Probing low temperature plasmas with structured light beams and derivative spectroscopy

This talk will introduce structured light beams and derivative spectroscopy for plasma diagnostics. Utilizing structured light beams enhances signal-to-noise ratio (SNR), simplifies optical setup, and enables high-resolution measurements of velocity-distribution functions (VDF) in plasma devices with limited access. Derivative spectroscopy offers advantages for signal quantification.

Structured light beams, featuring spatial and spatiotemporal structures like orbital angular momentum,[1] have inspired innovations in various fields, but their potential for plasma spectroscopy is largely unexplored. Recently these concepts have been applied in several diagnostic setups at PPPL.

Laser-Induced Fluorescence (LIF) typically requires dual-sided plasma access[2] which is not always feasible. The proposed confocal LIF configuration uses Laguerre-Gaussian annular laser beam from axicon lenses. It achieves spatial resolution of ~5mm at 300mm focal distance potentially reaching 1mm resolution akin to conventional LIF[3]. This approach could benefit other methods (e.g. Thomson or Raman scattering) making them applicable to industrial plasma sources or electric propulsion devices.

Building on the confocal approach wavelength modulation (WM) spectroscopy[4] applications to plasma diagnostics will be discussed. WM spectroscopy, derivative spectroscopy technique, measures spectral line profile derivatives offers high SNR and background-free signal. While common in laser absorption spectroscopy few works explored WM laser-induced fluorescence spectroscopy (WM-LIF). WM-LIF results in derivatives of VDF thus enhancing spectral resolution separating overlapping spectra components emphasizing sharp spectrum features by eliminating broadband background signals. This technique helps the confocal LIF approach e.g., in spatially averaged detection helping identify VDFs from adjacent regions.

References

[1] L. Allen M. W. Beijersbergen R. J. C. Spreeuw and J. P. Woerdman Phys. Rev. A 45 8185 (1992)

[2] P. Svarnas. I. Romadanov A. Diallo and Y. Raitses IEEE Trans. Plasma Sci. 46 3998 (2018)

[3] I. Romadanov Y. Raitses arXiv preprint arXiv:2303.12580. (2023)

[4] E. I. Moses C. L. Tang. Opt. Lett. 1 115 (1977)