An investigation into the mechanisms occurring in the discharge of an ion source

An experimental study of the mechanisms occurring in the constricted type of discharge used in ion sources has been performed. The variation on the radius of the fireball forming at the constriction, together with the plasma potential, electron temperature and density variation through the discharge, were measured as a function of the discharge current and neutral gas flow rate and pressure. The studies showed that the fireball grows out of the background plasma whenever local conditions require that the electron drift velocity exceed the ion sound speed. Large jumps in potential and electron temperature are found to occur across the sheath surrounding the fireball in a hydrogen discharge. In addition a maximum discharge current limitation in terms of the neutral gas flow rate was observed.