Demonstration of ripple reduction by ferritic steel board insertion in JFT-2M

In the JFT-2M tokamak, the application of low activation ferritic steels to plasmas has been investigated (the so-called Advanced Material Tokamak Experiment (AMTEX) programme). In the first stage, toroidal field ripple reduction was examined by ferritic steel boards (FBs) inserted between the toroidal field coils and the vacuum vessel. It is demonstrated that FB insertion reduced toroidal field ripple and the losses of fast ions produced by tangential co-NBI. By optimizing the FB thickness, such that fundamental mode ripple is minimized to 0.07% at the shoulder of the inside wall, ripple trapped loss is reduced to an almost negligible level. It is determined that the reductions of the fundamental mode ripple and the ripple banana diffusion coefficients at the shoulder are most effective in reducing ripple ion losses. Ripple loss reduction by FBs is also confirmed with perpendicular beam injection. The insertion of FBs causes no undesirable effects on plasma production and control.