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.