An integrated system to measure the effective charge of fusion plasmas in the ASDEX Upgrade tokamak

At the ASDEX Upgrade tokamak a new approach has been developed to determine the profiles of the effective charge (Zeff) from bremsstrahlung emission in hot fusion plasmas. The signals of all diagnostics measuring bremsstrahlung have been incorporated into one evaluation algorithm, resulting in the possibility to check whether or not signals are disturbed during each discharge. Especially helpful to this aim is a recently installed diagnostic measuring the bremsstrahlung spectrally resolved by using a spectrometer and a charge-coupled-device (CCD) camera as a detector. The experimental results are verified qualitatively and quantitatively using a discharge with silicon laser blow-off. Modeling of the bremsstrahlung as measured with the CCD camera using the routines and data of the ADAS package reveals that the background of the spectra is dominated by bremsstrahlung for most sight lines. Only those passing mainly outside the last closed flux surface measure a considerable amount of line and molecular band radiation. This edge emission can be considered as a radiative mantle. Subtracting it from all other signals results in reduced statistical errors of the deduced Zeff profiles while not changing the profile shape inside a normalized minor radius of 0.8.