Investigations of helium transport with charge exchange recombination spectroscopy
Sufficiently large helium transport in the plasma will be critical for the successful operation of future fusion reactors, as helium “ash” accumulation dilutes the fusion fuel.
The helium density in the plasma can be measured with charge exchange recombination spectroscopy at a neutral beam. However, the evaluation is complicated by the fact that recombined helium ions, which move along the magnetic field lines, add emission, which is not localised at the beam (“plume” effect). To interpret the measured spectra correctly, a model for the helium plume emission was implemented at ASDEX Upgrade. The injected beam neutrals and the spread of plume ions in the plasma can be seen in the Figure. The model is shown to describe the underlying physics mechanisms, as it can reproduce the experimental spectra.
As accurate helium densities can now be derived, investigations of helium transport have been undertaken. The helium density profiles were found to follow the shape of the electron density, while differences are observed between helium and boron. Theoretical predictions of turbulent transport, as calculated via gyrokinetic modelling, showed good agreement with the general trends of the measured profiles but no detailed validation of the predicted helium gradients could be found. For this work, Athina Kappatou was awarded a PhD degree at the University Eindhoven.