Measurement of the motional Stark effect (MSE) for Balmer alpha light emitted from heating or diagnostic neutral beams is a standard technique for estimating plasma toroidal current density in tokamaks. Most techniques typically rely on a determination of the polarization angle or relative intensities of the multiplet components which are spectrally resolved using either an array of interference filters or a high-throughput grating instrument. Neither of these approaches is amenable to two-dimensional MSE imaging. This paper proposes an alternative measurement scheme that is suitable for single snapshot two-dimensional imaging of the current distribution. This is achieved using a spectro-polarimeter that encodes the polarization and spectral information upon spatially orthogonal linear interference fringe patterns illuminated by the image of the Stark-shifted H-alpha emission from the neutral beam. The new technique opens the possibility to use synchronous detection methods and gated cameras to resolve the magnetic structure of periodic tokamak phenomena including sawteeth and magnetohydrodynamic activity.

http://iopscience.iop.org/article/10.1088/0741-3335/50/12/125003/pdf

Snapshot-imaging motional Stark effect polarimetry

A high-precision spectral motional Stark effect (MSE) diagnostic provides internal magnetic field measurements for Madison Symmetric Torus (MST) plasmas. Currently, MST uses two spatial views—on the magnetic axis and on the midminor (off-axis) radius, the latter added recently. A new analysis scheme has been developed to infer both the pitch angle and the magnitude of the magnetic field from MSE spectra. Systematic errors are reduced by using atomic data from atomic data and analysis structure in the fit. Reconstructed current density and safety factor profiles are more strongly and globally constrained with the addition of the off-axis radius measurement than with the on-axis one only.

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Two-point motional Stark effect diagnostic for Madison Symmetric Torus.

The specific size and structure of the edge current profile has important effects on the MHD stability and ultimate performance of many advanced tokamak (AT) operating modes. This is true for both bootstrap and externally driven currents that may be used to tailor the edge shear. Absent a direct local measurement of j(r), the best alternative is a determination of the poloidal field. Measurements of the precision (0.1-0.01{sup o} in magnetic pitch angle and 1-10 ms) necessary to address issues of stability and control and provide constraints for EFIT are difficult to do in the region of interest ({rho} = 0.9-1.1). Using Zeeman polarization spectroscopy of the 2S-2P lithium resonance line emission from the DIII-D LIBEAM, measurements of the various field components may be made to the necessary precision in exactly the region of interest to these studies. Because of the negligible Stark mixing of the relevant atomic levels, this method of determining j(r) is insensitive to the large local electric fields typically found in enhanced confinement (H-mode) edges, and thus avoids an ambiguity common to Motional Stark Effect (MSE) measurements of B. Key issues for utilizing this technique include good beam quality, an optimum viewing geometry, and a suitable optical pre-filter to isolate the polarized emission line. A prospective diagnostic system for the DIII-D AT program will be described.

/pdf/prospects-for-edge-current-density-determination-using-li-1gy7ijghze.pdf

Prospects for Edge Current Density Determination Using Li beam on DIII-D

An overview is given of the present design of the active beam spectroscopy diagnostics for the ITER. Present spatial resolution and signal-to-noise indicate that, in principle, all proposed measurements are possible covering the entire plasma minor radius. Calculations show that the mirror based periscopes and the impurity coating of the first mirror affect the signal strength and the polarization characteristics of the measured spectra having an impact in the measurement accuracy. On-line calibration techniques and methods to access the first mirror status are addressed.

Active beam spectroscopy diagnostics for ITER: Present status (invited)

The specific size and structure of the edge current profile has important effects on the magnetohydrodynamic stability and ultimate performance of many advanced tokamak (AT) operating modes. This is true for both bootstrap and externally driven currents that may be used to tailor the edge shear. Absent a direct local measurement of j(r), the best alternative is a determination of the poloidal field. Measurements of the precision (0.1°–0.01° in magnetic pitch angle and 1–10 ms) necessary to address issues of stability and control and provide constraints for EFIT are difficult to do in the region of interest (ρ=0.9–1.1). Using Zeeman polarization spectroscopy of the 2S–2P lithium resonance line emission from the DIII-D LIBEAM [D. M. Thomas, Rev. Sci. Instrum. 66, 806 (1995); D. M. Thomas, A. W. Hyatt, and M. P. Thomas, Rev. Sci. Instrum. 61, 340 (1990)] measurements of the various field components may be made to the necessary precision in exactly the region of interest to these studies. Because of the negli...

/pdf/prospects-for-edge-current-density-determination-using-53gsjd4ot7.pdf

Prospects for edge current density determination using LIBEAM on DIII-D

Magnetic field diagnostics in tokamaks using the motional Stark effect in fast neutral beams have been based on two kinds of polarimetry which we call ‘‘static’’ and ‘‘dynamic.’’ A detailed analysis shows that static polarimetry presents a number of advantages over dynamic polarimetry, provided it is made complete in the sense that a sufficient number of polarization analyzers are installed and different parts of the spectrum are explored to yield full information on the set of unknowns inherent in the problem. A detailed scheme of complete static polarimetry is proposed, including the case where an in‐vessel mirror with changing characteristics (coating by impurities) is placed in front of the optical detection system. The main merit of this scheme relies on the fact that it is self‐calibrating with respect to both the characteristics of the mirror and the transmission of the different polarization channels, the latter item implying that it is uniquely based on relative measurements of spectra. Further a...

D. Voslamber

Papers

Self‐calibrating magnetic field diagnostics in beam emission spectroscopy