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Description
Tungsten (W), chosen as the plasma-facing material for ITER, offers high thermal robustness and low tritium retention, but its erosion and penetration into the plasma core can degrade the confinement through dilution and radiation. The net W content depends on the interplay of erosion, divertor screening and transport, which vary with plasma species. To resolve these dependencies, we investigate W sources in DT plasmas at JET-ILW using optical emission spectroscopy, focusing on intra-ELM erosion and the effect of impurity seeding.
The identification of tungsten (W) atomic sources in deuterium-tritium (DT) plasmas has been improved through optical emission spectroscopy performed at JET-ILW. Intra-ELM erosion dominate the total W gross erosion source in DT plasmas. Notably, the sputtering yield (Y_W) is found to be roughly twice as large in H-mode neon (Ne)-seeded pulses, with concentrations of n_Ne⁄n_e ≈1.0-1.6%, compared to unseeded pulses. This increase is due to (a) the additional sputtering caused by Ne ions and (b) the higher T_(e,ped), which leads to a corresponding increase in impact energy. Although the sputtering yield is lower in the inner divertor compared to the outer divertor, W erosion is still dominated there. Lower Y_Wduring the ELMs in the inner divertor aligns well with predictions from the free-streaming model, which assumes that electrons transfer their parallel energy E_(e,∥)to ions as they stream toward the divertor target during ELMs, causing the resulting sputtering to depend mainly on n_ped and T_ped.
In unseeded plasmas, T ions are primarily responsible for W erosion during the ELM phases, whereas in Ne-seeded plasmas, both T and Ne ions contribute significantly to intra-ELM W erosion, with marginal contributions from D and Be ions. The contribution of W self-sputtering to the overall W erosion is limited to a maximum of ≈10%, consistent with predictions from the PIC/MC code BIT1 and from Monte Carlo simulations that account for multiple ionizations and electric fields in the Magnetic Pre-Sheath (MPS).
The W source in Ne-seeded plasmas is larger compared to unseeded reference DT plasmas. On the other hand, Ne-seeded plasmas show a decreased W density in the confinement region, indicating significantly improved W divertor screening. This can be explained by the fact that in Ne-seeded H-mode plasmas, increased Ti and reduced ne in the pedestal cause the drift direction to reverse, pointing outward as predicted by neoclassical transport (|R/Ln|<|R/2LT|, where Ln and LT are the density and temperature gradient scale lengths in the pedestal)