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

An impurity ion toroidal rotation profile has been observed in the ohmic discharges of the 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 a 1 m multi-track spectrometer coupled with a charge coupled device (CCD) detector. In typical discharges with central chord-averaged electron density less than or similar to 2.5 x 10(19) m(-3), the maximum toroidal rotation velocity at the plasma core is found to be -20 km s(-1) in the counter-current direction. Reversal of plasma toroidal rotation is observed for high electron density discharges with a central chord-averaged density of similar to 5 x 10(19) m(-3). During the rotation reversal, the direction of toroidal rotation changes to the co-current direction in the core. Interestingly, in the discharges with a central chord-averaged electron density of less than or similar to 2.5 x 10(19) m(-3), where neon gas is injected at the plasma current flat-top, the toroidal rotation reversal has been observed after the gas puff.