Tests of the critical bolted connection of the Wendelstein 7-X coils

The Wendelstein 7-X (W7-X) superconducting coils will be subjected to high electro-magnetic forces and moments during the stellarator operation. It is therefore essential to prove that the mechanical connections which keep the coils joined together and linked to the magnet system Central Support Structure (CSS) are capable to take the operational loads during the machine lifetime. Critical connections identified by the finite element (FE) analysis of the magnet system are the bolted Central Support Elements (CSE) between coils and CSS. The most critical one, called NPC1Z1, was studied by using mock-ups. The tests were carried out at 77 K and consisted in applying the forces starting from zero and ramping them up to a given set point, so simulating the coils' energization, and back to zero. The loading steps were repeated for more than 3600 cycles in order to reproduce the W7-X lifetime, including the high load cycles, where the reference loads were exceeded with the aim to explore the limit of such bolted connection. The tests showed that the mock-ups could take the nominal loads, but at 120% of such loads plastic failure on the mock-ups occurred. This paper presents an overview of the design and assembly of the CSE, the test mock-ups and test device, the execution of the test as well as the results and main conclusions of the experimental campaign. (C) 2007 Elsevier B.V. All rights reserved.