THE ROLE OF THE PERTURBED PLASMA POTENTIAL IN THE DYNAMIC PLASMA RESPONSE TO PHOTODETACHMENT

The destruction of negative ions by laser photodetachment suddenly mod ifies the electron and negative ion densities in a localized region. T he subsequent recovery of negative ion density n-(t) may be measured w ith a second photodetaching laser pulse. These measurements, compared with a ballistic model for n-(t), are the basis of a negative ion temp erature diagnostic, which is being developed and employed in H- and D- sources. In this paper the accompanying plasma potential perturbation and its effect on the negative ion response is discussed. The perturb ation can introduce an error in negative ion temperature measurements. Late in time, the negative ion density is observed to recover more sl owly than indicated by the ballistic theory. An intriguing mechanism i s proposed for the late slow recovery of negative ion density, based o n a small lingering potential perturbation and a hole in negative ion velocity space. These different theoretical approaches are illustrated with a detailed analysis of an example case from a hydrogen discharge .