Proposed mechanism for the formation of cold dense structures in plasmas.

The formation of cold dense structures in a hydrogen isotope plasma, caused by the penetration of impurities from a localized source, is modelled numerically. It is shown that a bubble structure with very high densities and low temperatures of all plasma components, i.e. electrons, main and impurity ions, arises if the density of impurity neutrals in the source exceeds a critical level. The major cause of this condensation phenomenon are Coulomb collisions between main and impurity ion species which, on the one hand, cool down the main ions and, on the other hand, confine impurity ions near the source. This mechanism is efficient even if the impurity radiation, considered usually as the most important cause of condensation instabilities, is completely switched off in the calculations. On the contrary, without ion-ion Coulomb collisions the radiation losses result only in a transient cooling of electrons with a later recovery of their temperature to a level comparable with the initial one.