Scanning polarimeter for measurement of the poloidal magnetic field in a tokamak

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Abstract
The internal magnetic field in a magnetically confined plasma may be deduced from the analysis of circular polarization of spectral lines emitted by the plasma. The theory of the measurement and a detailed design of a polarimeter constructed to measure the poloidal field profile in the Texas Experimental Tokamak (TEXT) are presented. The instrument measures the difference between left-hand and right-hand circularly polarized line profiles, a quantity directly proportional to the magnetic field component in the direction of observation. The high throughput of the Fabry–Perot interferometer employed in this design, combined with efficient light-collecting optics and lock-in detection of the polarization signal, allows measurement of the fractional circular polarization of the magnetic dipole line Ti xvii 3834 Å with an accuracy on the order of 10−3. The line-of-sight averaged poloidal field is determined with uncertainty as small as 50 G. The line emission used in the present measurement is not well localized in the plasma, necessitating the use of a spatial inversion procedure to obtain the local values of the field.