Negative ion density fronts

Negative ions tend to stratify in electronegative plasmas with hot electron s (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 w ithout negative ions, is usually thin so that the negative ion density fall s rapidly to zero-forming a negative ion density front. Theoretical, experi mental, and numerical results giving the spatio-temporal evolution of negat ive ion density fronts during plasma ignition, the steady state, and extinc tion (afterglow) are reviewed. During plasma ignition, negative ion fronts are the result of the break of smooth plasma density profiles during nonlin ear convection. In a steady-state plasma, the fronts are boundary layers wi th steepening of ion density profiles due to nonlinear convection also. But during plasma extinction, the ion fronts are of a completely different nat ure. Negative ions diffuse freely in the plasma core (no convection), where as the negative ion front propagates towards the chamber walls with a nearl y constant velocity. The concept of fronts turns out to be very effective i n the analysis of plasma density profile evolution in strongly nonisotherma l plasmas. (C) 2001 American Institute of Physics.