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 strongimpact on the evolution of the plasma discharge. It is, therefore, important to properly account forMHD activity when modelling the tokamak plasma behaviour.In 2011 the inner wall of JET was covered with Tungsten and Beryllium tiles, replacing the previousCarbon wall with the ITER Like Wall (ILW). In some discharges with ILW, Tungsten accumulates in thecore of the plasma, leading to plasma radiation collapse and ultimately disruption. This accumulationseems exacerbated by the presence of a magnetic island [1].Among the various MHD instabilities, a magnetic island can be detected by magnetic coils placedwithin 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 thecorrelation between the fluctuations of coil and ECE signals. The island width can be determined bycomparing 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 plasmaparameters in a time dependent axisymmetric MHD equilibrium configuration) in interpretive modeand its impurity component SANCO in predictive mode with transport coefficients calculated by thecodes 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 awidth determined by diagnostics as previously detailed [3]. The output of the simulations is studiedin different ways: reconstructed SXR emissivity, line integrated emissivity profiles, time evolution ofSXR channels intensity, Tungsten density and radiation. When opportune, comparison betweensimulated and experimental data is performed.Although not reproducing quantitatively the experimental result, the simulated emissivity at thefinal state reproduces qualitatively the trend of the experimental one. In particular if the island issituated in the inner, convex part of the Tungsten density gradient, the impurity is channelledtoward 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)