THE POTENTIAL FOR SAGBO CRACKING IN A NB3SN WIND-AND-REACT SOLENOID USING A CICC WITH INCOLOY-908 JACKET AND GLASS FABRIC INSULATION

The 45-T Hybrid Magnet System being constructed for the National High Magnetic Field Laboratory (NHMFL) in collaboration with the Francis Bi tter National Laboratory is composed of a resistive insert and superco nducting outsert. Two of the outsert solenoids use Nb3Sn and will be w ound with cable-in-conduit conductors (CICCs) using the wind-and-react technique. A candidate for the jacket material is Incoloy 908, but an obstacle to this choice is its sensitivity to Stress Accelerated Grai n Boundary Oxidation (SAGBO). The conditions for SAGBO - residual stre ss, elevated temperature, and exposure to oxygen (even in extremely lo w levels)- are difficult to avoid in coils fabricated by the wind-and- react process. The glass fabric insulation, applied during winding, is at high temperature, both a source of oxygen (primarily in the form o f absorbed water vapor) and, being highly compacted by the winding, an impediment to its removal. In attempt to solve this problem NHMFL has developed a pre-baking procedure in vacuum at 730 C, to be applied to the glass fabric before to insulate the coil. In order to test the ef ficiency of pre-baking and to check the existence of possible other fe atures that also lead to SAGBO, a dummy coil, wound with a CIC conduct or jacketed with Incoloy 908, has been manufactured. The coil has been heat treated in very high purity helium atmosphere. The results are r eported.