LIF measurement of the ion velocity distribution in an ICP

After the work done by M. van de Grift and N. Sadeghi on the ion velocity distribution in an ICP, some new measurements are planned for the future to extend their results. In this paper we will describe these results and the drawn conclusions which motivate our work. Velocity distribution functions (VDF’s) of 3d’G912 metastable argon ions in an ICP reactor were determined using Doppler Shifted Laser Induced Fluorescence (DSLIF). In the direction perpendicular to the reactor axis, the VDF was found Gaussian shaped with no average velocity, whereas along the reactor axis, the VDF was showing a clear acceleration of the ions in the presheath towards the Bohm velocity when reaching the sheath edge. The kinetic energy, gained in the electric field was found to be transferred by collisions to directions perpendicular to the reactor axis, especially for high pressures. A Monte-Carlo model has been used to’ determine the relation between the metastable ion VDF and ground state ion VDF. It was shown that at high pressures they are quite similar. Nevertheless, at low pressures, the two profiles do not correspond any more: Quenching of metastable ions lead to a depletion of the low velocity component of the metastable ion VDF and so that metastable ion VDF is no longer an image of ground state ion VDF.