Comparison of three devices for measuring the second derivative of a langmuir probe curve

Abstract

We have compared three devices, employing different principles, which plot the second derivative (SD) of the current voltage curve of a Langmuir probe. The probe measurements were performed in the positive column of a helium discharge. Two devices, using low modulation frequencies, produced within 1% the same shape of the SD curve. The third (200 kHz) yielded 10 to 25% lower values in the maximum of the SD curve; the discrepancy depended upon the pressure and diminished towards more negative probe potentials. The influence on the SD curves of the magnitude of the modulation voltages (up to 1 V peak to peak) is negligible in the case of helium. The determination of the space potential using the SD curves is discussed.