Sheath dynamics during the vapor shielding at a plasma-facing wall

Erosion of wall materials during transient events is a large concern for ITER and future devices with high fusion power. Enormous heat loads at the transient event cause melting, vaporization, and ablation of wall materials. The generated vapor from the surface strongly interacts with the incoming plasma and dissipates its energy. Then, vapor shielding can be an inherent protection for this wall erosion. We have developed a particle-in-cell simulation code, called PIXY, and have applied it to simulations of the vapor shielding in fusion devices. During the vapor shielding, the vapor emission rate significantly spreads over a very wide range. Thus, in order to treat the sufficient numbers of numerical super-particles, a weighted particle model is adopted in the PIXY code. Behaviors of incoming plasma, generated vapor, and ionized impurities are solved in a consistent manner with the sheath and pre-sheath formation. The sheath potential affects the heat transmission to the wall and resultantly the vapor emission. In this presentation, we focus on the dynamics of the sheath during the vapor shielding, and highlight the influence of sheath dynamics on the shielding efficiency. Overall simulation model and estimated total erosion during the transient heat loads will also be presented.