Analysis of impurity transport on WEST using the reciprocating collector probe with LA-ICP-MS measurements and 3DLIM modelling*

WEST provides a unique platform to study the behavior of a tokamak with tungsten divertors and first wall components in preparation for ITER operation. In order to achieve a high-performance operating scenario, the plasma facing components (PFCs) are boronized to limit erosion by reducing the concentration of impurities in the plasma. The boron films deposited from glow discharge boronization (GDB) are lost from the most exposed PFCs (SP areas, antenna limiters etc), necessitating repeated GDBs. During the 2019 WEST experimental campaign, the reciprocating impurity collector probe (CP) was employed to measure the boron content in the far-Scrape-off Layer region. Ex-situ analysis of the CP using Laser-Ablation Mass Spectroscopy (LAMS) is performed to obtain the areal density of deposited boron on the probe face. A synthetic version of the CP is created in the 3D impurity transport tool, 3DLIM, which is constrained by far-SOL plasma temperature, density profiles and magnetic connection lengths. The boron impurity distribution and magnitude in the far-SOL is interpreted from these measurements to assess the rate at which boron coatings from GDB is lost from the PFCs at multiple total plasma fluences since the last boronization. To alleviate the need for frequent GDB, an impurity powder dropper (IPD) has been successfully used on WEST to inject boron during discharges to maintain and extend the lifetime of the boron coating on the PFCs. The workflow developed here is then able to be extended to model the boron powder distribution in the vessel.