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.

I. Introduction

The rapid charging and discharging of electrical current on the magnet coils during the ITER Tokamak operation result in heat pulses which in turn induce pressure variation in the SHe cooling circuits where the pressure head for forced convection is generated by cold circulators. In particular, in the CS cooling circuit, the asymmetry of induced pressure rise at the suction and discharge of the circulator can result in pressure head values beyond the surge limit [1]. Such a phenomenon was not evident during the ITER CS model coil tests [2], mainly due to the fact that coils of different size (diameter) were wound layer by layer and connected in parallel. There the flow channels of the outer layers are less affected by the heat pulses than the inner ones and SHe flow across the circulator could persist. However, in case of the ITER CS system, all coils are wound pancake by pancake which dimensions are more or less identical (same flow channels in parallel) [3]. With such geometry the flow impedance of the inner layers will dominate throughout the whole pancakes during current shots and result in blocking of SHe flow across and trip of the circulator.