Measurement of wave-particle interaction and metastable lifetime using laser-induced fluorescence.

Extensive information, such as ion temperature and wave-particle interactions, can be obtained by direct measurement of ion distribution functions using laser-induced fluorescence (LIF). This nonintrusive plasma diagnostic provides an important window into the ion motions in phase-space. Previous simulation results suggest that LIF measurements, which are performed on metastable ions produced directly from neutral gas particles and also from ions in other electronic states, place restrictions on the metastable lifetime. In the case where metastable population is produced from direct ionization of neutral atoms, the velocity distribution measured using LIF will only faithfully represent processes which act on the ion dynamics in a time shorter than the metastable lifetime. In this paper, the metastable lifetime effects are explored experimentally for the first time using wave-particle interaction in an Argon multidipole plasma. We demonstrate that this systematic effect can be corrected using the theory addressed in the paper based on the metastable lifetime and relative fraction of metastables produced from pre-existing ions.