Probe Measurements in a Negative Ion Plasma

A plasma with negative ions (negative ion plasma), which is generated by introducing a small amount of SF₆ gas into a surface ionized plasma (e.g., Q-machine plasma), is investigated by electrostatic probe measurements. A theoretical analysis has been made by considering the presence of a negative ion sheath (NIS) in addition to the usual positive ion sheath (PIS), where the thermal velocity of negative ions is less than that of positive ions and the electron number density n_e is much less than the positive ion n₊. The ratios of saturation current of electrons and negative ions to that of positive ions, which are experimentally obtained by probe measurements, suggest that the sheath has a property of the change from PIS to NIS at the ratio of electron to positive-ion number densities n_e/n₊ below 10^-2.