Conductor and joint test results of JT-60SA CS and EF coils using the NIFS test facility

In 2007, JAEA and NIFS launched the test project to evaluate the performance of cable-in-conduit (CIC) conductors and conductor joints for the JT-60SA CS and EF coils. In this project, conductor tests for four types of coil conductor and joint tests for seven types of conductor joint have been conducted for the past eight years using the NIFS test facility. As a result, the test project indicated that the CIC conductors and conductor joints fulfill the design requirement for the CS and EF coils. In addition, the NIFS test facility is expected to be utilized as the test facility for the development of a conductor and conductor joint for the purpose of the DEMO nuclear fusion power plant, provided that the required magnetic field strength is within 9 T

振動式ヒートパイプによる核融合用超伝導マグネットの高性能化研究

  The superconducting magnets for fusion experimental devices are used in the condition of high magnetic field, high electromagnetic force, and high heat load. The pool boiling liquid helium cooling outside of the conductor or the forced flow of supercritical helium cooling inside of the conductor, such as the cable-in-conduit conductor, are used so far for the cooling method of the superconducting magnet for the present fusion devices. The pool cooling magnet has the disadvantages of low mechanical rigidities and low withstanding voltages of coil windings. The forced flow cooling magnet with cable-in-conduit conductors has the disadvantages of the restriction of the coil design because of the path of the electric current must be the same as that of the cooling channel for refrigerant. The path of the electrical current and that of the cooling channel for refrigerant can be independently designed by adopting an indirect cooling method that inserts the independent cooling panel in the coil windings and cools the conductor from outside. In this study, this indirect-cooling method is adopted as a promising candidate for fusion magnets.  Improvement of superconducting magnets for high magnetic field and high heat load is an important subject to achieve the early realization of nuclear fusion power generation. Application of high-temperature superconductors (HTS) to magnets has widely been studied, ever since the discovery of the high temperature superconductivity in late 1980s. HTS magnets may achieve higher magnetic fields with less operation cost and higher stability against a coil quench compared to low-temperature superconducting magnets. In HTS magnets, the stability of winding conductors is assured by the rise of operating temperatures. However, it is difficult to remove the local heat generated in an HTS magnet because the thermal diffusivity of each component material used in the magnet, such as copper, aluminum alloy, epoxy resin, GFRP, etc., decreases as the operating temperature increases. When a part of the windings turns into the normal-conducting state, therefore, large temperature gradients are easily produced in magnets, which could cause degradation of superconducting properties and mechanical damages by thermal stresses. In other words, the protection of magnets becomes more difficult than the case for low temperature superconducting magnets.  We propose a new method of including cryogenic oscillating heat pipes (OHPs), which is also called pulsating heat pipes (PHPs), in the HTS magnet windings as a heat transfer device. The OHP is a highly effective two-phase heat transfer device which can transport several orders of magnitude greater heat flux than the heat conduction of solid metals and be formed in a thin plate structure. In these respects, we consider that the OHPs imbedded in the coil windings can enhance the heat removal characteristics in HTS magnets.  The OHP is a wickless and typically exists as a serpentine-arranged tube where the tube forms a closed or open loop. The OHPs in the form as they are being investigated today have been first proposed and patented in 1990 by H. Akachi. The OHP is partially filled with a working fluid and the inner diameter of the tube is made sufficiently small in order to induce surface tension allowing formation of liquid slugs and vapor bubbles. Successful OHP operation occurs by an oscillatory pressure field and via the constant phase change of the internal working fluid. A pressure change induces a pseudo-chaotic displacement and circulation of the internal working fluid. The performance of an OHP is known to depend on thermo-physical properties of working fluid, filling ratio (the total internal liquid volume divided by the total internal channel volume), channel geometry (i.e. hydraulic diameter and length), number of turns, operating orientation and length of heating and cooling areas.  A modest number of studies about OHPs have been performed at room temperature and some types of OHP for electronics products have been already used as a high performance heat transfer device. The result of the fundamental experiment that uses the cryogenic loop heat pipes for the cooling between superconducting magnet and cryocoolers was reported in few literatures. However, there is a limitation in the orientation of the installation of the looped heat pipe and its shape is not applicable to the usage imbedded in magnets as a cooling panel. In this study, firstly proof-of-principle experiments of cryogenic OHP has been conducted using nitrogen, neon, and hydrogen as working fluids of OHP.  Prototype cryogenic OHPs to be used by being imbedded in superconducting magnets have been designed and manufactured. A stainless-steel pipe of 1.59 mm(1/16 inch) in outer diameter and 0.79 mm in inner diameter is bent 10 times at both ends with the straight sections of 160 mm in length. Two Cu blocks of 8 mm in thickness and 30 mm in length having grooves according to the pipe positions and are soldered with the pipes. A experimental apparatus for cryogenic OHP testing has been prepared, which consists of a cryostat, a GM cryocooler, a vacuum pump, gas cylinders (for nitrogen, neon and hydrogen), etc. The testing OHPs are placed in a vacuum chamber in the cryostat enclosed by the 60–80 K radiation shields and the working fluid is vacuum-encapsulated into the OHP and the OHPs are isolated by closing a valve on an inlet pipe. One of the two Cu blocks of OHP which works as a condenser is connected to the cold head of the cryocooler and the other Cu block which works as an evaporator attached with a foil heater. In the experiment procedure, the temperature of the condenser is maintained at a prescribed point which is below the condensation temperature, and the temperature of the evaporator is raised by the heater. The heat transport characteristics of the OHP have been measured by the temperature difference between the heating part (evaporator) and the cooling part (condenser) of the OHP. The effective thermal conductivity of portions of the fluid path in the pipes is calculated by each experimental data.  In the proof-of-principle experiments of cryogenic OHPs, the measured effective thermal conductivities have been measured to be 500–3500 Wm-1K-1 for H2, 1000–8000 Wm-1K-1 for Ne and 5000–18,000 Wm-1K-1 for N2 at the operating temperature ranges of 17–25 K, 26–32 K, and 67–80 K, respectively. These effective thermal conductivities are all larger than those of high-purity metals which are used as components of the conduction at low temperature engineering. It is,consequently, suggested that cryogenic OHPs can be applied to cooling of superconducting magnets. As a reference, the thermal conductivity of Cu with RRR (Residual Resistivity Ratios) = 100 at the magnetic field of 1 T and 20 K is about 2000 Wm-1K-1.  Having been encouraged by this successful experimental results, the performance characteristics of OHPs have been intensively examined, furthermore. The additional experimental parameters are the liquid filling ratio, pipe diameter, inclination angle and length of the heat transfer of the OHP. Here in this abstract, the two experiments on the effect of pipe diameter and inclination angle are introduced. The effect due to the inner diameter of the OHP has been examined by changing the outer diameter from 1.59 mm to 3.18 mm and the inner diameter from 0.79 mm to 1.59 mm. The effective thermal conductivities of this OHP have reached to 11,000 Wm-1K-1 for H2 and 19,000 Wm-1K-1 for Ne. The measured effective thermal conductivities of this OHP have been two times larger than those of the proof-of-principle experiments OHP for both H2 and Ne.  In order to effectively cool HTS magnets, it is required that cryogenic OHPs can operate in a variety of installation orientations. In this respect, the operating characteristics of the OHPs have been examined by changing the inclination angle a. The installation orientation is set at the following four angles: horizontal (a = 0), vertical with the evaporator located at the bottom (a = +90 degrees), diagonal with the evaporator at the bottom (a = +45 degrees) , vertical with the evaporator at the top (a = -90 degrees)and diagonal with the evaporator at the top (a = -45 degrees) . For the orientations with the evaporator located at the bottom (a = +90 and +45 degrees) and for the horizontal orientation (a = 0), the OHP has operated stably with an effective thermal conductivity observed at 2,000–11,500 Wm-1K-1 for H2 and 5,100–19,500 Wm-1K-1 for Ne. For the orientations with the evaporator located at the top (a = -45 and -90 degrees), however, the OHP has not worked stably. There have been many reports that OHPs can work also with these orientations at room temperature. Further optimization is necessary in order to operate cryogenic OHPs in various configurations, especially the turn number and the inner diameter of pipes. In order to mitigate the problem associated with the installation orientation, we propose a modified-type of OHP, with both ends cooled (condenser) and the center heated (evaporator), and stable operations have been confirmed experimentally. However, the measured effective thermal conductivity was found to be been smaller than that observed in the conventional type OHP. We consider that the effective thermal conductivity can be further improved by incorporating an optimized configuration for the OHP structure.  It is generally convenient to analyze the thermo-hydrodynamic properties for heat transfer using dimensionless quantities. In this work, it has been also attempted to get a comprehensive understanding of cryogenic OHPs by the semi-empirical correlations, which are based on values of thermo-hydrodynamic dimensionless numbers of the internal fluid. Using the model, of which the heat flux is expressed by the Karman number Ka, the Prandtl number Pr , the Jacob number Ja and inclination angle, a correlation has been formulated for the heat flux in OHPs, which is stated as follows: 97.077.005.005.0Pr))(exp(61.2−=JaKaaq. A total of 59 experimental data sets is used to make a fitting by means of multi-regression analysis. It is considered that this modeling with non-dimensional quantities is useful for the design of cryogenic OHPs.  The cryogenic OHPs used by being imbedded in superconducting magnets as a heat transfer device has been demonstrated for the first time in the world, and high heat transport properties of the cryogenic OHPs have been experimentally confirmed. A modified-type OHP, with both ends cooled and the center heated, has been proposed to reduce the negative effect of installation orientation and it has been tested successfully. We consider that it is possible to dramatically improve the performance of HTS magnets by using cryogenic OHPs

Performance tests of JT-60SA Helium Refrigerator System

JT-60SA is a fully superconducting tokamak, which uses magnetic field to confine hot plasma in the shape of a torus. JT-60SA magnet system is composed of 18 Toroidal Field coils, 4 Central Solenoid modules and 6 Equilibrium Field coils. These coils need to be cooled by forced flow supercritical pressure helium at 4.5 K. The thermal shields surrounding the coils have to be provided with pressurized helium at 80 K. High temperature superconducting current leads require helium gas at 50 K. Cryopump panels in the plasma vacuum vessels need to be kept at 3.7 K for hydrogen and helium adsorption. The equivalent refrigeration capacity of the helium refrigerator system is about 9 kW at 4.5 K. The nominal compressor flow of 672 g/s is compressed from 0.1 MPa to 1.45 MPa by 8 screw compressors running in parallel. Liquid Nitrogen is used for pre-cooling from 300 K to 80 K. Two turbines in series are operated at about 10 K. Part of flow is expanded in third turbine to supply supercritical pressure helium at 5 K. In order to manage the 12 kW heat pulse from the coils by plasma operations, 7000 liter liquid helium bath is equipped as a thermal damper. The manufacture and installation of the helium refrigerator system were conducted in 2015. The commissioning was finished in October 2016. Performance tests after the commissioning were conducted annually, in order to debug the system and train operators before the first plasma campaign of JT-60SA. During these performance tests, automatic cool-down and warm-up, automatic purification, the total cooling capacity, and stable operation in several operation modes were demonstrated. In this presentation, we represents the summarized result of these performance tests.10th Asian Conference on Applied Superconductivity and Cryogenics (ACASC), 2nd International Cryogenic Materials Conference in Asia (Asian-ICMC), and the CSSJ meetin

Design of JT-60SA Cryodistribution components

JT-60SA is a fusion experiment tokamak device using superconducting magnets to be built in Japan. This joint international project involves Japan and Europe. In this work, we presents the design of cryodistribution and its components which are composed of a main transfer line (TL) and valve boxes (VB).Five coolant loops are distributed between a helium refrigerator system (HRS) and cold components. Super critical pressure helium (SHe) of 4.5 K and 0.5 MPa supplied to 18 toroidal field coils, 6 equilibrium field coils and 4 central solenoid modules (LOOP1 & 2). SHe of 3.7 K and 0.5 MPa is supplied to divertor cryopumps (LOOP3). Gaseous helium (GHe) of 80 K and 1.4 MPa is supplied to radiation thermal shields (LOOP4). GHe of 50 K and 0.4 MPa is supplied to cold ends of high temperature superconducting current leads (LOOP5).TL is a vacuum heat-insulation multiple piping, of which the length is about 45 m, and connects between HRS and the tokamak cryostat. All 5 supply lines, 4 return lines and 2 control valves are installed in TL. The outer vacuum pipe diameter is 965.5 mm and the inner coolant pipe diameter are 108.3 mm for LOOP 1/2/4 and 59.0 mm for LOOP 3/5. A vacuum partition between HRS and the tokamak cryostat is located near the middle of TL in a longitudinal direction.VB contains cryogenic valves and measurement devices to control the cold helium flow. Eleven VBs are installed around the tokamak cryostat. Dimensions of VB body are 2 m in height and 1.4 m in diameter. Almost all cold helium lines from HRS are firstly into VBs through TL. Impulse lines, orifice plates, and resistor elements are installed at the pipes in VB for measurement of the pressure, the flow rate, and the temperature of coolant helium.第26回国際磁石技術会議（MT26 International Conference on Magnet Technology

The quench recovery analysis of the JT-60SA superconducting magnets

JT-60SA is one of the experimental nuclear fusion reactors with superconducting magnets. It is a joint international research and development project involving Japan and Europe. The temperature distribution changes in recovery is investigated.The quench recovery period is necessary to be confirmed. Generally, the maximum temperature drop of magnets is able to be confirmed by checking the thermometer attached to the outlet of the helium flow path. However, the maximum temperature of the JT-60SA central solenoid (CS) is not able to be measured during quench recovery. The flowing paths of CS is C-shaped and both of the outlets and the inlets of helium are on the outer periphery surface of the CS modules. Due to this C-shaped flowing path, heat exchanges between the inlet flow paths and the outlet flow paths. The CS outer periphery side becomes colder than the inner periphery side. The typical issue is the CS inside temperature is not able to be measured by the thermometers on the flowing paths. In this work, the CS temperature distribution changes during quench recovery is calculated and the period necessary for recovery is investigated.A CS module is composed of the 52 layers pancake coils. The 26 helium flowing paths are in a one module. The refrigerator supplies helium at 4.4 K to each flowing paths in nominal operation. In case of a quench, the refrigerator stops helium supply in order to shut out large heat load from the quenched magnet. The temperature distribution of the quenched CS will be smoothed by a heat conduction between each pancake coils while helium is stopped. Helium will be supplied again when the magnet pressure become low enough.The temperature distribution changes are calculated by using the thermal fluid simulation codes