The effect of impurity seeding on edge toroidal rotation in the ADITYA-U tokamak

Intrinsic toroidal rotation velocity (V_φ ) has been measured from the Doppler shift of C⁵⁺ carbon spectral lines (at 529.05 nm) in the edge region of the ADITYA-U tokamak without any auxiliary torque input in an ohmically heated pure hydrogen (H₂) plasma as well as in H₂ plasmas seeded with medium-Z (neon and argon) impurities . The toroidal rotation in the edge region is observed to reverse its direction from the counter-current to the co-current direction with an increase in plasma current beyond I _p ∼ 145-150 kA. Furthermore, a systematic decrease in the co-current V_φ has been observed with the edge density, which tends to decrease to almost zero velocity with an increase in the edge density. The injection of medium-Z (neon and argon) impurities is observed to influence the edge toroidal rotation significantly. In low I _p discharges, argon injection leads to a reversal of edge intrinsic rotation from the counter-current to the co-current direction. In high I _p discharges, both neon and argon seeding enhance the co-current rotation by about ∼5-10 km s⁻¹, at a constant I _p compared to pure H₂ discharges. Simultaneous measurements of the edge radial electric field, E _r, shows that the E _r × B_θ flow seems to be driving the edge toroidal rotation in ADITYA-U. With impurity injection, the E _r also gets modified, leading to an observed increase in the edge toroidal rotation.