged and levitated in a weakly ionized argon plasma, reveal a hexagonal crystal structure. The crystal is visible to the unaided eye. The particles are cooled by neutral gas to 310 K, and their charge is $>9800e$, corresponding to a Coulomb coupling parameter $\Gamma >20\mathrm{} 700$. For such a large $\Gamma$ value, strongly coupled plasma theory predicts that the particles should organize in a Coulomb solid, in agreement with our observations.