S. Baang

Fabrication of cables for the background-field magnet system of SSTF

The Korea Superconducting Tokamak Advanced Research (KSTAR) device has a fully superconducting magnet system consisting of 16 Toroidal Field (TF) and 14 Poloidal Field (PF) coils. For the test of the KSTAR cable-in-conduit conductor (CICC), an ambient magnetic field of /spl plusmn/8 T with a maximum change rate of 20 T/s is required and a background-field magnet system is being developed for the Samsung Superconductor Test Facility (SSTF). The CICC for PF1-5 is used as the conductor for background-field coils to check the validity of the PF CICC design. Two pieces of cables have been fabricated and the cable has the length of 870 m and the diameter of 20.3 mm. Pitches are 40, 80, 145, and 237 mm at each cabling stage. The void fraction of the CICC is expected to be more than 36%.

The test facility for the KSTAR superconducting magnets at SAIT

SSTF (Samsung Superconductor Test Facility) has been built with the primary goal of testing the KSTAR TF (Toroidal Field) and PF (Poloidal Field) magnets as well as CICC (Cable-in-Conduit Conductor) and superconducting strands in the most relevant manner. The facility is located at SAIT (Samsung Advanced Institute of Technology) near the KSTAR project home site. Two helium liquefiers of 120 liter/hr capacity have been utilized as refrigerators demonstrating simultaneous double mode operation of refrigeration and liquefaction. A forced flow supercritical helium cooling circuit allows the test facility to be operated at temperatures down to 4.5 K. Other major SSTF components are a large vacuum vessel (6 m diameter and 7.3 m height) with liquid nitrogen temperature shield, data acquisition and control system with EPICS (Experimental Physics and Industrial Control System), current leads, and 50 kA modular power supply with fast dump quench protection circuitry. SSTF has been used for the first test-phase of KSTAR CICC sample. The current status of SSTF as the KSTAR magnet test facility for components and qualification test is presented in detail.

The magnetic properties of the ferromagnetic materials used for HTS transformers at 77 K

The properties of the magnetic materials, such as the amorphous alloys and the grain-oriented silicon steels, are essential to the design of high-temperature superconducting transformers. In this paper the magnetic properties and the loss characteristics of different ferromagnetic materials are measured at liquid nitrogen temperature. It is shown from experimental results that at 77 K the saturation flux density of the amorphous metal is about 0.3 T lower than that of the silicon steel, and the loss of amorphous metal is 4 times less than that of the silicon steel. Since the losses of materials at 77 K are higher than those of at room temperature, it is not economical to immerse the transformer core in liquid nitrogen.

Magnetic field analysis of HTS transformer windings with high currents

The leakage magnetic field in HTS transformer windings fabricated by Bi-2223 tapes decreases the critical current and increases the ac losses in the tapes. Because of the anisotropy of the HTS tape, the numerical analysis of the radial component of the leakage field is especially important. In this paper the influence of the core structure and the winding configuration on the stray field is studied by finite element method, and some suggestions for improving the leakage field distribution are presented to make HTS transformers more efficient.

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.

The results of performance tests of superconducting wires for KSTAR magnets

The Korean Superconducting Tokamak Advanced Research device consists of 16 TF coils and 14 PF coils. All TF and PF coils are superconducting. Among them, 16 TF coils and 10 PF coils are made from Nb3Sn and the other 4 coils (PF 6 and 7, up/down) are made from NbTi. It is estimated that total 27 tons of Nb3Sn and 11 tons of NbTi are required for the fabrication of KSTAR magnets. The superconducting 8 mm rods are supplied by Mitsubishi Electric Corporation (MELCO) and Intermagnetics General Corporation (IGC) and processed to the final chromium plated 0.78 mm diameter strands by Nexans Korea and KISWIRE. Currently, about 6 tons of Nb3Sn strands have been produced, and the performance tests for them have been carried out. The specimens for the strand performance tests are taken from the both ends of the strand lot. In this paper, we present the latest test results of the current carrying capacity of non-copper volume, hysteresis loss of non-copper volume and residual resistivity ratio using statistical analysis.

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.

Simulation of dynamic stress in PF superconducting magnets for KSTAR under normal operating conditions

The PF (polodial field) superconducting magnets for the KSTAR (Korea Superconducting Tokamak Advanced Reactor) are operated in the high rates of change in the operating currents. Under the normal discharging condition of plasma, the current in superconducting magnets can generate high electromagnetic stress in CICC (cable-in-conduit-conductor) and its support structure. In order to simulate the peak value stress in PF superconducting magnet so that the support structure can be designed to bear the stress, ANSYS is used to study the variation of stress and strain distribution with time. In the paper, the detailed finite element model and numerical results are presented.

Thermohydraulic simulation on CIC conductor with adaptive mesh finite volume method for KSTAR tokamak superconducting magnet

To study the quench in the CICC, the numerical analysis code was developed. The fully implicit time integration of upwind scheme for finite volume method is utilized to discretize the equations on the staggered mesh. The scheme of adaptive mesh is proposed for the moving boundary problem and the time term is discretized by the /spl theta/-implicit scheme. The discretized equations are solved by the IMSL. The error analysis of this method is performed by various step-sizes of time and space. The thermal hydraulic behavior of the CICC used in KSTAR is studied.