Recent insights into the role of light impurities in the erosion of the all-W wall surfaces in WEST*

A collaborative research initiative on the WEST tokamak aims to characterize the balance of W sources between the divertor and the main chamber.  With the current proposal to change the ITER main chamber wall material from Be to W, this study on WEST is becoming of increased interest. A key finding in the initiative thus far has been that light impurities (O, C, B) play a large role in the gross erosion of W, in the WEST all-W wall environment, partly by the fact that they arrive at the wall predominantly at very high stages of ionization [1]. This finding, resulting from interpretive modeling of dedicated plasma pulses in the WEST C4 campaign, also raised further questions, now leading to new and ongoing efforts. One is to find a model for visible range spectral emission from low ionization stages, e.g., from O¹⁺ ions, as they play a lesser role in W gross erosion [2, 3].  Current focus is on self-consistent validation of O II and W I emission through comparison of 2D simulations using an iterative synthetic diagnostic workflow with experimental measurements [4]. Another is to gain more insight into the impact of ICRF-enhanced sheaths by deploying a WEST-antenna specific model for 3D RF rectified voltages at the antenna structures, including the guard limiter tangency surface when measurements were made with RF both on and off [5].  Previously, the measured >3x RF-on enhancement in local gross erosion was simulated by simply adding assumed rectified potentials to the sheath [1].