Analysis and model validation of tungsten prompt redeposition in tokamak divertor

Mechanisms governing W prompt redeposition in divertor are analyzed. At the divertor targets, the width of the Chodura sheath is comparable to the sputtered W ionization mean-free path. As a result, W prompt redeposition and net erosion are strongly related to the sheath properties. When 90{\%} or more of sputtered W are ionized within the sheath, the fraction of promptly redeposited tungsten impurity is determined by the multiple ionizations of W impurities outside of the sheath, and weakly depends on other sheath features. Furthermore, we show that the SXB coefficient used to spectroscopically estimate W gross erosion is significantly reduced due to the ionization of W within the sheath when the plasma density is large. The validity of the model describing W prompt redeposition is examined using W net erosion measurements in DIII-D. A reduced model is presented to quantify redeposition and net erosion on W samples exposed to uniform attached divertor plasma conditions. The ratio of W net erosion rates measured experimentally from W samples of different sizes exposed to the same attached plasma conditions [1] are well reproduced with this reduced model. [1] D. L. Rudakov, et al. Physica Scripta~T159 (2014)