q profile measurements with the motional Stark effect diagnostic in the DIII-D tokamak

The motional Stark effect (MSE) diagnostic has emerged as the most reliable technique for the measurement of q profiles in tokamaks equipped with neutral beams. In the DIII-D tokamak, two separate eight-channel MSE instruments are utilized, giving 16 radial measurements. The viewing geometry for each instrument is designed to optimize the radial resolution (1.5 cm < δR < 6 cm) over the measurement range. Dual-frequency polarization modulators combined with lock-in detection provide a maximum signal-to-noise ratio with a time response of about 1 ms. Special consideration has gone into the design of the collection optics to minimize undesirable polarization effects on reflectors and Faraday roation in lenses. Examples of data from negative central magnetic shear, VH-mode and sawtoothing discharges highlight the capabilities of the DIII-D MSE diagnostic system.