Modeling the effects of an NTM island on the transport of tungsten in JET

Since 2011, the inner wall of JET has been covered with tungsten (W) and beryllium tiles, replacing the previous carbon wall with the ITER like wall (ILW). In some discharges with the ILW, tungsten accumulates in the core of the plasma, leading to a plasma radiation collapse and possibly disruption. This accumulation seems exacerbated by the presence of a magnetic island. To get an insight into this behavior, we model a discharge with both an initially off-axis tungsten peak and a (m, n) = (3, 2) island (where m is the poloidal mode number and n the toroidal one). 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 set ad hoc for the off-axis W peaking. The island is modeled as an enhanced tungsten particle diffusion zone, with onset time, radial position, and width determined from experimental data. The underlying idea is that the onset of the mode suddenly changes the situation by short-circuiting the region where W has accumulated, with a more central region where inward neoclassical transport dominates. The outputs of the simulations are compared with experimental data like reconstructed soft x-ray (SXR) emissivity, line-integrated emissivity profiles, and time evolution of SXR channels intensity. The comparison is performed for the period shortly after the island has formed.