Performance evaluation and analysis of critical interface features of the National Compact Stellarator Experiment (NCSX)

The (18) modular coils for the National Compact Stellarator Experiment (NCSX) are joined at assembly by interfaces to form a toroidal shell which serves as the structural backbone of the device. There are six each of three coil types (A, B, and C); consequently, there are 4 distinct interface designs (A-A, A-B, B-C, C-C). This paper describes the performance evaluations and analyses used in the development of these critical interfaces. Initial analyses indicated that the baseline interface designs did not provide adequate shear capability along the inner (unbolted) legs between the modular coils and did not adequately address assembly tolerance requirements. Consequently a design effort was undertaken to develop interfaces with adequate shear capability and which would facilitate the achievement of assembly tolerances. Analyses indicated that a friction coefficient of 0.3 is necessary for “no-slip” joints with a preload value of ∼320 kN in the outboard regions. Two types of compatible segmented friction shims were developed to meet the friction requirement. One type uses alumina coated stainless steel shims and the other uses G-10/ stainless steel/ G-10 “sandwich shims.” Analyses indicated that the time constant requirements for induced currents in the shell could still be achieved with welds along all the inner (unbolted) legs except at the C-C interface. Consequently, welded interfaces utilizing alternating MIG fillet welds on each end of shims between coil castings were developed to react the shear loads. This configuration minimizes distortion since it avoids direct weld shrinkage stress across the interfaces. Analyses indicates that a 12.7 mm fillet weld has adequate shear capability, with average stress through the welds of 90–125 MPa, compared to a static limit of 217 MPa. Custom sized compression pucks located in the middle of the welded shims react the compressive loads and have average stresses less than 137 MPa. Fatigue acceptability of the welded joints was examined using crack growth data of welded specimens and estimates the maximum acceptable initial flaw size in 12.7 mm welds to be 3.2 mm. At this size, a crack would propagate to an unacceptably large size in 500,000 (5 X life).