Speaker
Description
Kinetic effects are found to significantly diminish the thermal force on W relative to conventional fluid modeling, underscoring the necessity of kinetic approaches for modeling W transport in boundary plasmas. A recently developed kinetic impurity transport model in DIVIMP has been used to investigate kinetic effects on W transport and screening across various divertor conditions. Results indicate that the kinetic correction to the W thermal force is significant not only in low-collisionality regimes (ν* < 20), but also under conditions of low ion-temperature (Ti < 20 eV) and high effective-charge (Zeff). In pure D discharges, the kinetic correction to the W thermal force is weaker in high-density H-mode plasmas than in low-density L-mode plasmas. However, the strong friction force in high-density H-mode cases makes the influence of kinetic effects on W screening more significant. These modeling results align with experimental observations on EAST, where increasing boundary plasma collisionality via D₂ puffing reduces core W density by more than 20%. With Ne-seeding, the higher plasma temperature in the main-SOL leads to a higher Zeff than in the divertor, resulting in a more pronounced kinetic correction on the W thermal force in the main-SOL. As divertor conditions transition from the high-recycling to detached regimes, the decrease in Ti and increase in Zeff further enhance kinetic effects on W screening. To further validate the role of Zeff-dependent kinetic effects, dedicated He-plasma experiments were conducted. As expected, superior W screening is observed in He plasmas, particularly under detached conditions, due to increased Zeff and the corresponding reduction in W thermal force.
The synergy between kinetic and E×B drift effects on W transport has also been delineated. With the reduction of the W thermal force by kinetic effects, E×B drifts are demonstrated to have a more significant effect on W leakage through a reversed near-SOL flow on EAST. In contrast, E×B drifts are weak in the large-size and high-Bt devices like CFETR, and W edge transport is governed by thermal and friction forces. Under Ne-seeding detached divertor conditions on CFETR, kinetic effects can even reverse the direction of the W thermal force from pointing upstream to pointing divertor target, thereby further enhancing divertor W screening. However, a clockwise plasma flow in the low-field-side SOL is observed as a potential W leakage pathway for CFETR. By increasing Ne injection rates, the reverse flow is effectively suppressed, which narrows the near-SOL leakage path and enhances W screening.