Characterization of the Full-Size Superconducting ASG Joint Designed for the DTT Toroidal Field Coils

The Divertor Tokamak Test (DTT) facility is a challenging high-field and compact tokamak, currently under construction at the Frascati ENEA Research Center. Its purpose is to significantly contribute to the exploration, design and assessment of systems intended for the management of heat exhaust. Its superconducting magnetic system consists of 18 Toroidal Field (TF) coils, 6 Poloidal Field (PF) coils and a Central Solenoid (CS). This work is focused on the superconducting praying-hands joint, manufactured by ASG-Superconductors for the DTT TF coils terminations and for the connection among the adjacent double-pancakes constituting the TF winding pack. The joint has been tested and fully characterized by dedicated 2-weeks cryogenic tests in the SULTAN facility at Swiss Plasma Centre (SPC, EPFL) in November 2023. The tests included 3000 EM cycles at the maximum foreseen operative load of 8.1 T - 21 kA and two Warm-Up-Cool-Down (WUCD) cycles, while reference DC measurements were done in normal and reversed polarity up to 42.5 kA and to 4 T, before, during and after cyclic loading and WUCDs. Assessment of AC losses, performed through bipolar sinusoidal pulsing at frequencies up to 2 Hz, and pressure drop measurements as a function of He mass flow rate, were conducted on the virgin sample and after full cyclic loading. The Minimum Quench Energy (MQE) has also been measured at different temperatures under nominal current conditions. All the conducted tests confirmed the efficacy and goodness of the joint design, with its resistance at self-field consistently remaining below the nominal allowable value of 2 nω.