DESIGN ISSUES OF THE PRE-COMPRESSION RINGS OF ITER

The pre-compression system is the keystone of ITER. A centripetal force of similar to 30MN will be applied at cryogenic conditions on top and bottom of each TF coil It will prevent the 'breathing effect' caused by the bursting forces occurring during plasma operation that would affect the machine design life of 30000 cycles. Different alternatives have been studied throughout the years. There are two major design requirements limiting the engineering possibilities. 1) the limited available space and 2) the need to hamper eddy currents flowing in the structures. Six unidirectionally wound glass-fibre composite rings (similar to 5 m diameter and similar to 300 mm cross section) are the final design choice. The rings will withstand the maximum hoop stresses <500 MPa at room temperature conditions. Although retightening or replacing the pre-compression rings in case of malfunctioning is possible, they have to sustain the load during the entire 20 years of machine operation. The present paper summarizes the pre-compression ring R&D carried out during several years. In particular, we will address the composite choice and mechanical characterization, assessment of creep or stress relaxation phenomena, sub-sized rings testing and the optimal ring fabrication processes that have led to the present final design.