The KSTAR superconducting magnetic coils, which are made of cable in-conduit conductor (CICC), maintain a superconducting state with forced-flow supercritical helium (4.5 K, 5.5 bar). During current changing of the superconducting magnetic coils, AC losses are generated in the CICC due to dI/dt, and the heat generated from the loss is removed by high heat capacity supercritical helium. At the same time, reversed flow of the helium occurs due to a rapid increase of the helium temperature and momentary changing of the pressure inside the CICC. This phenomenon has been detected in all of the poloidal field (PF) coils, especially in the upper (U) and lower (L) PF1 PF4 coils. The maximum change of the magnetic field in the PF1UL PF4UL coils is located near the inlet and outlet of the helium cooling channels, and that of the PF5UL 7UL coils is located at the center of the cooling channel. The temperature variation at the helium inlet was always measured to have a time delay after each shot. In the PF1 coil tests, it was measured to have a delay of 26 sec. During the first plasma campaign, this phenomenon was more severe in the case of all PF coils operating together than for a single PF operation. In this paper, we investigated the thermal-hydraulics of this phenomenon.Index Terms-CICC, inverse helium flow, KSTAR, superconducting magnet, supercritical helium.
To detect quenches in the Poloidal Field (PF) magnet system is more difficult than the Toroidal Field (TF) magnet system due to excessively high inductive voltages generated by PF pulsecurrents and plasma currents. According to reference scenarios being considered so far, the maximum voltage across the PF coils is inductively generated up to about 3.5 kV during the start of plasma (SoP) stage in a very short time period. The voltage measured by compensation of the inductive voltage should be below a certain level which is called as the quench voltage threshold. However, the compensated voltage might be higher than the threshold even with the well-designed compensation schemes. Accordingly, the quench voltage threshold and the quench protection delay time should be properly determined for the quench detection not to take a false action which could cause the fast energy discharge. From the quench simulation using the calculation of hot spot temperature and the resistive voltage growth as a function of time, the proper values of the quench detection parameters of the PF magnet system were derived for the maximum hot temperature rise to be limited within 150 K.
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