Impact of Failing Insert Coils on the Mechanical Design of the HFML 45 T Hybrid Magnet System

The High Field Magnet Laboratory of the Radboud University is constructing a 45 T hybrid magnet system. Large fault forces may arise in the not so unlikely event of failure of parts of the resistive insert magnet. The measures taken to accept the fault forces dominate the design of the support structures, in particular those coupling the room temperature resistive insert magnet with the superconducting coil in the cryogenic environment. The largest fault forces arise when half of one or several insert coils become electrically short-circuited in a burn-out accident. In case the complete upper or lower half of all the resistive coils collectively disappear, the so-called ultimate axial fault force on the superconducting outsert coil may amount to several MN - calculated in a static approach. Traditionally these ultimate fault forces are reference for the mechanical design to guarantee that the structures can sustain these forces preventing catastrophic failure of the magnet's housing. We will argue that if one takes into account the time dependent behavior of the powering and protection systems after detection of a coil failure, the resulting fault forces will be much smaller than the ultimate fault forces predicted by static approach.