Improvements in the evaluation of Thomson scattering data on ASDEX upgrade

The avalanche diodes used as light detectors in the Thomson scattering diag nostic on the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak show a reduced signal-to-noise ratio during plasma discharges with high ne utron rates and during plasma edge measurements. This often makes it imposs ible to determine the electron temperature from the ratios of the scatterin g signals of the different spectral channels analyzing the Thomson scatteri ng spectrum. With the use of the maximum likelihood method combined with li nearized electron temperature characteristics the electron temperature and density are determined directly from the scattering signals without calcula ting ratios between the signals. With this method the scattering signals fo r 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, respective ly, smaller than achieved with the previous method. (C) 2001 American Insti tute of Physics.