Polarimetry of motional Stark effect and determination of current profiles in DIII-D (invited)

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Abstract
The motional electric field E=v×B, where v is the velocity and B is the tokamak magnetic field, produces a strong Stark effect in spectral lines emitted by hydrogenic neutral beams. The tilt angle of the magnetic field line, a quantity related directly to the distribution of the plasma toroidal current, is deduced from a measurement of the direction of polarization of the Stark components. In the DIII‐D tokamak, the Balmer‐α line of deuterium emitted by one of the high‐power heating beams is analyzed. A multichord polarimeter measures the magnetic field pitch angle at eight spatial locations covering ∼0.6 of the nominal plasma diameter at the midplane outboard side. The diagnostic offers 2–8‐cm resolution in the major radius and 1‐ms integration time. The accuracy of the measurement of the polarization direction necessary for an adequate reconstruction of the current profiles is obtained with the use of active polarizing elements which produce high‐frequency intensity modulation with an amplitude related to the direction of linear polarization of the plasma radiation. The current profiles in highly shaped (noncircular) plasmas cannot be determined solely from the tilt angle measurements because they do not provide any information about the shape of magnetic surfaces. Thus, the polarization measurements are used in conjunction with a large set of external magnetic measurements (magnetic field and flux probes, diamagnetic loops, and Rogowski coils) by the magnetic field equilibrium code EFIT, and provide a constraint on the possible solutions for the current profile.