Analysis of the flow imbalance in the KSTAR PF cryo-circuit

• Investigate of flow imbalance trend for the KSTAR PF superconducting magnet.
• Flow imbalance is compared with individual magnet test and integration magnet test.
• Intensifying of flow imbalance is proven from the flow monitoring in the KSTAR PF circuit.
• Flow behavior is analyzed during magnet charging in the circulator circuit.
• Variation of magnet outlet temperature is analyzed due to flow imbalance.

The KSTAR PF cryo-circuit is a quasi-closed circulation system in which more than 370 g/s of supercritical helium (SHe) is circulated using a SHe circulator. The heated helium from superconducting magnet is cooled through sub cooler (4.3 K). The circulator is operated at 4.5 K and 6.5 bar, and the pressure drop of the circuit is kept at 2 bar in order to maintain the supercritical state and circulator stability. The circuit is connected with helium refrigerator system, distribution system, and supercritical magnet system. It has a hundred branches to supply supercritical helium to the poloidal field superconducting magnet. The branch was designed to optimize the operation conditions and they are grouped for one cryogenic valve has the same length within the cardinal principle of the optimization. Five cryogenic valves are installed to control the mass flow rate, and seven orifice mass flow meters, differential pressure gauges and temperature sensors were installed in front of the magnet in the distribution because upper magnet and lower magnet is symmetric theoretically. The cryogenic pipe line was manufactured with elevation about 10 m between upper magnet and lower magnet. The inlet and outlet helium feed-through were installed at the coil inside in case of KSTAR PF1–PF5 upper magnet and lower magnet. The flow imbalance is caused by void fraction and it could be changed due to manufacturing process even if it has the same length of cooling channel. This creates an imbalance among cooling channels and temperatures are slightly different. The flow was reduced and detoured due to the pressure rise rapidly at inlet and outlet during magnet operation. Therefore, the orifice mass flow meters are installed in front of the PF1–PF5 upper magnet and lower magnet in order to investigate flow trend and the thermos-hydraulic behavior is analyzed during PF magnet operation.