Negative Ions And Molecular Ions Observed In Ion Diodes

We observed, with a Thomson Parabola spectrometer, molecular ions H
₂
⁺
in an intense ion beam diode on the LION accelerator. This appears to be an early-time phenomenon that occurs close to the peak voltage. The current density in this 1.3 MV diode is about .5 kA/cm
²
, in comparison with ˜ 3KA/cm
²
typical of PBFA II proton diodes. We also observed on LION substantial differences in species accelerated from locations on the anode located only a few mm apart, indicating localized differences in temperature or density and possibly indicating substantial deviations in planarity on a mm scale. Geometric constraints on PBFA II make similar observations impossible using presently available diagnostics, but these observations suggest some possible contributions to divergence and energy spread. In an experiment unrelated to the foregoing, we placed conical apertures at several locations on the anode of the PBFA II accelerator at Sandia and recorded, using CR-39 track recording plastic, the presence of energetic negative ions during the early portion of the pulse. These ions, originating on the cathode surface or from the mylar membrane that separates the gas transport region from the acceleration gap, may play a role in the evolution of the anode-plasma-ion source. The number density and species identification await further results. We will present calculations on diode dynamics, assuming that negative ions are present in sufficient quantity to allow space charge limited flow.