Quantitative evaluation of laser-induced fluorescence in magnetized plasma accounting for disalignment effect
Abstract
Quantitative evaluation of tunable diode laser induced fluorescence (TDLIF) measurements in magnetized plasma take into account Zeeman splitting of energetic levels and intra-multiplet mixing defining the density distribution (alignment) of excited 2p8 multiplet is discussed in this paper. TDLIF measurements were used to evaluate light-transport properties in a strongly magnetized optically thick argon plasma under different pressure conditions. Therefore, a coupled system of rate balance equations were constructed to describe laser pumping of individual magnetic sub-levels of 2p8 state through frequency separated sub-transitions originating from 1s4 magnetic sub-levels. The density distribution of 2p8 multiplet was described by balancing laser pumping with losses including radiative decay, transfer of excitation between the neighboring multiplets driven by neutral collisions and quenching due to electron and neutral collisions. Resulting 2p8 magnetic sub-level densities were then used to model polarization dependent fluorescence, consider self-absorption, which could be directly compared with measured polarization resolved TDLIF measurements. This enables to obtain unique solutions for the 1s4 and 1s5 magnetic sub-level densities which were in good agreement with the densities obtained by laser absorption measurements. It is shown that LIF measurements in magnetized plasma conditions have strong pressure dependence that should be corrected consider effective disalignment rate. The presented measurement method and model can help further understanding and improve description of optical emission of argon in magnetized conditions.