Plasma-wall interaction studies in the WEST experiment and wall conditioning with the Impurity Powder Dropper

The Tungsten (W) Environment in Steady-state Tokamak (WEST) experiment is the only European tokamak equipped with superconducting toroidal magnetic field coils and metallic plasma-facing components (PFCs).  The first two campaigns were devoted to commissioning without boronization. From the third campaign, wall conditioning through deuterium (D₂), He, and diborane (B₂D₆) glow discharges was routinely performed, opening the operational space. Improved wall conditions allowed a long pulse scenario (~1 minute) and divertor peak heat load up to 5.5 MW m^-2 in L-mode, essential steps towards a steady state 10 MW m^-2 heat load to test ITER-like W PFCs. To further reduce impurity content and radiative losses, successful real-time wall conditioning using an Impurity Powder Dropper (IPD) was recently performed. As boron (B) powder (with grain sizes < 150 um) was injected for up to 16.5 s during L-mode plasma discharges, spectroscopic measurements showed a reduction of D and O signals at the outboard limiter and the lower divertor, suggesting reduced fuel recycling and improved light impurity screening. Furthermore, in the pre-drop phase, a promising reduction of the total radiated power and no sign of enhanced W sputtering were observed as the cumulated amount of dropped B was increased.