The role of SOL plasma in the confinement of NBI fast ions in W7-X

The impact of the scrape-off layer (SOL) plasma on deposition, confinement and losses of neutral beam injected fast ions was investigated in W7-X plasma. The effect of SOL width, density, temperature profiles, radial electric field, and charge-exchange reactions (CX) was explored. Ionization and slowing down partly counterbalance each other, as slowing down in cold SOL plasma compensates for ionization effects in radially decaying model profiles. However, the effect of SOL plasma on more vulnerable steel components is mitigated over a wide range of different profiles, because for those components the collisionality effect overrules the effect of SOL on ionization. The effect of the radial electric field is mitigated for steel components in the experimentally observed direction of the field. The effect of CX reactions is shown to lead to a widely spread low power load distribution with no clear effect on peak load. Statistical challenges caused by hugely varying triangle sizes in the discretization of walls are discussed.