A mechanical design for ferritic steels to reduce toroidal field ripple in the JFT-2M

A modification of JFT-2M was done in order to reduce the toroidal magnetic field (TF) ripple, using ferritic boards (FBs) between the TF coil and vacuum vessel. The arrangements (thickness in minor radius direction, toroidal position and toroidal width) of the FBs are determined so as to minimize the ripple at each TF coil under the restriction of existing ports, diagnostics, and heating systems. The FBs are supported from TF coil cases. The support structures are designed taking into account all coil current patterns of the JFT-2M. The number of the boards can be changed depending on the TF strength. Results of an electromagnetic force test after installation show no deformation of the support structure. The computations about FBs show a reduction of the fundamental mode ripple from 2.1% to 0.8% at (R,Z)=(1.6 m,0.0 m) for a TF strength B_t=2.2 T