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The ITER superconducting (SC) magnet system, which consists of central solenoid coils, toroidal field (TF) coils, TF structures, and poloidal field and correction coils, is cooled by supercritical helium (SHe) circuits located in dedicated auxiliary cold boxes (ACBs) of the cryodistribution (CD). Due to the increase in the nuclear heat load during the deuterium-tritium plasma phase, the necessary cooling power to maintain the thermal stability of the magnet system can be beyond the presently designed and contracted specification of the cryoplant (LHe and liquid nitrogen plants). Increasing the cryoplant capacity or performance in turn will significantly increase the project cost apart from the impacts on the utility infrastructures (cooling water, electricity, civil works, etc.). In this paper, we will present arrangements within the CD for an efficient and flexible operation. Instead of a common control, which is the present design, by individually controlling the LHe bath temperature of the ACBs, the cooling power can be concentrated to the magnet system in need (reduction in the SHe circuit temperature). In addition to this, a proposal to minimize the heat of compression of the cold rotating machines in order to further allocate the cooling power to the SC magnet system will be introduced.

The pressure head necessary for the forced convection of supercritical helium (SHe) across the ITER superconducting (SC) magnet system is generated by centrifugal-compressor type cold circulators. From the viewpoint of the cryogenic system, simulation results indicate that the main challenge of the ITER Central Solenoid (CS) operation is the peaking pressure head of SHe across the cold circulator induced during the rapid ramping of electrical current on the coils. Excess variation in pressure head will push the operation point of the rotating machine beyond the surge line in the compressor map which can trip the circulator and in the worst case result in damages. Therefore, in order to avoid the occurrence of a surge, it has been proposed to open adaptively a cryogenic valve, installed in parallel with the CS cooling channels, during the high pressure head instances. Such an action creates an additional flow channel which consequently suppresses the pressure head peak. The proposed strategy has been numerically benchmarked and tests have been performed in existing cryogenic facilities. In this paper, we will present the test results of pressure head mitigation across the cold circulator dedicated to the KSTAR CS and Poloidal Field (PF) coil cooling circuit with the main purpose of demonstrating the safe operation of the ITER CS circulator as well as cooling circuit during plasma shots. The tests were performed by progressively increasing the coil by-pass valve opening during identical PF/CS current shots and monitoring the tendency of the maximum pressure head variation across the cold circulator. Also, preliminary results of pressure head mitigation during real plasma experiments will be introduced.

The cryogenic system for the KSTAR superconducting (SC) magnets has been commissioned. It consists of a cold box, distribution boxes (DB) and cryogenic transfer lines. The cold box and DB #1 provide 600 g/s of supercritical helium to cool the SC magnets, their SC bus-lines, and the magnet support structures. It also provides 17.5 g/s of liquid helium to the current leads and supplies cold helium flow to the thermal shields. The main duties of the DB #2 are the relative distribution of the cryogenic helium among the cooling channels of each KSTAR cold component and the emergency release of over-pressurized helium during abnormal events such as quenches of the SC magnets. After individual commissioning, the system was integrated and cooled down with the KSTAR device. In this paper, the construction and commissioning results of the KSTAR cryogenic system will be introduced. In addition, we will present the cool-down results of the KSTAR device.

The dynamic simulation of a sub-scale ITER Central Solenoid (CS)/STRucture (STR) cooling loop has been performed to investigate the cryogenic control strategies against pulsating heat loads. HELIOS facility has been modified for this particular study, which consists of a Liquid Helium (LHe) reservoir with two immersed heat exchangers, a circulation pump and an approximately 130 m long piping with three evenly distributed heated sections. The setup provides the equivalent thermo-hydraulic configuration of the ITER CS/STR, cooled by the forced-flow Supercritical Helium (SHe). The dynamic simulator, C-PREST, has been utilized to model the HELIOS test loop and to compare the results with the experimental data sets. This paper presents the dynamic simulation results and discusses the control strategy to minimize the cooling power requirements, to have high reliability.

One of the main challenges of the ITER cryogenic system is to manage the large pulsed heat load variation generated by the superconducting magnet system during the fusion experiments. A solution proposed to smooth such pulses and verified by numerical simulations is to use the ITER magnet structure (ST) system as a thermal buffer by taking advantage of its large mass of about 5,000 tons and consequent thermal inertia. In order to validate the simulated predictions, tests have been executed on existing cryogenic facilities. In this paper we will present the results of the ITER cryogenic system feasibility tests performed at KSTAR. The tests were concentrated on the heat load mitigation scenario of the ITER ST via variation of the coolant mass flow rate by using a flow by-pass valve in parallel with an in-line valve.