Application of Low Activation Ferritic Steel in the JFT-2M Tokamak-Evaluation of Magnetic Effect to the Plasma
N. Isei, M. Sato, K. Tsuzuki, H. Kawashima, Y. Miura, H. Kimura
Japan Atomic Energy Research Institute
Abstract.
1. Introduction
Ferritic steel is a candidate material of first wall and blanket in the fusion reactor because of low activation, good thermal properties and swelling resistance. However it has not been used so far, because its ferromagnetic properties could degrade the control, stability and confinement of plasma. In order to examine the compatibility of ferritic steel with the plasma, Advanced Material Tokamak EXperiment (AMTEX) has been carried out in the JFT-2M Tokamak. In the AMTEX program, the ferromagnetism is used to reduce the toroidal field ripple. In this paper, experimental results of the ripple reduction and the magnetic effects on MHD behavior of the plasma are presented.
2. Ripple reduction
Fast ion losses due to toroidal field ripple in a tokamak is one of the crucial issues for the design of a fusion reactor, bacause the ripple induced excursion of energetic ions can result in serious localized heat deposition on the first wall. In the first phase of AMTEX, the ferritic boards (FBs) were inserted between the Vacuum Vessel (VV) and all the 16 toroidal field coils. Thickness of FBs can be adjusted for Bt below 2.2T. The experiments were performed in two types of thickness (FB:50mm, FB2:67mm). The reduction of ripple magnitude from 2.2% to 1.1% was confirmed by measurements using pick-up coils with FB1s at Bt=1.3T. Fast ion losses were substantially reduced with FB1s from measurements using infrared TV (IRTV). It was found that the ripple magnitude can be almost zero in the case of FB2 at Bt=1.0T, and that the increase of wall temperature by the ripple trapped loss was not observed by IRTV.
3. Effects on MHD and stability of the plasma
The operational region in tokamak experiments is usually limited by non-rotating helical magnetic perturbations called 'locked modes', which are caused by the error field. In order to examine the effects of error field by FBs, experiments were performed in the condition of unbalanced installation of FBs, where 40% of FBs continuously in the toroidal direction were removed to increase error field of n=1 mode artificially. Clear differences in the operational window and the MHD behavior were not observed in the experiments. Experiments in the second phase of AMTEX program, with FBs installed inside the VV in addition to the outside FBs, started from May in this year. Further investigations about the effects of FBs on MHD and stability of the plasma will be presented in the meeting.