Beurskens, M.

Bozhenkov, S.

Fuchert, G.

Höfel, U.

Pasch, E.

Wolf, R.

English

MPG.PuRe

Novel methods for evaluating Thomson scattering data to improve theaccuracy of the electron density and its error estimateG. Fuchert1, U. Höfel1,M.N.A. Beurskens1, S. Bozhenkov1, E. Pasch1, R.C. Wolf1, and the W7-X team1Max-Planck-Institut für Plasmaphysik, Wendelsteinstraße 1, 14791 Greifswald, GermanyLaser Thomson scattering is employed in fusion experiments to determine the electron den-sity and temperature from the measured spectrum of the scattered photons. However, the wholespectrum cannot be measured with sufficient accuracy for a single laser pulse. Instead, so-calledpolychromators are used. These split the full wavelength range into a number of channels andintegrate the spectrum over those channels. The spectrum is then reconstructed from those dis-crete measurements. Due to noise (electronic and photon shot noise) there is an uncertainty inthis reconstruction and, hence, in the density and temperature determiend from it. In reality,however, variations between individual laser pulses or adjacent spatial points are often largerthan the uncertainties arising from the reconstruction. This is especially true for the electrondensity. It is often challenging to identify the exact cause for such additional uncertainties and,consequently, to quantify a reliable estimate for the experimental error.In the first experimental campaigns of Wendelstein 7-X it was found that misalignment of thelaser beam was the dominant reason for uncertainties in the electron density. Besides mechani-cal improvements of the beam path, a novel calibration technique has been established, whichaccounts for any remaining alignment variations and substantially improves the profile quality.This technique may be crucial for larger fusion experiments like ITER, which necessarily havelong beam paths. But the question remains, how error bars can be quantified reliably, possiblyeven accounting for unknown error sources. For this, the distribution of measured signal levelsduring the absolute Raman calibration (with constant conditions) has been determined. Thisdistribution covers all fast changes to the overall sensitivity of the diagnostic (vibrations, de-tector gain, etc.) and in combination with the calibration uncertainties and the error due to theimperfect reconstruction of the spectrum, this distribution gives a realistic and experimentallyaccessible error estimate for the electron density. In this regard it was an interesting finding thatfor some polychromators the error distribution was not symmetric. For such a distribution, themean value is different from the maximum of the distribution (modus). This implies that thecalibration factor relating the intensity to the electron density, which is determined from themean of the distribution, yields slightly incorrect density values for most of the measurements.This effect can be mitigated by using the empirical distribution as error estimate.48th EPS Conference on Plasma Physics P1b.122

Novel methods for evaluating Thomson scattering data to improve the accuracy of the electron density and its error estimate

http://hdl.handle.net/21.11116/0000-000C-1695-3

Novel methods for evaluating Thomson scattering data to improve the accuracy of the electron density and its error estimate

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