Negative ion density fronts

Negative ions tend to stratify in electronegative plasmas with hot electrons (electron temperature T-e much larger than ion temperature T-i, T-e>>T-i). The boundary separating a plasma containing negative ions, and a plasma without negative ions, is usually thin so that the negative ion density falls rapidly to zero-forming a negative ion density front. Theoretical, experimental, and numerical results giving the spatio-temporal evolution of negative ion density fronts during plasma ignition, the steady state, and extinction (afterglow) are reviewed. During plasma ignition, negative ion fronts are the result of the break of smooth plasma density profiles during nonlinear convection. In a steady-state plasma, the fronts are boundary layers with steepening of ion density profiles due to nonlinear convection also. But during plasma extinction, the ion fronts are of a completely different nature. Negative ions diffuse freely in the plasma core (no convection), whereas the negative ion front propagates towards the chamber walls with a nearly constant velocity. The concept of fronts turns out to be very effective in the analysis of plasma density profile evolution in strongly nonisothermal plasmas. (C) 2001 American Institute of Physics.