Cheon Seog Yoon

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.

Quench characteristics in the multi-sectioned superconducting magnet impregnated with epoxy resin

A two-dimensional model is proposed to analyze the thermal and electrical behavior of quench in a multi-sectioned superconducting magnet impregnated with epoxy resin. In the simulation, the effect of AC losses due to the field variation is included. The three-coil system and NMR superconducting magnet system are studied. The temporal variations of current, voltage and hot-spot temperature rise of magnet system are analyzed.

Numerical analysis of stability margin and quench behavior of cable-in-conduit NbTi conductors for KSTAR

A numerical model has been proposed to analyze the stability margin and quench characteristics of the cable-in-conduit NbTi conductors for the KSTAR-PF (Korea Superconducting Tokamak Advanced Research) magnets. The dependence of the thermal, hydraulic and electrical properties on the external thermal disturbance was investigated. The algorithm of the program is based on the finite volume method which adopts space discretization and time integration by multi-step Runge-Kutta method to obtain stable numerical solutions. It was confirmed that the disturbance duration can influence the conductor stability and limiting current.

Operating temperature margin and heat load in PF superconducting coils of KSTAR

The code SAITOKPF has been developed for the design and analysis of the poloidal field (PF) coil system of KSTAR (Korean Superconducting Tokamak Advanced Research) device. The plasma operation is studied by consideration of flux conservation. An equivalent circuit model is used to simulate the magnetic coupling among the plasma current, superconducting coils, the tokamak supporting structure and the cryostat. Due to the high changing rates in the operating currents of the PF coils, the magnetic coupling can generate high AC losses including the hysteresis loss, the eddy current loss, and the coupling loss. The helium is forced-flowed through the cable in conduit conductor (CICC) to remove the losses and to keep the temperature rise in the superconducting cable lower than its current sharing temperature. The thermal coupling between pancakes, layers, and cooling channels in PF coils is simulated by a quasithree-dimensional thermo-hydraulic model. In this paper, the physical model, the numerical method and the structure of the simulation code are introduced. A nominal operating scenario of the KSTAR device is used to simulate these phenomena.

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.

Numerical model for thermal hydraulic analysis in cable-in-conduit-conductors

The issue of quench is related to safety operation of large-scale superconducting magnet system fabricated by cable-in-conduit conductor. A numerical method is presented to simulate the thermal hydraulic quench characteristics in the superconducting Tokamak magnet system. One-dimensional fluid dynamic equations for supercritical helium and the equation of heat conduction for the conduit are used to describe the thermal hydraulic characteristics in the cable-in-conduit conductor, The high heat transfer approximation between supercritical helium and superconducting strands is taken into account due to strong heating induced flow of supercritical helium, 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 a new adaptive mesh is proposed for the moving boundary problem and the time term is discretized by the-implicit scheme, It remarkably reduces the CPU time by local linearization of coefficient and the compressible storage of the large sparse matrix of discretized equations. The discretized equations are solved by the IMSL The numerical implement is discussed in detail, The validation of this method is demonstrated by comparison of the numerical results with those of the SARUMAN and the QUENCHER and experimental measurements.

Moving Mesh Application for Thermal-Hydraulic Analysis in Cable-In-Conduit-Conductors of KSTAR Superconducting Magnet

In order to study the thermal-hydraulic behavior of the cable-in-conduit-conductor (CICC), a numerical model has been developed. In the model, the high heat transfer approximation between superconducting strands and supercritical helium is adopted. The strong coupling of heat transfer at the front of normal zone generates a contact discontinuity in temperature and density. In order to obtain the converged numerical solutions, a moving mesh method is used to capture the contact discontinuity in the short front region of the normal zone. The coupled equation is solved using the finite element method with the artificial viscosity term. Details of the numerical implementation are discussed and the validation of the code is performed for comparison of the results with thse of GANDALF and QSAIT.

Simulation on helium expansion and quench in CICC for KSTAR with moving mesh methods

A numerical code, MFEM1D, has been developed to study the helium expansion and quench propagation in CICC. Since the heat induced flow gives cause to a high heat transfer coefficient between supercritical helium and the strands, the temperature difference between the strands and helium is assumed to be very small. The strong coupling of heat transfer at the front of the normal zone generates a contact discontinuity in temperature and density. In order to obtain convergence of numerical solutions, a moving mesh method is used to capture the contact discontinuity in the short front region of the normal zone. The coupled equation is solved by the moving mesh finite element method with artificial viscosity. Details of the numerical implementation are discussed and the quench study for the KSTAR coils is performed.

Analysis on stability margin of Nb/sub 3/Sn Cable-in-Conduit-Conductor

The stability margin of Nb/sub 3/Sn Cable-in-Conduit Conductor (CICC) has been analyzed by a numerical model. The numerical code is based on the finite volume method to discretize the one-dimensional conservation equations on the staggered mesh. The algebraic method is used to transform the non-uniform mesh in the physical plane to the uniform mesh in the computational plane. The numerical simulation shows that the faster supercritical helium mass flow rate can improve the stability and increase the limiting current. The stability and limiting current are decreased with the increasing the operating temperature and background field. The shorter disturbance duration time allows the higher stability in the well-cooled region and the lower stability in the ill-cooled region.

Thermo-hydraulic analysis of the KSTAR central solenoid model coil

The Korea Superconducting Tokamak Advanced Research (KSTAR) Central Solenoid Model Coil (CSMC) has been developed to validate the design of KSATR CS coil. The thermo-hydraulic characteristics were analyzed for the KSTAR CSMC. The major thermo-hydraulic parameters of the coil are AC losses, strand temperature, coolant temperature, pressure drop and temperature margin. A numerical code has been developed for the thermo-hydaulic analysis of the KSTAR CSMC according to the operating conditions. In this paper, the description of the thermo-hydraulic analysis models and analysis results of CSMC are presented. The total energy deposition due to AC losses in CSMC winding will induce a flow reversal of supercritical helium. The minimum temperature margin is estimated approximately 0.5 K. The maximum temperature in CSMC winding is about 7.6 K.