Trapped ions and dusty “quasi-atoms” polarization in an external electric field*

Micron-sized dust particles immersed in a low-density plasma acquire a large negative charge, Q_d = Z_d e₀ ~ 10³-10⁴e₀. A cloud of trapped ions with the total number of Z_tr ~ 0.5·Z_d can be formed around a dust particle. The trapped ions form a broad shell with the maximum at R_m ~ (0.3-0.5)·λ_i, which screens the proper charge of the dust particle from the influence of external electric fields. Only the effective charge Z_eff = Z_d - Z_tr manifests itself in the interactions of dust particles with other dust particles and with external electric fields. However, in the external electric field, the system of the dust particle and the cloud of trapped ions (the “dusty quasi-atom”) can be polarized, and thus the system acquires a dipole moment.