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

The spatiotemporal evolution of charged species densities and wall fluxes d uring 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 toward s 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].