Advanced Material Tokamak Experiment (AMTEX) Program with Ferritic Steel for JFT-2M.

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Abstract
To examine the effects of ferritic steel on error field, ripple reduction and plasma properties (especially for improved confinements), Advanced Material Tokamak EXperiment (AMTEX) is planned for JFT-2M. The AMTEX program consists of the test of toroidal field ripple reduction with ferritic inserts and that of the compatibility between ferritic steel and the tokamak plasma. To reduce fast ion losses due to toroidal field ripple, the ripple amplitude and its toroidal mode number have to be reduced. The guideline of the design for the reduction in ripple amplitude (δ) by inserting Ferritic Boards (FB) is as follows; the wider and thicker FB should be positioned far from the plasma and be located just under the toroidal field coil. Elimination of a central part of the FBs is effective in reducing a second harmonic ripple amplitude. Since the ferritic steel easily saturates by the toroidal magnetic field, the δ depends on the toroidal magnetic field. The JFT-2M Vacuum Vessel (VV) will be remodelled using ferritic steel for the plasma-matching test. Since the relative magnetic permeability of the ferritic steel is around two, the effect of the ferritic steel VV on plasma equilibrium is rather small. A simple port design can reduce any ripple amplitude of the fundamental mode but cannot reduce that of higher modes.