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Laser-induced fluorescence (LIF) measurements of Argon ions performed in the sheath/pre-sheath facing a floating metallic plate and a BNSiO2 ceramic one immersed in a low temperature plasma exhibit unexpected features. It appears that a strong fluorescence signal which could be unduly attributed to ions moving backward in the sheath is detected, even though the floating potential is far below the plasma potential. Moreover, this signal may be stronger than the one corresponding to ions having a forward motion. It is demonstrated that this abnormal measurement is due to the optical pumping saturation of the incident laser beam, while this saturation does not exist for the scattering of the beam at the sample surface. The reflected signal is unambiguously identified using a theoretical beam scattering model. It is also shown that the presence of the sheath/pre-sheath density gradient is able to trigger the LIF saturation effect, complicating ion density measurements.

Laser-induced fluorescence saturation effects on ion velocity distribution functions in the vicinity of reflecting surfaces