Observations of toroidal plasma rotation reversal in the Aditya-U tokamak

Impurity ion toroidal rotation profile has been observed in the ohmic discharges of Aditya-U tokamak from the Doppler-shifted passive charge exchange line emission of C5+ at 529 nm. . The line has been monitored using a space resolved visible spectroscopy diagnostic consisting of 1 m multi-track spectrometer coupled with a charge coupled device (CCD) detector. In typical discharges with central chord-averaged electron density 2.5 x 1019 m-3, the maximum toroidal rotation velocity at the plasma core is found to be -20 km/s in the counter-current direction. Reversal of plasma toroidal rotation is observed for high electron density discharges with central chord-averaged density of ~ 5 x 1019 m-3. During the rotation reversal, direction of toroidal rotation changes to co-current direction in the core. Interestingly, in the discharges with central chord-averaged electron density of 2.5 x 1019 m-3 where neon gas is injected at the plasma current flat-top, the toroidal rotation reversal has been observed after the gas puff.