q-profile measurements in the Tokamak Fusion Test Reactor*

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Abstract
The q‐profile plays a key role in determining plasma equilibrium and stability in tokamaks. With the development of the motional Stark effect (MSE) diagnostic, accurate q(R,t) profiles have been measured and utilized for equilibrium and stability analysis. A multichannel MSE polarimeter system on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 33, 1509 (1991)] has obtained data for several plasma conditions including L‐mode, supershot, current ramps, and high βp. For sawtoothing discharges on TFTR, it is found that q(0)∼0.7, well below one, and remains below one throughout the entire evolution of the sawtooth cycle with an increase in q(0) of ≤0.1 after a sawtooth crash. During high βp operation or coinjection of neutral beams on TFTR a significant broadening of the current profile and an increase of q(0) is observed.