Negative hydrogen ions and evidence for a potential dip in an electron cyclotron resonance discharge for highly charged ions

By means of photodetachment we have detected negative hydrogen ions in an electron cyclotron resonance (ECR) discharge under conditions where highly charged ions are present in the plasma if hydrogen is absent. From the time delay between the laser pulse and the photodetachment (PD) signals we obtained the location of the negative ion clouds in the halo of the plasma. It coincides with the location found for atomic hydrogen ions in previous ion cyclotron resonance heating experiments. The confinement of the ions to these places must be due to the presence of local potential maxima. These maxima imply a potential dip in the core of the plasma. Furthermore, these H- ions indicate the presence of cold electrons even under conditions where high charge states are produced. (C) 1998 American Institute of Physics.