Effects of plasma shape on laser blow-off injected impurity transport in TCV

The extreme shaping capabilities of the TCV tokamak have been used to investigate the effect of the plasma geometry on the confinement of non recycling trace impurities injected by means of the laser blow-off technique. The progression of the injected Silicon in the core of TCV Ohmic limiter plasmas was followed by the 200-channel soft x-ray photodiode array with good spatial and temporal resolution. The results show that the plasma triangularity and elongation play an important role in the impurity confinement time imp τ . Remarkably, the increase of elongation from 6 . 1 = κ to 3 . 2 = κ produces a threefold reduction of imp τ while the electron energy confinement time Ee τ remains almost constant. imp τ is fairly constant in the triangularity scan for 2 . 0 > δ while there is a marked increase for lower values, leading to 100 > imp τ ms for negative triangularities. The increase of the toroidal magnetic field T B from 92 . 0 = T B T to 47 . 1 = T B T produces a decrease in the confinement time by almost of a factor-of-two. Simulation of the evolution of the line integrated soft x-ray signals, performed by the 1-D code STRAHL, provided both central and peripheral values of the transport coefficients together with estimates of the radial profiles. The simulations show that anomalous transport is dominant over neoclassical transport, except near the plasma centre. Interestingly, the convective velocity is positive (outwardly directed) in all limiter cases.