Experience Gained during Fabrication and Construction of

Abstract. During the design and fabrication of the main components for the W7-X stellarator a number of tech-nical challenges and severe problems had to be overcome many of which are considered relevant for other ma-chines of similar size and complexity. Massive supporting structures are used to sustain the large electro-magnetic loads. Finite element analyses (FEA) were used to design the main support structures. However, when the original FEA models were improved and refined, structural weaknesses showed up requiring reinforcement after components had already been fabricated. Some highly loaded elements are qualified by testing of instru-mented prototypes. This proved to be very important to find and eradicate weaknesses. The coils for W7-X have been the subject of considerable delivery delays due to a variety of problems. In addition to the reinforcement of the coil support blocks on the non-planar coils, refined FEA showed that the planar coils undergo excessive out-of-plane bending. This has been corrected by additional shear pins to stiffen the casings. In an early stage it was decided that all 70 coils for W7-X should be cold tested. The cold tests have shown that of all the coils tested thus far the specified superconductive behaviour was according to specification. However, a number of faults have been found. These include cold leaks, poor performance of instrumentation, some of which is considered important for machine operation, as well as electrical insulation faults. After cool-down and warm-up in a test cryostat all coils are tested under Paschen conditions. Initially many non-planar coils did not pass this test. After returning to the factory detailed investigations have been carried out, repair procedures have been qualified and applied. Loss of time may partially be compensated by accelerating the assembly of the machine through the use of parallel assembly lines