ИСТИНА

A violent relaxation of the electron velocities, resulting in the sharp increase of their temperature, is a well-known property of the ultracold plasmas created by photoionization of a neutral gas. Surprisingly, a numerical modeling of this phenomenon substantially depends on the boundary conditions imposed on the simulated volume. In this work, we studied in detail three types of the boundary conditions: "free" (i.e., a finite-size bunch of the charged particles), periodic and reflective. It was found that the minimal increase of the electron temperature takes place in the case of free boundaries. This is not surprising because the electrons accelerated due to the multi-particle interactions escape outwards and begin to move in the "halo", thereby, not experiencing a further heating. The maximum increase in the temperature was obtained in the case of the periodic boundary conditions, but a considerable part of this increase comes from a permutation of the electrons between the opposite sides of the simulated volume (i.e., has a non-physical nature). At last, the reflective boundary conditions give the intermediate magnitude of the temperature increase. Since they are characterized by a small integration errors and do not suffer from other non-physical effects, we believe that just the reflective conditions are the most robust tool for simulation of ultracold plasmas.