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Development of tungsten brazing technology for the upgraded Korea superconducting tokamak advanced research divertor was launched in early 2013. ITER grade tungsten block was brazed on the CuCrZr alloy in vacuum at 980 °C for 30 min using silver free brazing alloy. An OFHC-copper was used as an interlayer between tungsten and the CuCuZr. The brazing alloy is a 0.05 mm thickness of plate of which component is a Ni–Cu–Mn. It is found that the optimal loading on mock-up is about 20 kPa. Ultrasonic test and shear strength test were carried out to check the strength between tungsten and substrate material. And the joint conditions were analyzed by scanning electron microscopy. Tungsten-brazed mock-ups with a cooling tube were tested at the electron beam facility, Korea heat load test facility-electron beam in Korea Atomic Energy Research Institute. The high heat flux test was performed for tungsten-brazed mock-ups under heat flux of about 5 MW/ \textm^2 with more than 2000 cycles. All the tungsten-brazed mock-ups met the requirements and there was no delamination or failure at the bonding joints. [ABSTRACT FROM PUBLISHER]

Tungsten (W) monoblock that consists of W according to ITER specifications, oxygen-free high conductive copper (Cu-OFHC), and CuCrZr has been designed in preparation for KSTAR upgrade for steady-state long-pulse operation with 27.5-MW heating power. Finite-element analysis has been performed by ANSYS Workbench. The hydraulic thermomechanical analysis is performed to investigate the optimal design parameters and the expected fatigue lifetime under the operating conditions with 5.4-MW/ \textm^2 incident heat flux at the divertor. For a single W monoblock, the optimization gives a W thickness of 5 mm, a side thickness of 2.5 mm, a thickness in the back part of 6 mm, and a Cu-OFHC interlayer thickness of 0.5 mm. For a mock-up consisting of five optimized W monoblocks, the thermal fatigue analysis is performed. As a result, the maximum value of equivalent total strain in the CuCrZr tube is found to be 0.266%, which corresponds to for a fatigue lifetime of 23 000 cycles. [ABSTRACT FROM AUTHOR]

Here we report our study on the lithium-doped ZnO films prepared by sol-gel method. The films are polycrystalline wurtzite structure and the average particle size is about 50 nm. All the films are ferromagnetic, and the film with Li content of 8 at. % has the largest saturation magnetization of about 55 memu/g. The photoluminescence spectra show that the films have weak emission near ultraviolet region and strong emission in visible regions. The Li atoms in Zn sites can induce Zn vacancies and a low acceptor level in the energy gap, which are required for ferromagnetism. The first-principles calculations demonstrate that the ferromagnetism in the Li-doped ZnO originates from the induced magnetic moment of oxygen atoms in the nearest neighbor sites to Li-Zn vacancy pair. [ABSTRACT FROM AUTHOR]

In- and outboard plasma facing components (PFCs) of Korea Superconducting Tokamak Advanced Research (KSTAR) have been fully installed in 2010 for D-shaped diverted plasmas. Before the start of plasma operation, the PFCs were baked up to 200 ^\circ\C by hot nitrogen gas circulation system to remove impurities including water. The surface temperature of the PFC tiles was monitored by 200 thermocouple sensors during the plasma operation [plasma shot or glow discharge cleaning (GDC)], and the temporal and spatial (poloidal) temperature profiles are obtained. Depending on the heat flux on each tile, the surface temperature shows time-dependent behavior. After 1-h morning He GDC, the temperature of the PFCs at the inboard side has reached to 40 ^\circ\C. After an H-mode shot, the temperature of divertor tiles around the striking points was substantially increased. The time-averaged total heat flux after a few of L- and H-modes in 2010 was estimated to be approximately 0.010 \MW/m^2. [ABSTRACT FROM PUBLISHER]

The central solenoid (CS) coil system of the KSTAR device consists of four pairs of coils with up-down symmetry and a set of support structure. The CS coil system should endure various loads including thermal and electromagnetic load during operation. One of the most important roles of support structure is to preserve the structural stability of coil system from vertical forces by applying the pre-compression on the coil stack. The main assembly procedure of the coil stack is as follows: 1) coil subassembly and stacking, 2) structure assembly, 3) preloading process by heating shells in the support structure, and 4) measurement on the final dimension. The special jigs and heating system for the preloading were designed to apply pre-compression on the coil stack during the assembly at room temperature. Moreover, a few tens of thermocouples and strain gauges were installed for measuring temperature and thermal expansion of preloading structures. Since the final assembly of the CS coil was successfully completed with about 7.3 MN of preloading in 2006, the CS coil system has been experienced four thermal cycles from 300 K to 5 K until 2011. This paper describes the main results of applying the pre-compression on the coil stack and mechanical behaviors of preloading structures during thermal cycles. [ABSTRACT FROM PUBLISHER]

The Cable-In-Conduit Conductor (CICC) for the KSTAR buslines is made of NbTi superconducting (SC) strands. A busline consists of several electrical joints, which are the major heat load contributors to the busline cryo-system. In the poloidal field (PF) busline helium circuit, the supercritical helium is fed to the electrical joint of current lead end and comes out to the magnet terminal joint. This helium flow configuration has been verified to maintain the cryogenic stability of the buslines through the KSTAR operation. During the normal operation of the KSTAR PF coil, the heated helium coming out to both the coil and the busline meets at the magnet terminal and exchange heat, but the busline outlet temperature still remained less than magnet outlet temperature. As the buslines for the electrical connection in series of the upper and lower coils for PF1 and PF2 have the helium path through the two terminal joints of magnet, they experience higher temperature than the other buslines mainly due to the larger heat exchange. In this case, the connection buslines are considered to have very low safety margin and have the strong possibility of quenches. [ABSTRACT FROM PUBLISHER]

The helium refrigeration system (HRS) of the Korea Superconducting Tokamak Advanced Research (KSTAR), which was designed to provide an energetic equivalent cooling power of 9 kW at 4.5 K, has been operated successfully during the last three years since 2008. For the operation of KSTAR, all of the cold components, such as the superconducting (SC) magnets, magnet structure, thermal shields, SC buslines, and current leads, were cooled down to the aimed cryogenic temperature by generated helium in the HRS. The supercritical helium (4.5 K and 5.5 bar) of 300 g/s was delivered for cooling of the SC magnet (poloidal field (PF) magnet: 14; toroidal field (TF) magnet: 16) and magnet structure and was circulated by the PF and the TF circulators. Thermal loads due to magnet operation are removed at the thermal damper installed in the distribution box of the HRS. Moreover, the pressure change in the magnet will directly affect the circulators. In particular, the pressure drop of the PF circulator drastically changed due to the PF magnet operation depending on the plasma generation scenario. In the 2010 campaign, the operating range of the PF magnet expanded from \pm4 to \pm10 kA, and the plasma scenario was improved. Accordingly, the mass flow rate of the PF circuit was increased from 350 to 369 g/s in order to achieve cryogenic stability. However, the operating range of the PF circulator exceeded the allowable range during shot #2526 and shot #3878. Nevertheless, the circulator successfully operated without any significant fault. The operating plan of the PF magnet that will be further expanded in the operating range may cause damages to the circulator continuously. Consequently, the influence of PF operating scenarios on the PF circulator was analyzed to investigate the safety margin of the HRS. [ABSTRACT FROM PUBLISHER]

The electric-field control of magnetization has many potential applications in magnetic memory storage, sensors, and spintronics. A new multiferroic spin-valve (SV) heterostructure was grown by combining a multiferroic \ BiFeO3 (BFO) and a \ Co90{\ {Fe}}10/{\ {Cu/Co}}90{\ {Fe}}10 spin valve on \ Pt/Ti/SiO2/Si substrate. Through the magnetoelectric coupling between a BFO layer and \ Co90{\ {Fe}}10 layer, the magnetoresistance of SV could be controlled by electric field applied on the BFO layer. The BFO film exhibits an obvious ferroelectric property with a remnant polarization of 17 \mu\ C/cm^2 and a coercive field of 400 kV/cm. The exchange bias of 6 Oe and magnetoresistance (MR) of 1.04% were observed in the heterostructure at room temperature. Moreover, an electric-field-induced change of MR was demonstrated when we employed 0.5- and -0.5-V voltage between top and bottom electrode layer. Furthermore, the values of MR of the spin were changed into 0.71% and 0.95%, respectively. This realization of the electric-field-induced change of MR indicates the great potential applications in future functional devices. [ABSTRACT FROM AUTHOR]

Superconducting magnets of the Korea Superconducting Tokamak Advanced Research (KSTAR) are cooled by supercritical helium with 4.5 K, which was supplied and recovered by the 9 kW of the Helium Refrigerator System (HRS). While current is being charged, the supercritical helium expands to both side of the helium inlet and the outlet of the magnets due to the generated AC losses. To maintain the pressure gradient, both the supply and the return pressures of the HRS are increased at the same time and the differential pressure of the HRS was reduced after the event. However, the pressure rising in the magnets may block the helium flow or create reversal flow of the helium. During unipolar experiment of PF1 magnet up to 2 kA with 1 kA/s of ramp-up rate, the mass flow rate was decreased at the PF1 cooling tube (manifold) in the helium distribution system (HDS), whereas the pressure is increased and the temperature is to be increased or decreased according to compression and expansion of the heated helium in the magnets. For the bipolar experiment of PF1 up to \pm2 kA with 1 kA/s ramp-up rate and 2 kA/s ramp-down rate, the conditions in the helium flow were drastically changed, especially the mass flow rate was measured to be maintained at zero for a few second (more than 4 s). This behavior could decisively affect the cryogenic stabilities in the magnet, and may impose a major limit on the long pulse operation of KSTAR. In this paper, we investigated this behavior and analysed by using 1-dimentional thermo-hydraulic code, GANDALF. [ABSTRACT FROM AUTHOR]

The AC loss in a large superconducting magnet coil shows a tendency to be changed after assembly. For the ac loss measurement of the Korea Superconducting Tokamak Advanced Research (KSTAR) superconducting coils after assembly, several current scenarios, trapezoidal pulses and a DC offset sinusoidal pulses, were applied to the PF1 Upper (U) and Lower (L) coils during the commissioning. The measurement was done once before and once after the plasma discharge experiments. All coil data were obtained by the tokamak monitoring system and helium distribution system which were designed to measure temperature, pressure, and mass-flow at both inlets and outlets of the coils. The PF1 coils of the cable-in-conduit conductor type were made of Nb3Sn superconducting strands, whose winding scheme is 20 layers with each layer having 9 turns. Each has 10 cooling channels for the heat removal by the supercritical helium at 4.5 K. For the trapezoidal pulse tests, the current was increased up to 4 kA with several different ramp rates and a 2 kA DC offset and 0.5 kA sine waves with different frequencies from 0.1 Hz to 0.3 Hz were applied to the coil. According to the analyses, the AC loss was slightly decreased for the same condition after 700 plasma shots. It was believe that such a result was due to reduced inter-strand resistances which changed the transverse resistance between the inter-strands. The coupling time constant was estimated to be 32 ms in the trapezoidal tests and 13.6 ms for the DC offset sinusoidal pulses. The former is larger than the latter because of the effect of the jacket eddy current loss due to Incoloy 908 which is a ferromagnetic material. Considering the jacket eddy current losses, the coupling time constant was recalculated and the value estimated to be about 13 ms for all current wave forms during first commissioning. [ABSTRACT FROM AUTHOR]

The KSTAR current lead system (CLS) was commissioned during the first campaign of the plasma experiments. The current lead system is divided into a toroidal field (TF) system and a poloidal field (PF) system. Two pairs of current leads for the TF system and 7 pairs for the PF system were installed. Instead of the copper leads, vapor-cooled type brass current leads (CLs) were mounted in order to minimize the cryogenic load. The CLs were installed after the verification of the components through helium leak tests, low current charging tests at room temperature, and high voltage tests. The stability of the CLS was inspected through LN2 cool down tests, helium leak tests, flow control tests, and temperature control tests using a ceramic heater after the installation all of their components. During the first campaign, it was cool down until liquid helium temperature after vacuum pumping and the helium flow rates of the CLs were controlled successfully at the optimum values. In case of the TF CLs were 0.20 g/s and 0.26 g/s at zero current and 15 kA respectively. On the other hands the PF CLs kept 0.2 g/s during PF coil operation at 4 kA and zero current. And a ceramic heater was reliably operated to prevent water condensation at the warm terminal of the CL. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The function of the current feeder system (CFS) is for conducting large currents from the power supplies to the KSTAR superconducting (SC) magnets. The CFS consists of SC bus-lines, joints, and current leads. The bus-line conductor is a circular cable-in-conduit conductor (CICC), which consists of a 4.5 mm thick stainless steel 316L seamless pipe containing 324 strands of chrome coated NbTi superconductor and 243 strands of OFHC. The ends of the CICC are assembled with specially designed lap joints. The joining resistance is controlled to less than 2.5 nano-ohm to minimize Joule heating. The outer surfaces of the CICC were electrically insulated up to 15 kV with jackets made of Kapton film and prepreged E-glass tape. Helically wrapped conducting fiber was used to measure the voltages of bus-line quenches. Two pairs of prototype brass leads for poloidal field (PF) and toroidal field (TF) coils have been fabricated and tested up to the currents of 26 kA for the PF leads and 35 kA for the TF leads. The test results satisfied all the requirements so that all 18 leads were manufactured and assembled on site. This paper will describe the detailed manufacturing progress and commissioning results of the KSTAR CFS. [ABSTRACT FROM AUTHOR]

The Korea superconducting tokamak advanced research (KSTAR), which is under construction at the National Fusion R&D Center, Korea Basic Science Institute, Daejeon, Korea, has the mission to develop a steady-state capable advanced superconducting tokamak to establish the scientific and technological bases for a fusion reactor. After an intensive R&D program, substantial progress of the KSTAR tokamak engineering had been made on major tokamak structures, superconducting magnets, in-vessel components, diagnostic system, heating system, and power supplies with industrial manufacturers by May 2002. The engineering design has been elaborated to the extent necessary to allow a realistic assessment of its feasibility, performance, and cost. Since May 2003, the project has been in the phase of procurement. The fabrication of main tokamak structure such as vacuum vessel, cryostat, and supporting structures is well progressed. The manufacturing work of superconducting coils is also proceeding favorably. The tokaniak assembly started in July 2003 after site preparation and assembly jig. The start of commissioning is scheduled for June 2006. This paper describes the key features and engineering progress of the KSTAR tokamak and elaborates the work currently underway. [ABSTRACT FROM AUTHOR]