Measurement of Sputtered Tungsten Flux Using Corona Model-Based Spectroscopic Diagnostics

Tungsten (W) is a promising armor material of the divertor in current and future tokamaks. The impact of sputtered W on the plasma performance is significant due to its strong radiative loss. In evaluating the sputtered W atom influx into the plasma, the spectroscopic diagnostics of the W influx using the S/XB method has attained recent attention. Here, S, X, and B denote the ionization and excitation rate coefficients, and the branching ratio for emission, respectively. The S/XB value, derived from the relationship between a specific line intensity of sputtered W atoms and the W particle influx, can be theoretically obtained from the corona approximation of W I. By multiplying the S/XB value with a specific spectral line intensity, the sputtered W flux can be determined. However, the S/XB calculation assumes a Boltzmann distribution for the density of metastable states of W I, introducing ambiguity due to the free parameter T_w (temperature of W atoms). In this study, we propose a method to directly obtain the metastable densities and calculate the sputtered W flux by combining multiple spectral line intensities and a set of the population kinetics equations for the level density, without using T_w. In addition, a metastable resolved ionization rate coefficient is used in our calculation. To validate the suggested method, we compare the sputtered W flux obtained from the spectroscopic method with the results obtained using sputtering yield by the argon ion bombardment using the KAERI plasma beam irradiation facility.