H.-S. Chang

Dynamic Simulation of Sub-Scale ITER CS/STR Cooling Loop

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.

Feasibility Studies of the ITER Cryogenic System at KSTAR

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.

COMMISSIONING RESULT OF THE KSTAR HELIUM REFRIGERATION SYSTEM

To keep the superconducting (SC) magnet coils of KSTAR at proper operating conditions, not only the coils but also other cold components, such as thermal shields (TS), magnet structures, SC bus-lines (BL), and current leads (CL) must be maintained at their respective cryogenic temperatures. A helium refrigeration system (RRS) with an exergetic equivalent cooling power of 9 kW at 4.5 K without liquid nitrogen () pre-cooling has been manufactured and installed. The main components of the KST AR helium refrigeration system (HRS) can be classified into the warm compression system (WCS) and the cryogenic devices according to the operating temperature levels. The process helium is compressed from 1 bar to 22 bar passing through the WCS and is supplied to cryogenic devices. The main components of cryogenic devices are consist of cold box (C/B) and distribution box (D/B). The C/B cool-down and make the various cryogenic helium for the KSTAR Tokamak and the various cryogenic helium is distributed by the D/B as per the KSTAR requirement. In this proceeding, we will present the commissioning results of the KSTAR HRS. Circuits which can simulate the thermal loads and pressure drops corresponding to the cooling channels of each cold component of KSTAR have been integrated into the helium distribution system of the HRS. Using those circuits, the performance and the capability of the HRS, to fulfill the mission of establishing the appropriate operating condition for the KSTAR SC magnet coils, have been successfully demonstrated.

Optimization of the ITER Cryodistribution for an Efficient Cooling of the Magnet System

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.

Qualification Test Results of the KSTAR Superconducting Coils From the Construction to the Commissioning Steps

To achieve the first plasma of the Korea superconducting tokamak advanced research (KSTAR), the KSTAR superconducting coils were tested in advance. As they should operate in excessively low temperature of 4.5 K and high magnetic field environment of 7.5 T, it is crucial to monitor the cryogenic and the structural behaviors of KSTAR device during the commissioning period including a cool-down. The temperatures of the KSTAR toroidal field (TF) coil and the poloidal field (PF) coils were measured during the entire operating period. The mechanical stresses on the TF and PF structures were continuously monitored to check if they go beyond the limiting value calculated through the simulation. The alignment of the KSTAR device was checked by using displacement sensors. The TF coils were successfully supplied with 15 kA DC current for 8 hours, and the maximum 5 kA/s current variation of the PF coils were tested. For the main experiment, the interlock test of the quench detection system for the KSTAR coils was carried out at reduced currents of 1 kA. From these results the quench protection circuit, and the current-flow of the KSTAR superconducting coils proved to be well performed for the first plasma operation.

Test Results of Pressure Head Mitigation of Supercritical Helium Across Cold Circulators at KSTAR for the Justification of the ITER Central Solenoid Cooling Circuit Design

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.