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Polarimetry measurements of the Doppler-shifted H_ emission from a neutral hydrogen beam on the PBX-M tokamak have been employed in a novel technique for obtaining q (r) and magnetic field pitch-angle profiles using the Stark effect. The resulting q (r) profile is very broad and its central value, q (0), is significantly below 1, which has important implications for theoretical models of sawteeth.
Levinton et al. (Mon,) studied this question.