Self-trapping of negative ions due to electron detachment in the afterglow of electronegative gas plasmas

The spatiotemporal evolution of charged species densities and wall fluxes during the afterglow of an electronegative discharge has been investigated. It was found that plasma decay crucially depends on the product of negative-ion-detachment frequency (gamma(d)) and diffusion time tau(d). If gamma(d)tau(d)> 2, negative ions convert to electrons during their diffusion towards the walls. The presence of detached electrons results in "self-trapping" of the negative ions, due to emerging electric fields, and the negative-ion flux to the walls is extremely small. Thus, negative ions can be extracted in the afterglow only if gamma(d)tau(d)< 2. (C) 2000 American Institute of Physics. [S0003-6951(00)00320-X].