Measurement of Negative Ion Density by an Electric Probe with a New Analytic Formula

Abstract

Absolute density of negative ions is measured in the DiPS(Diversified Plasma Simulator, B=2000 G) and MAP-II(Material and Plasma, B=200G) plasmas by a new method without knowing the sheath potential, sheath velocity, temperatures of positive and negative ions, and effective masses of positive ions when there are negative ions among normal positive ion-electron plasma. This new probe method is to be verified by comparing with the laser photodetachment method. This measurement is based upon an analytic formula, which is derived using the measured data of ion and electron saturation currents and electron temperature by assuming the linear dependence of the small change of negative ion densities upon small change of the pressure or flow rate of the added gases for the generation of negative ions along with the linearity between the negative ions and the positive ions generated by the added gases. With the measured I-V characteristics over three different pressures of mixed gases or flow rates of added gas for the generation of negative ions, the whole spectrum of negative ion density variations can be deduced by this three-point linearity.