Qualification of the Fastening Components of the Outer Intercoil Structure of the JT-60 SA Tokamak

Abstract

The 18 toroidal field coils of the JT-60SA are mechanically linked by the so called “Outer Intercoil Structure” (OIS). Each OIS is bolted to its neighbors by five splice plates: this presents the double advantage of facilitating the assembly of the structure as well as ensuring an efficient electrical insulation by the insertion of an epoxy-glass sheet between the clamping stainless-steel parts. Since strong electromagnetic loads are carried by these OIS, the bolting requires a high preload to provide a significant contact pressure and thus prevent the slippage of the splice plates. We identified some critical issues that are associated with this bolted joint: the sliding behavior at the cryogenic temperature, the risk of creep of the epoxy-glass spacers during the long period of the assembly phase, and the high stress levels developed in the bolting. In this paper, we present the results of the qualification tests for several components of the fastening parts of the OIS. Two mockups of the connection were tested at room temperature and at 4 K to measure the sliding coefficient between the stainless steel and the glass-epoxy faying surfaces. The loss of preload in the bolts due to the hydraulic tightening process has also been measured on different scaled mockups. The effectiveness of the titanium washers used to compensate the loss of preload due to the difference in the thermal shrinkage between the steel and the glass-epoxy components has also been validated. Finally, the creep behavior of the insulation has been investigated by testing the glass-epoxy samples on a specially designed testing device.