Investigation of a Drifting Plasma Containing Negative Ions by Cylindrical Langmuir Probe Ion Mass Spectrometer

Abstract

The study of drifting plasma in the laboratory or the ionsphere is not a straightforward problem because of the many variables such plasma will encounter. The presence of negative ions will add to the complication due to the detachment mechanism and other physical processes. It is not easy to separate the negative ion current component from the electron current component. To study negative ion population and mass spectra in the lower ion-sphere the value of the mean free path λ and the Debye shielding radius λo of the medium should be taken into consideration together with the collision frequency of the different charged particles. Cylindrical Langmuir probes have been used successfully as dianostic techniques for the drifting plasma. A cylindrical probe of suitable radius in relation to λ and λo was chosen. The probe was swept by a composite wave-form which contained a sawtooth sweep and two audio -frequency signals. The experiment was flown on board a stabilised rocket "Centaure" at high latitudes at Norway. The outputs yielded information about : 1. The first derivative of the current i with respect to the applied probe voltage -V 2. Percent modulation from which d2 i/dv2 was obtained. The second derivative is a function of the energy distribution function of the charged particles. The negative ion Gaussian peaks of energy are easily detached on the energy distribution function graph from those of thermal electrons.