Acceptance Test of Pre-Compression Rings for ITER Magnet System

In the ITER machine, two sets of three pre-compression rings will be installed at the top and bottom of the 18 toroidal field coil structures. The rings will tightly hold the coils with a radial force of 4,800 t per coil against the operating forces. Four other pre-compression rings will serve as spares. Weighing 3.4 t each, with an inner diameter of 5 m, the rings made of pultruded S-fiberglass composite are possibly the largest and most highly stressed composite structures to be designed for a cryogenic environment. All 10 rings have been delivered to ITER and tested at the ITER pre-compression ring facility at a double nominal stress level. The facility is used to measure the stress at rupture of sub-size rings and for acceptance tests of the full size rings. These tests are supplemented by the stress relaxation test at nominal load on a ring equipped with acoustic sensors, to confirm that the creep performances are also adequate. The paper discuss the design of the rings, the manufacturing route, and the test results including the analysis of the acoustic emission signals.