Effects of complex magnetic ripple on fast ions in JFT-2M ferritic insert experiments

In JFT-2M, ferritic steel plates (FPs) were installed on almost the whole inner surface of the vacuum vessel. This arrangement is called the ferritic inside wall (FIW), and is the third step of the advanced material tokamak experiment programme. The toroidal field (TF) ripple was reduced by optimizing the thickness of FPs but the total ripple structure has become more complex, with a non-periodic feature in the toroidal direction, because of the existence of ports and other components that limit the periodic installation of FPs. We investigated the effect of this complex ripple on the heat flux onto the first wall due to fast ion loss. The ripple trapped loss was reduced as a result of the reduced magnetic ripple of the FIW. Additional FPs were also installed outside the vacuum vessel to produce a localized larger ripple. A small ripple trapped loss was observed when the shallow ripple well exists in the poloidal cross section, and a large ripple trapped loss was observed when the ripple well extends deep into the plasma region. Experimental results were almost consistent with computation with a newly developed fully three-dimensional magnetic field orbit-following Monte-Carlo code including the three-dimensional complex structure of the TF ripple and the non-axisymmetric first wall geometry.