Role of pinch in Argon impurity transport in ohmic discharges of Aditya-U Tokamak.

We present experimental results of the trace argon impurity puffing in the ohmic plasmas of Aditya-U tokamak performed to study the argon transport behaviour. Argon line emissions in visible and Vacuum Ultra Violet (VUV) spectral ranges arising from the plasma edge and core respectively are measured simultaneously. During the experiments, space resolved brightness profile of Ar line emissions at 472.69 nm (3p4s P-3p4p D), 473.59 nm (3p4s P-3p4p P), 476.49 nm (3p4s P-3p4p P), 480.60 nm (3p4s P-3p4p P) are recorded using a high resolution visible spectrometer. Also, a VUV spectrometer has been used to simultaneously observe Ar line emission at 18.79 nm (2s2p P-2s2p P) and Ar line emission at 22.11 nm (2s S-2s2p P). The diffusivity and convective velocity of Ar are obtained by comparing the measured radial emissivity profile of Ar emission and the line intensity ratio of Ar and Ar ions, with those simulated using the impurity transport code, STRAHL. Argon diffusivities ~ 12 m/s and ~ 0.3 m/s have been observed in the edge (ρ > 0.85) and core region of the Aditya-U, respectively. The diffusivity values both in the edge and core region are found to be higher than the neo-classical values suggesting that the argon impurity transport is mainly anomalous in the Aditya-U tokamak. Also, an inward pinch of ~ 10 m/s mainly driven by Ware pinch is required to match the measured and simulated data. The measured peaked profile of Ar density suggests impurity accumulation in these discharges.