Spectral Motional Stark Effect Measurements on ASDEX Upgrade

Local measurements of the magnetic field are essential for the reconstruction of plasma equilibria. Typically, motional Stark effect (MSE) polarimetery is employed to derive pitch angles from the central s emission line [1]. This paper uses the full spectrum due to the Motional Stark Effect (sMSE) to derive information about both the magnetic field strength and its direction. The observation of the MSE spectrum is reported, e.g., in Ref. [2]. This work aims specifically at the assessment of measuring capabilities to investigate the plasma diamagnetism and the role of fast ions for the plasma equilibrium. A spectral MSE diagnostic has been implemented and discharges were performed to assess the measuring capabilities and the effects of different heating scenarios on the creation of fast ions. Spectral MSE diagnostic on ASDEX Upgrade Experimental set-up: The detailed features of the diagnostic set-up are described in Ref. [3]. The sMSE diagnostic observes the Doppler-shifted Balmer-a transition from deuterium neutral beam emission. The observation optics consist of a Czerny-Turner spectrometer and a CCDcamera to detect spectrally resolved data at different plasma radii. To suppress the unshifted high intensity neutral hydrogen and deuterium emission, a blocking wire has been placed in the focal plane of the spectrometer. Furthermore, the optical elements of the MSE polarimeter set-up are assessed with respect to their polarizing characteristics. Data analysis: The detailed forward model for the spectral MSE measurement is described in Ref. [4]. The magnetic field strength causes a splitting of the n = 3! 2 transition due to the induced Lorentz field ~L =~v◊~. The anisotropic intensity distribution of the differently polarized components of the Stark spectrum (p and s light corresponding to Dm = 0 and Dm = ±1 transitions) is reflected in intensity modifications captured in the applied forward model by an intensity ratio Tp = Â i Ipi /Â i Isi . In order to calculate absolute values of the pitch angle, non-statistical distributions of the Stark sublevels [5] need to be considered. This deviation can be incorporated by a correction cns. Furthermore, polarizing effects of the optical components