Optical diagnostics for main neutral species in iodine plasmas

Iodine will be an alternative propellant for plasma thrusters used in space. To understand the basic physics and chemistry of iodine plasma discharges, it appears necessary to measure the densities and temperatures of neutral species. Optical diagnostics offer many possibilities for making these measurements, but one difficulty, with thrusters, is the very low particle density. In order to develop and improve different diagnostics, we have used a plasma generated in a long quartz cell by RF capacitive coupling, with a pressure varied between 2 and 30 Pa. The feasibility of two-photon absorption laser induced fluorescence (TALIF) was demonstrated by the excitation of (³P₂)6p ²[1]^o_3/2 and (³P₂)6p ²[3]^o_7/2 levels around 300 nm and accurate measurements of atomic temperature were obtained. Absolute energy measurements of the two-photon excited levels revealed that all upper energy levels of I I have to be revised down, by -0.169(11) cm^-1. We also used optical absorption methods to measure the density of iodine molecules (@488 nm), and the density and the temperature of the two first levels of iodine atoms (@ 1315 nm), noted I(²P_3/2) and I(²P_1/2). It is shown that the molecules are easily dissociated (up to 85 % along the line-of-sight) and that the density of the I(²P_1/2) level is found to be a significant fraction (about 30%) of the total atomic density. Finally, the atom temperature increases both with RF power and with pressure and reaches 1000 K at 50 watts and 25 Pa.