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KSTAR project has required the new helium distribution box named upgraded distribution box (DBU) for the operation of the cryogenic components such as in-vessel cryo-pump (CPI), super-sonic molecular beam injector (SMBI), and deuterium pellet injection system (PIS). Two CPIs are inserted into the tokamak vacuum vessel and these components shall be operated at 90 K for the liquid nitrogen thermal shields and 4.5 K for the hydrogen cryo-panel. One hydrogen PIS was newly mounted on the tokamak for the 2016 KSTAR campaign. Liquid nitrogen shall be supplied to the one SMBI. For the operation of above mentioned 3 kinds of cryogenic components, a helium refrigerator, which had been used for the R&D in the KSTAR facility construction phase (2002–2013), was moved and inserted into the KSTAR 9 kW helium facility room. The cooling capacity of the refrigerator at 4.5 K is 1 kW and it was manufactured from the Linde Kryotechnik before 2002. After some maintenances in warm compressor, electrical power supply, oil-filter, and so on, commissioning of the refrigerator up to 4.5 K was accomplished successfully. From the beginning of 2015, design and fabrication of the DBU was started. It shall control the liquid nitrogen for the SMBI and CPI thermal shields whereas liquid helium for the CPI cryo-panel and PIS. To minimize the temperature of the liquid nitrogen to be supplied to SMBI and CPI, a thermal damper tank was inserted into the distribution box. Nitrogen return gases are to be warmed up to room temperature at the heater in the distribution box. A 1000 l of liquid helium vessel is located nearby the PIS to supply cold gas helium (∼5 K). Because the CPI cryo-panel requires regeneration up to 90 K, complex regeneration and re-cool down scenario was developed and applied to the DBU. Including operational results, details of the DBU progresses will be reported in this paper.

The cooling of superconducting (SC) magnet system for fusion device is the first procedure of the experiment. The Korea Superconducting Tokamak Advanced Research (KSTAR) device is fully SC tokamak that is made of NbTi and Nb 3 Sn. The KSTAR SC magnets are composed of 16 Toroidal Field (TF) coils and 14 Poloidal Field (PF) coils with 300 tons of cold mass. A Helium Refrigeration System (HRS) with 9kW@4.5 K was installed and has been operated since 2008 to cool the SC magnets and keep all magnets at appropriate temperature conditions. This paper describes and analyzes the operational result of HRS during the KSTAR cool-down in 2008 and 2013. According to the cool-down analysis, the larger cryogenic helium flow from HRS made more efficient cooling of SC magnets. Furthermore, the study of cool-down optimization based on practical approach is presented and the build-up of control logic by using of cool-down optimization is also discussed.

Thermo-acoustic oscillation is well-known phenomena which can generate quite a heat load to a cryogenic system. Here we report that thermally induced oscillation has been occurred in the KSTAR cryogenic system and furthermore that oscillation propagates the cryogenic circuit, affects strongly the overall supercritical helium flow and generates severe mechanical vibration on the system. Additional vibration sensors were attached on the cryogenic circuit to trace the origin of that abrupt thermal leak and detailed hydraulic data analyses have been carried out. It was found that test heaters of flange type inserted in directly the by-pass lines are the origin of the thermally induced unexpected oscillation and thereby by the dismantle of the heaters the oscillation can be permanently eradicated.

The KSTAR superconducting (SC) magnets are cable-in-conduit-conductor (CICC) type magnets, which have a hundred parallel and twisted SC strands in a square conduit with a porosity of 0.36. They are cooled by the forced flow of supercritical helium with a 2bar pressure drop. A flow imbalance test of the magnet cooling channels, one of the quality assurance processes, was conducted using Ar gas at room temperature during the magnet’s manufacture, and was carried out again after assembly. In those tests, the flow imbalance was measured to be within at most ±10% for the all cooling channels, which satisfied the criterion (±20%). During individual tests of the magnets, the tendency of the flow imbalance showed that the mass flow rate of the upper (U) magnets PF1, PF2, PF5, PF6, and PF7 was higher than that of the lower (L) magnets. On the other hand, the mass flow rate of the upper magnets PF3 and PF4 was lower than that of the lower magnets. After assembly, this imbalance became more serious, and the measured mass flow rate of the upper magnets was higher than that of lower magnet at the PF2, PF3 PF4, and PF5 magnets. This imbalance can influence the temperature and the pressure of supercritical helium, as well as accelerate reversal flow and limit operating performance. In this paper, the helium behavior is analyzed and is explained to be due to the flow imbalance in the PF magnet.

KSTAR in-vessel cryo-pump has been installed in the vacuum vessel top and bottom side with up-down symmetry for the better plasma density control in the D-shape H-mode. The cryogenic helium lines of the in-vessel cryo-pump are located at the vertical positions from the vacuum vessel torus center 2,000 mm. The inductive electrical potential has been optimized to reduce risk of electrical breakdown during plasma disruption. In-vessel cryo-pump consists of three parts of coaxial circular shape components; cryo-panel, thermal shield and particle shield. The cryo-panel is cooled down to below 4.5 K. The cryo-panel and thermal shields were made by Inconel 625 tube for higher mechanical strength. The thermal shields and their cooling tubes were annealed in air environment to improve the thermal radiation emissivity on the surface. Surface of cryo-panel was electro-polished to minimize the thermal radiation heat load. The in-vessel cryo-pump was pre-assembled on a test bed in 180 degree segment base. The leak test was carried out after the thermal shock between room temperature to one before installing them into vacuum vessel. Two segments were welded together in the vacuum vessel and final leak test was performed after the thermal shock. Commissioning of the in-vessel cryo-pump was carried out using a temporary liquid helium supply system.

Korea Superconducting Tokamak Advanced Research (KSTAR) has a helium refrigeration system (HRS) with the cooling capacity of 9 kW at 4.5 K. Main cold components are composed of 300 tons of superconducting (SC) magnets, main cryostat thermal shields, and SC current feeder system. The HRS comprises six gas storage tanks, a liquid nitrogen tank, the room temperature compression sector, the cold box (C/B), the 1st stage helium distribution box (DB#1), the PLC base local control system interconnected to central control tower and so on. Between HRS and cold components, there’s another distribution box (DB#2) nearby the KSTAR device. The entire KSTAR device was constructed in 2007 and has been operated since 2008. This paper will present the maintenance result of the KSTAR HRS during the campaign and discuss the operation record and maintenance history of the KSTAR HRS.

The Koreas Superconducting Tokamak Advanced Research (KSTAR) PF cryo-circuit is designed for cooling the fourteen superconducting magnets (Nb3Sn, NbTi) and structures. Those are cooled down by the supercritical helium (4.5 K and 5.5 bar) of a forced flow (pressure gradient: 2 bar) in order to maintain the supercritical state of the helium. To supply a large amount of supercritical helium (>370 g/s), a circulator was inserted into the PF cryo-circuit. The compressed supercritical helium is distributed to five helium manifolds with cryogenic valves and supplied to each PF magnet. While the PF magnets had been operating, the mass flow rate reduced and the pressure head of the circulator was fluctuated depending on the PF magnet operation scenario. These phenomena could damage the circulator and could stop it during operation. Therefore, by-pass valve, which is parallel with in-line valve and is connected with inlet and outlet of the magnet, was opened in order to reduce of the circulator's pressure head. In this paper, we focused on the helium behavior of the superconducting magnet when the by-pass valve was opened in order to release the pressure head of the circulator and the results will be presented.

KSTAR is a fully superconducting (SC) tokamak consisting of 30 magnet coils made of Nb 3 Sn and NbTi. To keep the SC magnets at proper operating conditions, all cold components of KSTAR such as the SC bus-lines, current leads, and thermal shields must be maintained at the respective cryogenic temperatures by using a helium refrigeration system (HRS). The main components of the HRS can be classified into the warm compression system (WCS) and the cryogenic systems according to their operating temperature levels. The HRS had been manufactured, installed and commissioned until March 2008 and has been operated for KSTAR campaign since then. In this paper, the result of the commissioning, operation and reliability record of the KSTAR HRS will be presented.

The cryogenic system for the KSTAR superconducting (SC) magnets has been commissioned. It consists of a cold box, distribution boxes (DB) and cryogenic transfer lines. The cold box and DB #1 provide 600 g/s of supercritical helium to cool the SC magnets, their SC bus-lines, and the magnet support structures. It also provides 17.5 g/s of liquid helium to the current leads and supplies cold helium flow to the thermal shields. The main duties of the DB #2 are the relative distribution of the cryogenic helium among the cooling channels of each KSTAR cold component and the emergency release of over-pressurized helium during abnormal events such as quenches of the SC magnets. After individual commissioning, the system was integrated and cooled down with the KSTAR device. In this paper, the construction and commissioning results of the KSTAR cryogenic system will be introduced. In addition, we will present the cool-down results of the KSTAR device.

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.

The KSTAR (Korea Superconducting Tokamak Advanced Research) superconducting magnet system adopts a superconducting CICC(Cable-In-Conduit Conductor) type conductor. It consists of 16 TF (Toroidal Field) coils and 14 PF (Poroidal Field) coils and it also uses two different types of CICCs-Nb3Sn cable with Incoloy 908 conduit and NbTi cable with 316LN stainless-steel conduit. A special CICC jacketing system is developed for the KSTAR CICC fabrication: the tube-mill process, which consists of forming, welding, sizing and squaring procedure. The cabling process for TF and PF superconducting cable and the fabrication process of each CICCs (TF CICC and PF CICC) is described. The welding of conduit materials are also discussed. The fabrication results such as the geometrical specification, micro structure and the void fraction will be discussed

The KSTAR superconducting magnet system consists of 16 TF (Toroidal Field) and 14 PF (Poloidal Field) coils. Both of the TF and PF coil systems use internally-cooled Cable-In-Conduit Conductors (CICC). The PF coil system, which consists of 8 coils in the CS (Central Solenoid) coil system and 6 outer PF coils, provides 17 V-s and sustains the plasma current of 2 MA for 20 seconds inductively. PF 1-5 coils use Nb/sub 3/Sn CICC in an Incoloy 908 conduit and PF 6-7 coils use NbTi CICC in a modified stainless steel 316LN(316LN+). The fabrication of PF7L/U & PF6L/U coils is completed at present. The engineering issues and fabrication process, which are related with the KSTAR PF 6-7 coil development, are presented and discussed in this paper. TF and PF coils are in the fabrication stage for KSTAR completion in the year 2006.