Using Structured Light for Multidimensional LIF Velocimetry of Plasma*

Laser-induced fluorescence (LIF) spectroscopy is a non-invasive diagnostic technique used to probe the internal properties of plasmas [1]. This method facilitates the characterization of the kinetic properties of ions and neutrals through the measurement of their velocity distribution functions (VDFs). The principle underlying this technique is the Doppler effect which manifests as follows. Ions and atoms typically occupy metastable excited states due to electron collisions and can absorb photons corresponding to specific resonant transitions to be further excited to higher energy levels, followed by fluorescence emission. When laser light illuminates the plasma, the wavelength perceived by the particles is Doppler-shifted due to their thermal or directed motion, causing the absorption profile to broaden. This is the main broadening mechanism at lower pressures and under moderate electric and magnetic fields. By varying the laser wavelength across the absorption profile and measuring the resultant fluorescence, it is possible to construct a VDF, thereby providing insights into the plasma components' average velocity, temperature, ionization rates, and electric potentials [2].

LIF typically employs a plane wave laser to measure 1D VDFs along the laser beam propagation direction. However, with spatially and temporally structured light, there are possibilities for new modalities to conduct LIF. Some example of structured light-based LIF include confocal LIF with Bessel beams [3] and a wavelength modulated approach [4]. Other promising variants include LIF with beams carrying orbital angular momentum (vortex beams) [5, 6, 7, 8]. We will present results on the investigation of vortex beams, their generation, property, and investigate their applications for probing plasma.