Magnetic Design Optimization and Quench Study of a Highly Saturated Superferric Quadrupole for FAIR Super-FRS Multiplets

The design of the FAIR Super-FRS superferric multiplets is under evaluation by CIEMAT, as part of the Spanish contribution to this facility. The most challenging magnet is the so-called Type 3 quadrupole, as it features a large warm bore aperture, 380 mm, with a gradient of 10 T/m and a magnetic length of 800 mm. Field quality request is quite tight, 8 units (1E-4) for gradients below 8 T/m. Our analysis is based on a preliminary design, whose main drawback was the large mass of the cold iron, about 6800 kg, which complicates cooling and assembly. A new design with lighter iron is presented and optimized in this paper. The most difficult problem is to achieve the requested field quality with a highly saturated iron yoke, which has been solved by drilling holes and shaping the pole ends of the iron yoke. On the other hand, reducing the iron mass requires increasing ampere turns and saturation, together with the stored energy and the maximum voltage and temperature in case of a quench. Thus, a good quench simulation is essential. A custom quench model has been developed using a Finite Differences Method in Matlab to solve the adiabatic heat balance equation inside a coil, taking into account the material properties dependence on temperature. The quench model has been improved and validated by several quench propagation tests on a fully impregnated and instrumentated NbTi solenoid.