Quench Protection of Fusillo Subscale Curved CCT Magnet

The Fusillo project at CERN aims to design and build a demonstrator magnet with multi-harmonic corrected fields in a 90° curved CCT magnet. In the first stage, a subscale magnet is built with 30° bending, about 1/30 of the demonstrator conductor length, and increased current to reach coil stresses equivalent to the demonstrator. The subscale magnet enables qualification of the technology developments, fabrication methods, winding and assembly procedures, and magnetic and quench protection design and measurement setups. The subscale magnet comprises two tilted solenoids with an opposite inclination on curved aluminium formers. Each solenoid has two channel turns with 70 Nb-Ti/Cu wires. The magnet is protected by an active quench detection with energy extraction (EE). EE causes current decay, which induces eddy currents in the formers. As a result, the differential inductance of the magnet is reduced, and the formers heat up, with the potential to strongly influence the quench behaviour of the windings. Calculation of the eddy currents and heat propagation in the formers with simultaneous quench propagation in the magnet windings requires a three-dimensional (3D) simulation. A cooperative simulation approach has been developed to simulate transients in this magnet. It involves two software tools developed at CERN as part of the STEAM framework: a finite element-based tool called FiQuS and a finite difference-based tool called LEDET. FiQuS calculates eddy currents in the formers and the temperature of the formers, whereas LEDET calculates windings' temperature, current and voltage. This approach enables a 3D quench simulation with great geometrical detail while maintaining reasonable computational cost. The simulation results are compared to measurement results from the forced EE. The agreement between the measurements and simulations is presented, and the key factors that affect magnet quench behaviour are identified.