Capacitively coupled plasma used to simulate Titan's atmospheric chemistry

A complex chemistry in Titan's atmosphere leads to the formation of organic solid aerosols. We use a radio-frequency (RF) capacitively coupled plasma discharge produced in different N(2)-CH(4) mixtures (from 0% to 10% of CH(4)) to simulate this chemistry. The work presented here was devoted to the study of the plasma discharge. In our experiment, the electron density is measured by the resonant cavity method and is about 10(15) m(-3) in pure N(2) plasma at 30 W excitation RF power. It decreases by a factor of 2 as soon as CH(4) is present in the discharge, even for a proportion as small as 2% of CH(4). An optical emission spectroscopy diagnostic is installed on the experiment to study the evolution of the N(2) bands and to perform actinometry measurements using Ar lines. This diagnostic allowed us to measure variations in the electron temperature and to show that a decrease in the density of the electrons can be compensated by an increase in their energy. We have also used an experimental setup where the plasma is tuned in a pulsed mode, in order to study the formation of dust particles. We observed variations in the self-bias voltage, the RF injected power and the intensities of the nitrogen bands, which indicated that dust particles were formed. The characteristic dust formation time varied, depending on the experimental conditions, from 4 to 110 s. It was faster for higher pressures and for smaller proportions of CH(4) in the gas mixture.