Analysis of the Early Quench Development in JT-60SA Toroidal Field Coils Tested in the Cold Test Facility

The toroidal field system of the JT-60SA tokamak is composed of 18 NbTi superconducting coils. Each TF coil is composed of six cable-in-conduit conductor lengths, wound in double-pancakes, and carrying a nominal current of 25.7 kA. These coils are being tested in the single-coil configuration at the so-called cold test facility (CEA/IRFU, Saclay, France). The test program includes for all coils a quench test at nominal current obtained by a progressive increase of the operating temperature at helium inlet up to quench (around 7.5 K). Thanks to the accuracy of the voltage measurements, it has been possible to follow the very early development of the quench at the scale of a few millimeters normal length. The paper presents the evolution of the resistive voltages measured on eight coils before the safety discharge is triggered. In addition, this early quench development over one conductor length was also simulated using the THEA code with relevant boundary conditions. Two different quenches, which occurred on two different coils, were analyzed, one starting on a central pancake (peak magnetic field) and the other starting on a side pancake (warmer conductor due to heat transfer from the casing). The simulations show good agreement with experiments, stressing particularly three phases in the quench development clearly identified in the measured resistive voltages of all coils.