Optical vortex laser-induced fluorescence measurement using an l=10 Laguerre-Gaussian beam*

We have been studying the application of optical vortex beams to plasma measurement. Due to the additional Doppler effect in the azimuthal direction, the optical vortex laser-induced fluorescence (OVLIF) spectrum can be deformed by the motion of the particles in the direction vertically across the beam [1]. A proof-of-principle experiment for the perpendicular argon-ion flow detection was performed with the HYPER-I device at the National Institute for Fusion Science. A Laguerre-Gaussian (LG) beam with the topological charge l=10 was produced by converting a Hermite-Gaussian beam using a computer-generated hologram on a spatial light modulator. The LG beam was injected parallel to the surface of an electrode inserted in the plasma, and the OVLIF was detected by a photomultiplier tube. An increase in the standard deviation of the spectrum was observed as the negative applied voltage to the electrode was increased, which is qualitatively consistent with the prediction of our numerical study.

[1] S. Yoshimura et al., Jpn. J. Appl. Phys. 59, SHHB04 (2020).