A 2.5-T, 1.25-m Free Bore Superconducting Magnet for the Magnum-PSI Linear Plasma Generator

DIFFER's main experiment, Magnum-PSI, is the only laboratory setup in the world capable of exposing materials to plasma conditions similar to those of future fusion reactors. The success of the Magnum-PSI experiment depends on the generation of a 2.5-T magnetic field without restricting the diagnostic access and operational aspects of the experiment. This has been achieved with a magnet consisting of five superconducting solenoids wound on a 2.5-m-long stainless steel coil former positioned in a cryostat offering a 1.25-m warm bore. A copper stabilized multifilamentary NbTi conductor with a 3.48-mm² cross section has been used; thus the magnet exhibits a total inductance of 500 H and a stored energy of 16 MJ. This presents quite a challenge for the protection scheme that has been implemented using a mix of back-to-back cold diodes and external dump resistors. The coils generate a plateau-shaped magnetic field adjustable up to 2.5 T while the distance between the coils allows for 16 room temperature view-ports. The coils are cooled with liquid helium using a recondensing system operated with cryocoolers, while the magnet system cycles between zero and full field up to once per day. The magnetic stray field is shielded down to 1 mT outside the experimental area by iron walls that flank the magnet.