The charge imbalance in ultracold plasmas

Abstract

Ultracold plasmas are regarded as quasineutral but not strictly neutral. The results of charge imbalance in the expansion of ultracold plasmas are reported. The calculations are performed by a full molecular-dynamics simulation. The details of the electron velocity distributions are calculated without the assumption of electron global thermal equilibrium and Boltzmann distribution. Spontaneous evolutions of the charge imbalance from the initial states with perfect neutrality are given in the simulations. The expansion of outer plasma slows down with the charge imbalance. The influences of plasma size and parameters on the charge imbalance are discussed. The radial profiles of electron temperature are given for the first time, and the self-similar expansion can still occur even if there is no global thermal equilibrium. The electron disorder induced heating is also found in the simulation.

Anisotropic ultracold plasma expansion

“…For electrons, disorder induced heating leads to a non self-similar expansion in the strongly coupled regime [16] and the quasi-neutrality breaks down. Thus, ultracod plasma expansion evolves a change imbalance leading to slowdown the outer plasma expansion [17]. However, expansion remains self similar despite a non global thermal equilibration.…”

Electron trapping in freely expanding ultracold neutral plasmas

“…This generates a weak electric field that traps the remaining electrons. It has been observed, however, that such local charge imbalance does not affect the self-similar expansion (Coulomb explosion) of the plasma [14]. The ion radial distribution can be obtained by fluorescence techniques [15].…”