Measurements on argon ion by tunable diode-laser induced fluorescence in a low magnetic field helicon configuration reactor

This paper deals with the characterization of a RF reactor with a helicon configuration by studying a pure argon plasma. Tunable diode-laser induced fluorescence (TD-LIF) is performed in the center of the diffusion chamber to determine relative Ar+ metastable density as a function of RF power and static magnetic fields. A diode laser tuned at λ0 = 668.429 nm excites argon ions from the 3d4F7/2 metastable state to the 4p4D5/2 state, which decays to the 4s4P3/2 state with 442.60 nm fluorescence emission. Visible camera imaging, optical emission spectroscopy (OES) and RF compensated Langmuir probe measurements are used as complementary diagnostics. The results show that under our conditions, it is possible to work in three different regimes: the pure inductive, the simple Trivelpiece–Gould (TG) mode and the resonant TG mode. Moreover, these three regimes are very sensitive to the experimental plasma parameters. The decrease in the TD-LIF signal under particular conditions is associated with an increase in electron density and in argon ion emission lines in the 400–500 nm range. This phenomenon is assumed to be the consequence of the destruction of metastable states by electronic collisions.