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A maintenance method for the breeder blanket of the LHD-type helical reactor FFHR was considered. A simple segmentation in a plane with a constant toroidal angle was proposed by utilizing the flexible arrangement capacity of the coolant path in the blanket module with a molten salt tritium breeder. Moreover, it was found that all blanket modules can be extracted by a combination of uniaxial movements and poloidal rotation without cutting of the coolant pipes inside the vacuum vessel., source:https://doi.org/10.1585/pfr.11.2405047, identifier:0000-0001-6028-8980

FFHR-d1 is a conceptual design of the Large Helical Device-type fusion reactor. Several design optimizations for FFHR-d1 have been conducted under a multipath strategy. The structural design began from a radial build design of the components using a system code analysis and considering a shielding/breeding blanket design. The support structure must be sufficiently rigid to hold a constant geometric position of the radial build components. Additionally, large apertures are required for the maintenance work of in-vessel components. The shape of the support structure was carefully chosen and the analytically determined stress level was within the permissible limit. Thus, the basic design of the structural components of FFHR-d1 was established. To accelerate the design activity and achieve a consistent helical reactor system, the high-temperature superconducting joint-winding, liquid metal divertor, a supplemental helical coil addition, a novel divertor structure, and other challenging options are investigated alongside the basic design. This paper describes the structural design status of FFHR-d1 by focusing on the latest radial build design, construction scheme, and effectiveness of the novel divertor structure., source:https://doi.org/10.1585/pfr.11.2405061, identifier:0000-0003-4210-9859

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., source:https://doi.org/10.1585/pfr.12.1405017, identifier:0000-0002-7901-6077

The required pumping power for the liquid metal ergodic limiter/divertor REVOLVER-D (Reactor-oriented Effectively VOLumetric VERtical Divertor), which is considered as an optional divertor design for the LHD-type helical fusion reactor FFHR-d1, has been estimated with a consideration of the change in the strength of transverse magnetic field along the flow channel. It is found that the pressure drop and the required pumping power can be suppressed to an acceptable level by using insulated ducts., source:https://doi.org/10.1585/pfr.12.1405016, identifier:0000-0001-6028-8980

Recently, a new limiter concept with liquid metal for a helical fusion reactor was proposed. This work present a modeling study to understand the influence of the limiter to the plasma and to find critical parameters for the liquid limiter concept. EMC3-EIRENE code is applied to a limiter configuration of a vertical column formed with plane surfaces at the inboard side of the Large Helical Device (LHD). The calculation results indicate that heat load on the helical divertor plates is significantly reduced when the limiter is inserted into the ergodic layer even without impurities. However, the influence of impurity radiation is too large in some cases. In order to achieve a large reduction of divertor flux and to avoid a large reduction of core electron temperature, parameter scans with limiter position and sputtering yield were performed. The results suggest that the limiter position is a critical parameter for acceptable degradation of the core and sufficient reduction of divertor flux., source:https://doi.org/10.1585/pfr.13.3403034, identifier:0000-0002-9117-5898

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., source:https://doi.org/10.1585/pfr.13.1405003, identifier:0000-0001-5079-6391

A cost model for a tokamak fusion power plant (FPP) is improved to evaluate material cost and manufacture cost, separately. Then, the improved cost model is applied to a commercial tokamak FPP, and reduction of FPP construction cost is investigated considering learning effect on manufacture of the fusion island part and the advanced manufacture of toroidal field (TF) coils. Finally, a development scenario of a tokamak FPP is proposed to contribute substantially to global climate stabilization under the framework of the Paris Agreement., source:https://doi.org/10.1585/pfr.14.1305047, identifier:0000-0001-7201-4311

A magnetic field is used to confine the plasma to achieve controlled fusion. Therefore, since the movement of the plasma follows magnetic field lines, a plurality of magnetic field lines is calculated from electromagnetic field simulation results in a fusion reactor. Because of the complicated distribution of magnetic field lines in three-dimensional (3D) space, existing analysis measures which are mostly based on two-dimensional poloidal plasma cross-sections are unsatisfactory for domain experts. To solve this problem, we propose a technique for reconstructing a regular scalar field from the magnetic field lines. First, on poloidal plasma cross-sections, intersection points of magnetic field lines are used to make annotations of learning the plasma shape. Then, a deep neural network is built to approximate the scalar field that represents the probability of the existence of magnetic field lines. Consequently, a 3D model of plasma shape has managed to be constructed by applying the marching cubes method. The effectiveness of the proposed method is demonstrated by comparing it with the conventional method and domain experts’ reviews., source:Hu, K., Koyamada, K., Ohtani, H. et al. Visualization of plasma shape in the LHD-type helical fusion reactor, FFHR, by a deep learning technique. J Vis 24, 1141–1154 (2021). https://doi.org/10.1007/s12650-021-00768-w, source:https://doi.org/10.1007/s12650-021-00768-w

MHD effect of a liquid metal sheath jet, which is the jet stabilized by an inserted chain, has been experimentally investigated to examine its applicability to the divertor target of a fusion reactor. The deflection of the liquid metal jet together with the chain was observed when the electric current was applied to the jet under the presence of the magnetic field perpendicular to the jet flow direction. This means that the stabilization of the liquid meal jet by the chain is maintained when the jet is deformed by the magnetohydrodynamics (MHD) effect. However, the magnitude of the jet deflection agrees with the calculation result of the deflection of jet by the Lorentz force when the jet is regarded as a conductor. This indicates that mitigation of the Lorentz force and further investigation of the behavior of the liquid metal sheath jet under the exposure of divertor plasma is necessary to realize the divertor target using the liquid metal sheath jet., source:https://doi.org/10.1585/pfr.14.1405092, identifier:0000-0001-6028-8980

Present plasma direct energy conversion (DEC) system has been developed since proposal of ARTEMIS. The system for D-3He reactor is composed of cusp-type DEC (CuspDEC) for particle discrimination, traveling wave DEC (TWDEC) for recovery of high energy protons, and secondary electron DEC (SEDEC) for recovery of extremely accelerated protons. Studies on each device are in the third stage, where higher capability of each device will be derived. Various proposals and examinations were reported and the present paper treats the researches comprehensively and shows explanation and discussion for some researches: separation of high density plasma and ion-ion separation in CuspDEC, studies on modulation in TWDEC, and improvement of electron collection in SEDEC., source:https://doi.org/10.1585/pfr.14.2405013, identifier:0000-0002-3458-1267

A fusible metal pebble divertor system named the REVOLVER-D2 is newly proposed here, based on the molten tin jet divertor REVOLVER-D and the pebble divertor concepts. In the REVOLVER-D2, solid tin pebble flows are used as the divertor target. Molten tin pools receive the falling tin pebbles to mitigate the impact of the heavy metal shower. All pebbles are melted in the molten tin pool and recycled into the tin pebbles again. The solid tin pebbles are formed inside the vacuum vessel by the shot tower method using silicone oil pools. This concept enables simultaneous achievement of a high heat load tolerance and a high evacuation efficiency, while mitigating the MHD effects. A high heat load can be removed by the pebble flow if the pebble size is large enough and the velocity is fast enough as with the molten tin jet. Although the charge up of pebbles affects the pebble motion due to the Lorentz force and the Coulomb force, the Lorentz force is negligible if the injection velocity is fast enough. For example, center-to-center pebble distance due to the Coulomb force will be ∼ 4 mm in the case of the injection velocity of 10 m/s if the molten tin pool is placed 2 m under the point where pebbles enter the ergodic layer., source:https://doi.org/10.1585/pfr.14.3405050, identifier:0000-0001-5079-6391

The cartridge-type helical blanket system called the CARDISTRY-B has been proposed for the helical fusion reactor FFHR-d1. The CARDISTRY-B is aimed at easy construction and maintenance. However, there remain many issues in the design. For example, these include a large number of cartridges, no tangential ports, low compatibility with the helical divertor, and others. To solve these issues, an improved design of the cartridge-type blanket is proposed and named the CARDISTRY-B2. This is composed of the tritium breeding blanket (BB) filled with the flowing molten salt and the neutron shielding blanket (SB) filled with the tungsten-carbide. The number of the BB (or, SB) cartridges in the 1/10 section of the full torus are reduced from 32 to 17 (or, from 44 to 17). Tangential ports for the neutral beam injection heating are newly equipped together with the multi-purpose blanket cartridge. Compatibility with the helical divertor is also improved. The BB cartridges are fixed on the fixing units set on the top of the SB. No other part of the BB is touching the SB except the fixing ribs. The SB cartridges surround the superconducting magnet coils and work as the cryostat inside the vacuum vessel. This makes the large bellows for the large maintenance ports unnecessary. Details of the CARDISTRY-B2 designed for a small helical fusion reactor FFHR-b1 are described in this paper., source:https://doi.org/10.1585/pfr.14.1405163, identifier:0000-0002-7901-6077

A new divertor concept REVOLVER-D2 that uses metal pebbles as a divertor target has been proposed for the LHD-type helical fusion reactor FFHR-c1. Demonstration of sufficient plasma shielding rate is one of the important issues to show the feasibility of the REVOLVER-D2. For this purpose, an experiment on the plasma irradiation to the metal pebble flow has been conducted in the sheet plasma device TPDsheet-U in Tokai University. Reduction of the ion flux and electron density in the metal pebble flow was observed and it was confirmed that the metal pebble flow can play the role of the divertor target. The measured shielding rate is consistent with the prediction by the model based on the Lambert-Beer-Bouger’s law. Achievement of the shielding rate of > 72 % is expected in the FFHR-c1 using the nozzle with the size of > 310 mm × 310 mm and the pebble flow with the diameter of 5 mm., source:Takeru Ohgo, Takuya Goto, Toshikio Takimoto, Akira Tonegawa, Junichi Miyazawa, Plasma irradiation experiment on the metal pebble flow in the TPDsheet-U, Fusion Engineering and Design, Volume 165, 2021, 112236, ISSN 0920-3796, https://doi.org/10.1016/j.fusengdes.2021.112236., source:https://doi.org/10.1016/j.fusengdes.2021.112236

We developed an analysis model to predict and describe neutron emission anisotropy caused by anisotropy of energetic deuterons in the large helical device (LHD), by considering energetic-ion distribution functions, the differential cross-section of fusion reactions, and the exact shape of the vacuum vessel of the LHD. Neutron emission anisotropy that can be observed by the neutron activation system is evaluated numerically assuming deuterium-beam-injected deuterium plasmas confined by the LHD using the model. We demonstrate the dependence of neutron emission anisotropy on the beam-injection direction and the electron temperature. Based on this estimation, we propose an experimental scenario to validate the analysis model and easily understand energetic-ion anisotropy caused by the neutral beam injection., source:https://doi.org/10.1585/pfr.14.3403123

The importance of the parallel momentum balance on the bootstrap current evaluation in non-axisymmetric systems is demonstrated by the benchmarks among the local drift-kinetic equation solvers, i.e., the Zero-Orbit-width (ZOW) model, DKES, and PENTA. The ZOW model is extended to include the ion parallel mean flow effect on the electron-ion parallel friction. Compared to DKES code in which only the pitch-angle-scattering term is included in the collision operator, PENTA code employs the Sugama-Nishimura method to correct the momentum balance. The ZOW model and PENTA codes, both of which conserve the parallel momentum in like-species collisoins and include the electron-ion parallel frictions, agree each other well on the calculations of the bootstrap current. The DKES results without the parallel momentum conservation deviates significantly from those from the ZOW model and PENTA. This work verifies the reliability of the bootstrap current calculation with the ZOW model and PENTA for the helical plasmas., source:https://doi.org/10.1585/pfr.12.1203004

0000-0003-4210-9859, Superconducting magnets in fusion reactors are subjected to a huge electromagnetic force of >100 MN/m. The magnets have to be sustained with a strong-body structure to avoid high stress and deformation. The total weight of the magnet system in the fusion reactor is estimated to be more than 20,000 tons. We applied topology optimization technique to the magnet support structure to reduce the weight of fusion reactors. Compared with the conventional design, we achieved a weight reduction of >25%. Static and seismic analyses were carried out to validate the soundness of the topology-optimized design. Consequently, the stress against the electromagnetic force in the structure was within the permissible range. It was discovered that using seismic isolation structure can adequately prevent the damage to the magnet system even when directly subjected to a massive earthquake.