Spectroscopic investigation of the fast ion transport in ASDEX Upgrade
The distribution function of fast, supra-thermal ions has been measured for the first time at ASDEX Upgrade with high spatial and temporal resolution. A newly developed spectrometer and optics in the vacuum vessel of ASDEX Upgrade were installed to investigate Balmer-α radiation that is emitted by fast ions after charge exchange reactions. The measured Doppler shifts and the intensity of this radiation contain information on the density and velocity distribution of the fast particles and have been compared to theoretical predictions. During experiments with only minor plasma instabilities the measurements are described best when assuming that the fast ion transport is mainly caused by collisions with thermal electrons and ions. However, in the presence of instabilities, such as sawtooth crashes, the measurements deviate strongly from this classical description. As can be seen in the figure, a very strong change of the fast ion density is observed in the presence of sawtooth instabilities. With every crash, the central fast ion density drops, accompanied with an increase further out.
GEIGER, B. et al., Nuclear Fusion 54 (2014) 022005.
With this work Benedikt Geiger graduated at the LMU München with summa cum laude in January 2013. For his outstanding scientific achievement, the Max Planck Society has awarded Benedikt Geiger the Otto Hahn Medal in May 2014.