Measuring the Scrape-Off Layer Electric Field in LTX-β Using Doppler-Free Saturation Spectroscopy

The Lithium Tokamak eXperiment-β (LTX-β) provides a unique environment to study the impact of liquid lithium walls on plasma behavior, particularly in reducing recycling and enhancing plasma confinement. These lithium coatings lead to a significantly reduced recycling rate in the scrape-off layer (SOL), creating a low-collisionality regime where a potential forms along open field lines to maintain ambipolarity and retain electrons. This mechanism has been hypothesized to generate a poloidal electric field in the SOL, which plays a crucial role in modifying transport and confinement properties [1, 2].



To verify this effect, we employ a newly installed Doppler-Free Saturation Spectroscopy (DFSS) diagnostic system in LTX-β. DFSS enables high-resolution spectroscopy by eliminating Doppler broadening through counter-propagating laser beams, resolving fine structure effects with exceptional precision [3, 4, 5]. This technique allows for direct measurement of the poloidal electric field in the SOL, providing a new capability to explore the influence of lithium coatings on edge plasma behavior. These initial results set the stage to test hypotheses about low-recycling regimes and their role in shaping tokamak confinement.



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[2] Maan, A., et al. (2024). Estimates of global recycling coefficients for LTX discharges. Physics of Plasmas, 31(2), 022505.

[3] Ferguson, A. I., & Tolchard, J. M. (1987). Laser spectroscopy of atomic hydrogen. Contemporary Physics, 28(4), 383-405.

[4] Preston, D. W. (1996). Doppler-free saturated absorption: Laser spectroscopy. American Journal of Physics, 64(11), 1432-1436.

[5] Martin, E. H., et al. (2016). Applications of doppler-free saturation spectroscopy for edge physics studies (invited). Review of Scientific Instruments, 87(11), 11E402.