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The absolute calibration of the Thomson scattering diagnostic for electron density measurements on the DIII-D tokamak has traditionally relied on Rayleigh scattering from neutral monatomic gases [1]. However, in regions with significant stray laser light, such as the divertor, Rayleigh scattering proves unreliable. To address this, a Raman scattering-based calibration method has been implemented across all Thomson scattering measurement locations, including the divertor.
This study presents the results of the Raman calibration and compares them to Rayleigh-based results where overlap exists. The Raman calibration generally produces higher plasma density values compared to Rayleigh, though not uniformly across all locations. Potential causes for this discrepancy are explored alongside proposed solutions. The influence of spectral channel selection for Raman wavelengths and of point-to-point variability on calibration outcomes is discussed.
Additionally, cross-calibration using electron cyclotron emission (ECE) cut-off measurements has been refined, leveraging an updated plasma shape to improve spatial coverage, and is used to correct the Raman calibration factors. In the divertor region, a combination of Raman-based calibration, ECE-based corrections, and plasma pressure-based analysis is performed.
Work supported by U.S. D.O.E. under DE-FC02-04ER54698.
[1] B. Bray, C. Hsieh, T. N. Carlstrom, and C. C. Makariou (2001) “Upgraded calibrations of the Thomson system at DIII-D,” Review of Scientific Instruments, vol. 72, no. 1, pp. 1115–1117.