Monte Carlo simulations of tungsten redeposition at the divertor target

Recent modeling of controlled ELMs in ITER with tungsten (W) divertor target plates by SOLPS code package predicted high electron temperatures ( > 100 eV) and densities ( > 1×10 21 m -3 ) at the outer target.Under certain scenarios W sputtered during ELMs can penetrate into the core in quantities large enough to cause deterioration of the discharge performance, as was shown by coupled SOLPS5.0/STRAHL/ASTRAruns.The net sputtering yield, however, was expected to be dramatically reduced by the 'prompt redeposition' during the first Larmor gyration of W +1 (Fussmann et al., 1995).Under high n e /T e conditions at the target during ITER ELMs prompt redeposition would reduce W sputtering by factor p -2 ~ 10 4 (with p ≡ gyro ion ω τ ~ 0.01).This relation however doesn't include effects of multiple ionizations of sputtered W atoms and electric field in the magnetic pre-sheath (MPS, or 'Chodura sheath') and Debye sheath (DS).Monte-Carlo simulations of W redeposition with the inclusion of these effects are described in the paper.It is shown that for p << 1 inclusion of multiple W ionizations and electric field in the MPS and DS changes physics of W redeposition: from geometrical effects of circular gyro-orbits hitting the target surface to mainly energy considerations: potential barrier for ions escaping into the main plasma.The overwhelming majority of ions are being drawn back to the target by a strong attracting electric field.It is also shown that a possibility of W self-sputtering avalanche can be ruled out due to smallness of sputtered W energies compared to incident energies (as neutrals/ions circulate in the MPS) which doesn't compensate for the kinetic energy gain of ions in the MPS/DS, leading to a sharp reduction in sputtering yields.Results of simulations are applicable to a wide range of plasma conditions at the target plates that can be encountered in various magnetic confinement fusion devices.