Modelling the effects of NTM islands On the transport of heavy impurities in a tokamak

In tokamaks, magnetohydrodynamic (MHD) instabilities are frequently observed to have a strong impact on the evolution of the plasma discharge. It is, therefore, important to properly account for MHD activity when modelling the tokamak plasma behaviour. In 2011 the inner wall of JET was covered with Tungsten and Beryllium tiles, replacing the previous Carbon wall with the ITER Like Wall (ILW). In some discharges with ILW, Tungsten accumulates in the core of the plasma, leading to plasma radiation collapse and ultimately disruption. This accumulation seems exacerbated by the presence of a magnetic island [1]. Among the various MHD instabilities, a magnetic island can be detected by magnetic coils placed within the torus wall and by temperature measurements provided by Electron Cyclotron Emission (ECE) diagnostic. The island appearance time and position can be measured by exploiting the correlation between the fluctuations of coil and ECE signals. The island width can be determined by comparing the first and the second harmonic of ECE fluctuations [2]. We use JETTO (a one-and-a-half-dimensional transport code calculating the evolution of plasma parameters in a time dependent axisymmetric MHD equilibrium configuration) in interpretive mode and its impurity component SANCO in predictive mode with transport coefficients calculated by the codes NEO and GKW [1]. We model a discharge with both an initially off-axis Tungsten peak and a (3, 2) island. The island is modelled enhancing Tungsten diffusion, in a position, time instant and with a width determined by diagnostics as previously detailed [3]. The output of the simulations is studied in different ways: reconstructed SXR emissivity, line integrated emissivity profiles, time evolution of SXR channels intensity, Tungsten density and radiation. When opportune, comparison between simulated and experimental data is performed. Although not reproducing quantitatively the experimental result, the simulated emissivity at the final state reproduces qualitatively the trend of the experimental one. In particular if the island is situated in the inner, convex part of the Tungsten density gradient, the impurity is channelled toward the inner core as observed in experiments [4]. [1] Angioni C. et al., Nucl. Fusion 54, 083028 (2014) [2] Baruzzo M., et al., Plasma Phys. Control. Fusion 52 075001 (2010) [3] Marchetto C., et al., 41st EPS Conference on Plasma Physics, ECA Vol. 38, P1.018 (2014) [4] Hender T. C., et al., 41st EPS Conference on Plasma Physics, ECA Vol. 38, P1.011 (2014)