Modelling the thermal buffer behavior of JT-60SA in case of pulsed heat loads

Abstract

JT-60SA is a joint international fusion experiment built and operated by Japan and Europe in the framework of the Broader Approach agreement. Integrated commissioning held in 2021 and 2023 provided valuable experimental data, particularly a data set depicting the reaction of the cryogenic system to a Toroidal Field (TF) coil fast safety discharge (FSD). When a TF FSD occurs, the actual protocol imposes at high currents a disconnection of the TFC hydraulic circuit from the cryoplant and releases the heated and pressurized helium in a quench tank to avoid overpressure in the refrigerator and the magnet system. To shorten the time for cryoplant recovery, disconnection should be avoided in the absence of tripping risk for the whole cryoplant. In this regard, Simcryogenics modelling tool was used to represent the cryodistribution and simulate TF FSD and explore the possibility to avoid disconnecting the cryoplant after any TF FSD. One of the driving factors in the decision is the pressure rise observed in the buffer bath of the refrigerator during a FSD, preventing the Warm Compression Station (WCS) from being overloaded, otherwise inducing a severe tripping of the cryogenic system. In this paper, we present a model that reproduces the experimental signals of the buffer bath during and after controlled FSDs from 15, 18 and 20 kA and it also extrapolates for the nominal current of 25.7 kA. We conducted several parametric studies to explore the impact of some drivers of bath pressure rise: the increasing rate of the pressure setpoint and a mitigation of contributors to the total WCS mass flowrate. We show here some analysis and interpretations of the resulting trends obtained in the study and further conclude with a proposal path to manage the cryomagnet system during a nominal TF FSD avoiding disconnection.