Motional Stark Effect measurements and modelling at Wendelstein 7-X

The motional stark effect (MSE) is an effective method to assess the rotational transform in mag-
netic confined plasmas by measuring the polarisation angle of the emitted light. It relies on the
Stark-splitting of the Balmer alpha emission, caused by the interaction between the injection of
neutral beams and the background plasma. A prototype MSE diagnostic system, which relies on
spectrum measurements taken using different linear polarising filters, has been installed on the
Wendelstein 7-X stellarator (W7-X), and its diagnostic capabilities have been demonstrated. In
this work we present an upgraded MSE diagnostic system and discuss the comparison between the
experimental data and the forward model originally developed for the beam emission spectroscopy
measurements at W7-X and modified to include the polarising optical elements. The experimen-
tal data and the model results show a good agreement for relative changes of the polarisation angle
for different plasma β. However, the evolution of the polarisation angle during electron cyclotron
current drive experiments could not yet be resolved due to limited signal-to-noise ratio. ... mehrPossible
upgrades to improve diagnostic performance are outlined