Improvements in the Evaluation of Thomson Scattering on ASDEX Upgrade

The avalanche diodes used as light detectors in the Thomson scattering diagnostic on the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak show a reduced signal-to-noise ratio during plasma discharges with high neutron rates and during plasma edge measurements. This often makes it impossible to determine the electron temperature from the ratios of the scattering signals of the different spectral channels analyzing the Thomson scattering spectrum. With the use of the maximum likelihood method combined with linearized electron temperature characteristics the electron temperature and density are determined directly from the scattering signals without calculating ratios between the signals. With this method the scattering signals for nearly all timepoints can be evaluated and the statistical errors of the electron temperature and density are up to factors of 2 and 1.4, respectively, smaller than achieved with the previous method.