Although the q profile plays a key role in theories of instabilities and plasma equilibrium, it has been quite difficult to measure until the recent development of the motional Stark effect (MSE) diagnostic. A multichannel motional Stark effect polarimeter system has recently been installed on the Tokamak Fusion Test Reactor (TFTR). The diagnostic can measure the magnetic field pitch angle [γp=tan−1(Bp/BT)] at ten radial locations. The doppler shifted Dα radiation from a TFTR heating beam is viewed near tangential to the toroidal magnetic field via a re‐entrant front surface reflecting mirror. The field of view covers from inboard of the magnetic axis to near the outboard edge of the plasma with a radial spatial resolution of 3–5 cm. A high throughput f/2 optics system results in an uncertainty for γp of ∼0.1°–0.2° with a time resolution of ∼5–10 ms. Initial pitch angle profiles from TFTR have been obtained. The MSE data is consistent with the estimated magnetic axis position from external magnetic measurements and the q=1 radius is in good agreement with the inversion radius from the electron cyclotron emission temperature measurements.

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