Measurement of the Tungsten Gross Erosion Rate of Divertor Armor using Multiple Atomic Spectral Lines in KSTAR

The S/XB method, an optical emission spectroscopic technique, has attained recent attention for its ability to measure in-situ tungsten erosion from tokamak divertors. Unlike conventional methods using a single spectral line and theoretical S/XB value, our approach utilizes multiple W I spectral lines and solves a set of photon emission equations within the collisional-radiative model [1]. This enables direct calculation of line-averaged W I metastable level densities and simultaneous inference of the local electron temperature on the divertor surface, enhancing accuracy and self-consistency. To apply the developed method in KSTAR, the line of sight for W I was aligned with the central divertor, and Langmuir probe on the divertor surface was operated at the same poloidal location. An outer strike point was scanned to measure spatial variation of W erosion, under the fixed diagnostic point. From the measurements, the maximum W gross erosion rate near the outer strike point was evaluated as 2.5×10¹⁹ atoms∙m^-2s^-1 under a plasma current of 0.48 MA, a toroidal magnetic field of 1.9 T and a heating power of 7 MW.