Second derivative Langmuir probe measurements of negative oxygen ionconcentration taking into account the plasma depletion caused by negative ionsink on the probe surface

Abstract

Taking into account the effect of plasma depletion caused bynegative ion sink on the probe surface, a refinement of the second derivativeLangmuir probe method for the determination of negative ion concentration isproposed. The concentration of the negative ions is obtained by comparing theexperimental data with the calculated second derivative probe current signalsat a finite ratio of probe radius and negative ion free path.In a specially constructed dc discharge tube with coaxial electrodes, secondderivative Langmuir probe measurements are performed in pure oxygen (gaspressure 30-110 Pa, discharge current 40-80 mA). The negative ionconcentrations at different discharge current and gas pressure values areobtained. The factors affecting the uncertainties of the results arediscussed.The values of the negative oxygen ion concentration obtained by laserphotodetachment measurement are in good agreement with those obtained by thesecond derivative Langmuir probe measurements.The proposed processing of the experimental data extends the applicability ofthe second derivative Langmuir probe method for accurate measurements of thenegative ion concentration at higher gas pressures and finite probe radii.