[en] A cryopump for divertor with closed structure in the large helical device (LHD) vacuum vessel is exposed to various heat loads such as radiant heat and conductive heat from the surroundings. If the water cooling shield or liquid nitrogen cooling shield (hereinafter referred to as LN2 cooling shield) is heated beyond cooling capacity, cryopanel temperature rises and activated carbon starts to release the adsorbed gas, and then the cryopump loses the function as an exhaust pump. In particular, evaluation on the heat load on the cryopanel and the resulting temperature rise of the activated carbon are important because these matters directly affect the performance. In this study, the thermal load on various shields and the thermal load on the cryopanel were evaluated. The thermal analysis of the heat load of the water-cooled shield and the liquid LN2 cooling shield was carried out using the finite element method, and it was confirmed that these shields had a high heat shielding function to the radiant heat from the high temperature part of the divertor heat receiving plate. A simplified model was used to calculate the radiation heat from the LN2 cooling shield, the heat load due to heat conduction from the jig supporting the cryopanel, and the heat load due to the heat of adsorption generated when the activated carbon adsorbs hydrogen gas. The thermal analysis of the cryopanel was conducted again using the finite element method with boundary conditions set, and it was confirmed that the activated carbon surface temperature of the cryopanel can maintain 20K or under which guarantees the function as an exhaust pump even under the maximum exhaust of hydrogen gas. (A.O.)

[en] The world's first quasi-axisymmetric stellarator CFQS (China First Quasi-axisymmetric Stellarator) will be constructed as the joint project of National Institute for Fusion Science (NIFS) in Japan and Southwest Jiaotong University (SWJTU) in China. Physics design of CFQS plasma was completed, and numerous efforts are now being made to finalize engineering design of CFQS. The CFQS vacuum vessel (VV) will be made of thin plates of SUS316L with thickness of 6 mm. The thickness is determined by considering fabricability and manufacturing cost. In this report, analyses have been performed to confirm the structural reliability of VV and evaluate the influence of eddy current using finite element method software ANSYS/Mechanical^TM and ANSYS/Maxwell^TM. (author)

[en] The authors have developed a cryo-adsorption pump that can be installed in an LHD vacuum vessel, in an attempt of installing in a closed structure divertor of large helical device (hereinafter, LHD). In order to implement the cryo-adsorption pump under the severe thermal and spatial constraints of the divertor, this project conducted a design study by mutually complementing the 3D-CAD structural design and the radiation heat analysis based on finite element method. As a result, it became possible to obtain a structure having sufficient heat resistance and heat shielding performance even under high heat load from a divertor heat receiving plate or plasma at 1000degC or higher. At the same time, while utilizing the thermal analysis, it was possible to obtain a structure where the heat load to the adsorption panel due to radiation and gas/solid heat transfer could be suppressed to the allowable temperature or under. Through this thermal design, this project obtained the prospect of the cryo-adsorption pump that can perform a role of a vacuum pump even under the high heat load environment of the LHD divertor section. The results of placing the developed cryopump in the LHD torus and testing its exhaust capacity revealed that the exhaust speed is about 1.5 to 2 times the design target, and the exhaust capacity is corresponding to the number of particles for one week used in an LHD high density experiment. Thus, the sufficient exhaust capacity was verified. (A.O.)

[en] The authors developed a cryo-sorption pump aimed at installing in a closed structure divertor for a large helical device (LHD). In addition to newly developing an inorganic bonding method that does not contaminate the vacuum container, they quantitatively clarified the pore characteristics of activated carbon and the exhaust performance of the cryopump via experiments, and selected high performance activated carbon suitable for the cryo-sorption pump. Furthermore, by interconnecting structural design based on 3D-CAD and radiant heat - heat transfer simulation based on finite element method, they designed thermal structure that is thermally robust even under high radiant heat environment and also able to keep an adsorption panel below acceptable temperature. Through this thermal structural design, they obtained the prospect of a cryo-sorption pump that can work as a vacuum pump even under the high heat load environment of LHD divertor part. The developed cryopump was placed in the toroidal section inside the LHD torus, and its exhaust performance test was conducted. It could be confirmed that the exhaust speed and the exhaust capacity have sufficient performance for the high density test of LHD. (A.O.)

[en] The manufacturing of tungsten (W) - graphite bonded divertor components for the Large Helical Device (LHD) has been investigated. The spark plasma sintering method was used to bond W and graphite with titanium (Ti) interlayer. Small specimens were fabricated to investigate the bonding strength and to diagnose the bonding interface. The granular structure was formed in the grooved area on the graphite surface. It was suggested that this granular structure had affected the bonding strength. (author)

[en] The stabilization effect of the Internal Flow Resistances (IFR) inserted into water jets has been investigated in order to apply the liquid metal divertor to the helical fusion reactor, FFHR. A high heat load exceeding 20 MW/m² is one of the important issues for the divertor in the nuclear fusion reactor. A new concept of ergodic limiter/divertor called the REVOLVER-D has been proposed for FFHR. This divertor is expected to tolerate a high heat load larger than a few tens of MW/m². Jets narrow after acceleration by gravity and transform to droplets due to the surface tension instability. Stabilization of jets is an important issue for realizing the REVOLVER-D. In this paper, stabilization of jet by various IFR has been examined. Here, the 'sheath jet' is defined as a jet with an IFR inside. Experiments on sheath jet have been carried out and the stabilization effect of IFR has been confirmed. Numerical simulations on sheath jet using ANSYS have been carried out. It is shown that the velocity of the sheath jet reaches the terminal velocity, which is proportional to 0.36 ± 0.01 power of the flow rate and -0.26 ± 0.01 power of the outer circumference of the IFR. (author)

[en] Divertor pumping was applied to plasma discharges for superior fuel particle control in the Large Helical Device (LHD). The LHD is equipped with two different pumping systems. One is the main pumping system, in which the pumping speed is 260 m³/s in hydrogen. The other pumping system is the divertor pumping system in which the pumping speed is 70 m³/s in hydrogen. Divertor pumping was applied to 40-second long pulse Electron Cyclotron Heating (ECH) discharges to assess the improvement in particle control provided by divertor pumping. The results show that without divertor pumping, the electron density was not controlled by gas puffing using the feedback signal of line-averaged electron density. Then, the plasma confinement deteriorated, finally leading to radiation collapse. On the other hand, with divertor pumping, the density was well-controlled by gas puffing using the feedback signal. The results indicate that divertor pumping is one of the key tools for controlling the particles in fusion plasmas. (author)

[en] The in-vessel cryo-sorption pump for the Closed Helical Divertor (CHD) in the Large Helical Device (LHD) has been developed at the National Institute for Fusion Science (NIFS). An organic adhesive-free bonding technique for attaching activated carbon pellets to a copper cold panel was invented, which employs the indium solder with intermediate materials. The prototype of the CHD with the newly developed cryo-sorption pump was installed in the LHD. Performance of the cryo-sorption pump was estimated in the LHD vacuum vessel. A satisfactory result of the maximum pumping speed up to 9 m"3/s was obtained with one divertor module in one toroidal section (10% of the torus), which is equivalent to the required pumping speed of the CHD. (author)

[en] A new cartridge-type blanket named the CARDISTRY-B is proposed for the helical fusion reactor FFHR-d1.This blanket is composed of the neutron shield and the tritium breeder using molten salt. Both of these are toroidally segmented every two degrees. At each toroidal angle, the segmented parts are divided further into several cartridges in order to make it possible to assemble these cartridges after completion of the superconducting magnet coils. The neutron shield is basically assembled by using mortise and tenon prepared on each of the cartridges and the lower port, instead of the wide area welding. After assembly, the plasma side of the neutron shield is welded to form a vacuum vessel. Another side of the neutron shield facing on the superconducting magnet coils is covered with the thermal shield, which was already attached before assembly. The tritium breeder cartridges can be replaced without cutting or welding of cooling pipes inside the vacuum vessel, where severe radiation dose is expected. Details of the CARDISTRY-B, including the results of motion analysis for all cartridges and estimation of the cartridge weight, are discussed. (author)

[en] An innovative color measurement technique is employed in the Large Helical Device (LHD). This study provides a method for obtaining in broad spatial extent and in great detail the color information of the first wall relating to the thickness of the deposition layer. The RGB (Red, Green, and Blue) value, mainly of the stainless steel plates on the helically twisted coil, is measured by a color analyzer equipped with an integrated sphere light source. On the outer torus side, the colors of almost all stainless steel plates are close to carbon black, which suggests that deposition is dominant. On the inner torus side, all plates except for those neighboring the carbon divertor plates are almost white, as in the case of the stainless steel substrate of the first wall, which suggests that erosion is dominant. The relationship between the color and the distance from the stainless steel plates to the plasma is investigated. (author)