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 meas urements. A theoretical analysis has been made by considering the pres ence 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 muc h less than the positive ion n(+). The ratios of saturation current of electrons and negative ions to that of positive ions, which are exper imentally obtained by probe measurements, suggest that the sheath has a property of the change from PIS to NIS at the ratio of electron to p ositive-ion number densities n(e)/n(+) below 10(-2).