Hyun-ki Park

The background magnets of the Samsung Superconductor Test Facility (SSTF)

The background magnet system of SSTF (Samsung Superconductor Test Facility) for KSTAR (Korea Superconducting Tokamak Advanced Research) is now under design. The main coil (MC) is split solenoids and the gap can be changed from 0 to 750 mm. The ID of MC is 750 mm. It will be wound using a CICC (cable-in-conduit conductor) designed for the central solenoid of KSTAR. The central field is 8 T at 22.5 kA when the gap is 250 mm. The ramp rate of MC is 3 T/s. A pair of blip coils will simulate (during the discharge) 1 T amplitude and 20 T/s rate electromagnetic disturbances expected from the KSTAR operation. To compensate the inductive interaction between MC and blip coils during the discharge of the blip coils, a pair of cancellation coils is foreseen. Both blip and cancellation coils (BCC) are fed in series and generate 1 T central field at 7 kA and 250 mm gap. The BCC are wound with CICC and cooled internally and externally.

Heating surge and temperature oscillation in KSTAR PF and TF coils for plasma disruption under continuous plasma discharging conditions

The operating characteristics in the poloidal field (PF) and toroidal field (TF) coils of KSTAR (Korean Superconducting Tokamak Advanced Research) for given operating scenarios are analyzed. In order to control the plasma shaped in KSTAR to realize the thermal nuclear fusion reaction, the operating currents in PF coils are controlled with high ramp rate with respect to time. The induced currents in the support structure and cryostat generate high eddy losses. They also produce a large hysteresis, eddy and coupling losses in superconducting PF and TF coils. The supercritical helium with high velocity through the cable-in-conduit-conductor (CICC) removes the heat load to keep the temperature of superconducting cable lower than its current sharing temperature. The maximum temperature rises in PF and TF are calculated under the continuous operating scenario. The simulation shows that the maximum temperatures in TF and PF are about 5.7 K and 5.9 K, respectively.

The superconducting transformer of the Samsung Superconductor Test Facility (SSTF)

In the frames of designing the SSTF (Samsung Superconductor Test Facility) for the KSTAR (Korea Superconducting Tokamak Advanced Research), the 50 kA transformer charging a CICC (cable-in-conduit conductor) short sample for one second is now under design. The primary winding conductor consists of six NbTi and six stainless steel strands tabled around a low RRR rectangular copper core, which was used by Kurchatov Institute in small SMES (superconducting magnetic energy storage) windings. The secondary winding consists of 24 subcables wrapped around and soldered to a low RRR copper strip. Each subcable consists of six NbTi strands cabled around a copper strand. The strands for primary and secondary windings are 0.85 mm diameter NbTi wires with six micrometer 8910 filaments. Both primary and secondary conductors have large current and temperature margins to ensure a reliable operation of the superconducting transformer. The primary coil is placed in a cylindrical LHe vessel. The four secondary turns are glued to the outer surface of the LHe vessel. The joints between the transformer and the sample are described.

AC losses and heat removal in three-dimensional winding pack of Samsung superconducting test facility under pulsed magnetic field operation ☆

The Samsung superconducting test facility (SSTF) will be operated under the highly pulsed field to simulate the operating conditions of KSTAR. An analysis has been performed to study the transient heat removal characteristics and temperature margin for the main, blip and compensating coils in the SSTF. This method is based on a quasi-three-dimensional model, which the thermal coupling of turn-to-turn, pancake-to-pancake and channel-to-channel is taken into account, to simulate the conductor temperature rise and the thermal expansion of supercritical helium due to the high AC losses under the pulsed field. The local AC losses, which include coupling loss, eddy current loss and hysteresis loss in the cable-in-conduit conductor, are estimated. The temperature margin, mass flow rate, distribution of AC losses are studied under the given operating scenario. The mass flow reduction and peak temperature rise depending on the inlet pressure and inlet position of CICC are studied. It is shown that the initial mass flow rate remarkably influences on the peak temperature of superconducting strands. The large mass flow rate can reduce the temperature rise when the inlet of helium is located at the high field region. By contrast, because of heat induced flow to improve the cooling condition of the superconducting strands, the small initial mass flow rate results in the low peak temperature in strands when the inlet of helium is located at the low field region.

Mechanical and Thermal Characteristics of Insulation Materials for the KSTAR Magnet System at Cryogenic Temperature

The KSTAR(Korea Superconducting Tokamak Advanced Research) superconducting magnet is electrically insulated by the composite material of epoxy resin and glass fiber (2.5 kV/mm) and Kapton (8 kV/mm). The insulation composite material of epoxy resin and glass fiber is prepared using a VPI (Vacuum Pressure Impregnation) process. The superconducting magnet is under mechanical stress caused by the large temperature difference between the operation temperature of the magnet and room temperature. The large electro‐magnetic force during the operation of the magnet is also exerted on the magnet. Therefore, the characteristics of the insulation material at cryogenic temperatures are very important and the tensile stress and thermal expansion coefficient for the insulation materials of the KSTAR superconducting magnet are measured. This paper presents results on mechanical properties of the insulation material for KSTAR magnets, such as density, ultimate tensile stress and thermal contraction between room temperatur...

Operating characteristics of the KSTAR superconducting TF coil

The Korea Superconducting Tokamak Advanced Research (KSTAR) device, a steady-state-capable advanced Tokamak, is to be built in Korea. The KSTAR device is made up of 16 toroidal field (TF) and 14 poloidal field (PF) superconducting coils. A simulation code for the transient operation of the KSTAR TF and PF coils is developed. For stable Tokamak operation, the operating characteristics of the KSTAR superconducting magnet system are studied for operation scenario. We calculated the temperature margin of superconducting cables, cryogenic flow parameters and the heat deposition in TF and PF coils. The effect of the 3D heat conduction and the eddy current loss in the supporting structure and vacuum chamber of KSTAR is also included.

Influence of inlet pressure and mass flow rate on the temperature rise of superconducting strands in SSTF under the normal operating conditions [of KSTAR]

In order to estimate the operating characteristics of the main coils, blip and compensating coils of SSTF (Samsung superconducting test facility) which will be operated under the pulsed field to simulate the operating scenarios of KSTAR (Korean Superconducting Tokamak Advanced Research), an analysis has been performed to study the influence of cryogenic parameters, such as inlet temperature, pressure and mass flow rate on the temperature margin for the main, blip and compensating coils. The temperature margin is studied under the given operating scenarios. The reduction of mass flow rate and peak temperature rise, which are strongly depending on the inlet pressure and inlet position of supercritical helium in the CICC are studied. It is noticed that the initial mass flow rate remarkably influences on the peak temperature of superconducting strands. The large mass flow rate can reduce the temperature rise when the helium inlet is installed at the high field region. On the other hand, the small initial mass flow rate results in the low peak temperature in strands when the helium inlet is located at the low field region, since the heat induced flow occurs to improve the cooling condition of the superconducting strands.

The conductors of the 50 kA superconducting transformer for SSTF

The 50 kA transformer for Samsung Superconductor Test Facility (SSTF), which will charge the Korean Superconducting Tokamak Advanced Research cable-in-conduit conductor samples for 1 s is under design. The NbTi based conductors for primary and secondary windings are described. The primary winding conductor consists of six NbTi and six stainless steel strands cabled around rectangular copper core. Such a design was previously used by Kurchatov Institute in small SMES windings. The secondary winding conductor consists of 24 subcables wrapped around and soldered to a copper strip. Each subcable consists of six NbTi strands cabled around one copper strand. NbTi strands for both primary and secondary windings are 0.85 mm in diameter. NbTi wires have 8910 6-μm filaments. Both primary and secondary winding conductors have large current and temperature margins to ensure a reliable operation of the superconducting transformer.

Three-dimensional microstructures fabricated by multi-step electrochemical etching of aluminum sheet

We present a fabrication process for three-dimensional (3D) microstructures using multi-step electrochemical etching of metal foils. The present process offers advantages of simple, cose-effective, and fast process with uniform and well-controlled material property. In the experimental study, we performed single-step electrochemical etching of 2D cantilever array and multi-step electrochemical etching of 3D micro probe array. The average depth etch rate and the average bias etch rate were measured as 1.50±0.10µm/min, and 0.77±0.03 µm wide structure. The surface roughness of 11.34nm has been measured. The demonstrated multi-step electrochemical etching process results in a simple, cost-effective, and timesaving fabrication of 3D metallic structures.

A study on the forming of CICC for the superconducting Tokamak device with post-forming predictions via virtual manufacturing

To construct the magnet assembled with the CICC (Cable-In-Conduit Conductor) for the superconducting Tokamak fusion device, the 3-roll bending, that inherently has a difficulty to form the coil with accurate radius of curvature, is used for continuous winding. This difficulty is mainly caused by the spring back after forming. In order to obtain precise dimension, a trial-and-error operation is inevitable. To reduce the effort of tryout, a relation between travel of the bending roller and post-forming displacement was obtained via virtual manufacturing. The radius of CICC after forming was expressed as a function of the bend-roll travel. In addition, the variation of the CICC cross-section (reduction of the conduit cross-section) was investigated during the first turn and during conduit bending with largest curvature to check if the strand can have enough space. To ensure the validity of the computation, prototype coils were manufactured and the coil radii after forming were measured. The data showed similar pattern with some discrepancy for large-sized coils. The residual stress generated by jacketing on the coil before roll bending was measured to investigate if its existence influences the final deformation behavior. A mapping function was proposed to compensate the error caused by the numerical assumptions.