First motional stark effect measurements at Wendelstein 7-X

Abstract The rotational transform plays a crucial role in magnetic confinement devices and its profile is directly related to stability and confinement properties. A popular way to assess the rotational transform profile is using a Motional Stark Effect diagnostic system, which relies on the splitting of the Balmer alpha emission, due to the interaction between a particle beam and the plasma. A prototype diagnostic was installed at the stellarator Wendelstein 7-X, in order to demonstrate that such a diagnostic can be operated in this device. This work presents the diagnostic design and preliminary measurements as well as the calibration procedures necessary to deal with the extra complexity of measuring polarisation via an aluminium mirror. Qualitative agreement with simulations is shown in the variation of the measured polarisation angle over wavelength. A small but significant change in the measured angle is seen with changes in the plasma beta. While of a similar order to that expected, insufficient sensitivity of the prototype precludes a quantitative match to the prediction. The required changes to sufficiently improve the sensitivity to achieve this in future experiments is discussed.