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The High-Temperature Superconducting (HTS) magnet option has been explored for fusion reactors as well as for next-generation fusion experimental devices. The Stacked Tapes Assembled in Rigid Structure (STARS) conductor uses HTS tapes with simple stacking without twisting and transposition. A practically applicable STARS conductor is presently being developed with an operating current of 18 kA at 20 K temperature and ~15 T magnetic field. The conductor is required to have a high current density of 80 A/mm2. For the second stage of the conductor development, internal electrical insulation is applied between the copper stabilizer casing and the outer stainless-steel jacket, and a 6-m conductor sample was fabricated in a solenoid coil shape with a 600-mm diameter and three turns. The coiled sample was tested in 8 T, 20 K using a facility equipped with a maximum 13-T, 700-mm bore solenoid coil. Excitation up to the rated current of 18 kA was successfully attained with stable operation. The characteristics of the conductor observed during the excitation test are discussed.

In Large Helical Device (LHD), the detached plasma is obtained without external impurity gas feed by supplying an m/n = 1/1 resonant magnetic perturbation (RMP) field to a plasma with an outwardly shifted plasma axis position of Rax = 3.90 m where the magnetic resonance exists in the stochastic magnetic field layer outside the last closed flux surface. The plasma detachment is triggered by the appearance of an m/n = 1/1 island when the density, increased using hydrogen gas feed, exceeds a threshold density. The behavior of intrinsically existing impurities, in particular, carbon originating in the graphite divertor plates, is one of the important key issues to clarify the characteristic features of the RMP-assisted plasma detachment although the particle flux still remains on some divertor plates even in the detachment phase of the discharge. For this purpose, vertical profiles and two-dimensional (2-D) distributions of edge carbon emissions of CIII to CVI have been measured at extreme ultraviolet wavelength range, and the results are compared between attached and RMP-assisted detached plasmas. It is found that the CIII and CIV emissions located in the stochastic magnetic field layer are drastically increased near the m/n = 1/1 island O-point and in the vicinity of both inboard and outboard edge separatrix X-points during the RMP-assisted detachment, while those emissions are only enhanced in the vicinity of the outboard edge X-point in attached plasmas without RMP. The result clearly indicates a change in the magnetic field lines connecting to the divertor plates, which is caused by the growth of the m/n = 1/1 edge magnetic island. In contrast, the intensity of CVI emitted radially inside the magnetic island significantly decreases during the detachment, suggesting an enhancement of the edge impurity screening. The measured carbon distribution is analyzed with a three-dimensional edge plasma transport simulation code, EMC3-EIRENE, for the attached plasmas without R, source:Physics of Plasmas 24, 022510 (2017); https://doi.org/10.1063/1.4976989, source:https://doi.org/10.1063/1.4976989, identifier:0000-0003-0407-101X

Effects of low-n magnetohydrodynamic instabilities on plasma performance have been assessed in the regime where an achieved beta value is limited by instabilities. The unstable regime of an ideal interchange mode is characterized by enhanced magnetic hill and reduced magnetic shear. Experiments have clarified that (i) low-n modes are significantly destabilized in the ideal-unstable configurations and lead to degradation of central beta by at most 60%, and (ii) the degree of their damages strongly depends on the mode rotation velocity. The occurrence of the minor collapse is independent of an existence of an error field., source:S. Sakakibara et al 2015 Nucl. Fusion 55 083020, source:https://doi.org/10.1088/0029-5515/55/8/083020, identifier:0000-0002-3306-0531

The internal mode structure of a precursor with the tearing-parity structure of the locked-mode-like instability is investigated. For the first time, the Thomson scattering system with high temporal and spatial resolution enables us to find the non-rotating temperature flattening region in the torus outboard side in addition to the rotating island during the slowing-down phase. Additionally, the width of the flattening region is 10% normalized by the plasma minor radius. Furthermore, the radial profiles of the pressure degradation during the slowing-down phase are evaluated. At the beginning of the slowing-down phase, the pressure degradation area is located slightly inside the resonant surface. After that, the peak location moves to the core region. Finally, the degradation area peaks at the plasma center., source:https://doi.org/10.1585/pfr.16.1402091, identifier:0000-0003-3754-897X

High-temperature superconducting (HTS) conductor is a feasible candidate to make magnets for the next generation fusion devices because of its higher temperature margins and higher critical current in a high magnetic field in comparison to low-temperature superconducting (LTS) conductors. The recently proposed concept of the HTS-WISE (Wound and Impregnated Stacked Elastic tapes) conductor was studied to clarify its characteristics under certain magnetic fields. The WISE conductor, including 30-stacked REBCO (Rare-Earth Barium Copper Oxide) tapes, was fabricated and energized in a 9-T test facility which produced the condition of magnetic field B = 5 - 8 T and a temperature T = 30 - 50 K. Obtained critical currents (5.4 - 10.8 kA) increased with a decreasing magnetic field and/or temperature under the condition of T > 40 K. The maximum current of 16.9 kA was obtained at T = 30 K, which corresponded to the engineering current density jE = 60 A/mm2. Experimental results showed qualitative agreement with numerical calculations of the critical current. We confirmed the operation of the WISE conductor under a high magnetic field and low temperature., source:https://doi.org/10.1585/pfr.17.2405006, identifier:0000-0002-3541-6298

A new type of self-sustained divertor oscillation is discovered in the Large Helical Device stellarator, where the peripheral plasma is detached from material diverters by means of externally applied perturbation fields. The divertor oscillation is found to be a self-regulation of an isolated magnetic field structure (the magnetic island) width induced by a drastic change in a poloidal inhomogeneity of the plasma radiation across the detachment-attachment transitions. A predator-prey model between the magnetic island width and a self-generated local plasma current (the bootstrap current) is introduced to describe the divertor oscillation, which successfully reproduces the experimental observations, source:T. Kobayashi, M. Kobayashi, Y. Narushima, Y. Suzuki, K. Y. Watanabe, K. Mukai, and Y. Hayashi Phys. Rev. Lett. 128, 085001, source:https://doi.org/10.1103/PhysRevLett.128.085001, identifier:0000-0001-5669-1937

Effects of poloidal shear flow on the stability of interchange modes in a Large Helical Device (LHD) configuration are numerically studied. Three-dimensional (3D) numerical codes are utilized for the equilibrium and stability calculations. A static equilibrium is employed and a model poloidal flow as a flux function is incorporated in the initial perturbation. The results show that the initially applied flow can suppress the growth of the interchange mode if the flow is sufficiently large., source:https://doi.org/10.1585/pfr.11.2403035, identifier:0000-0002-7698-0223

In order to clarify the mechanism responsible for slowing down the precursor of an instability in Large Helical Device (LHD), whose behavior is similar to the locked mode instability in tokamaks, the spatial structure of the precursor of the locked-mode-like instability, and the relationship between the rotation of the precursor and the E × B rotation were experimentally investigated. The precursor rotates together with the E × B rotation at the resonant surface, and the precursor rotation slows down because of a decrease of the E × B rotation. The multi-channel fluctuation measurement of the precursor suggests that the precursor has a magnetic island, which may be related to the decrease of the E × B rotation. In addition, the reason for the appearance of the precursor with a magnetic island is discussed. The precursor appears when a magnetic island grows initially without rotation but then shrinks and begins to rotate., source:https://doi.org/10.1585/pfr.12.1402028, identifier:0000-0003-3754-897X

Low-Temperature Superconductors (LTS) are sensitive to non-uniform current distribution, which produces quenching. So, transposition of strands is indispensable in LTS cables to help current redistribution. In contrast, High-Temperature Superconductors (HTS) have higher thermal stability, which is expected to help current redistribution among strands (tapes) without quenching. Generally, HTS cable designs consider transposition to reduce quench likelihood and better handling AC operation. However, transposition causes mechanical strain in the tapes, reducing their performance. Recently, a 20-kA-class Stacked Tapes Assembled in Rigid Structure (STARS) conductor is being developed at NIFS, for the next-generation helical devices. To weigh the simple stacking feasibility of HTS tapes, a previous experiment confirmed, that 5 non-transposed HTS tapes can stably conduct a worst-case non-uniform current distribution without quenching. This further suggests that when using HTS tapes for DC HTS cables, transposition may be optional, but not strictly required. A numerical simulation was developed, dealing with the current distribution among the HTS tapes in a worst-case scenario, reproducing the previous experimental observation, and a second experiment was performed to give insights into the contact resistance between HTS tapes. The self-magnetic field effect and temperature fluctuations are to be explored for quench scenarios., source:https://doi.org/10.1585/pfr.17.2405066, identifier:0000-0002-0687-7191

A High-Temperature Superconducting (HTS) magnet is being considered to for use in the next-generation helical experimental devices. Three types of large-current HTS conductors are being developed, and one of them is the STARS (Stacked Tapes Assembled in Rigid Structure) conductor which uses HTS tapes with a simple stacking technique. Following the proof-of-principle experimental results obtained in the former 100-kA-class prototype hand-made conductor sample, an actually applicable conductor is being developed with a rated current of 18 kA at a temperature of 20 K and a magnetic field of ∼10 T. One of the crucial requirements for this conductor is to have a high current density of 80 A/mm2. In the first phase of the development, a 3-m short sample was fabricated by applying laser-beam welding to the stainless-steel jacket. It was tested in liquid nitrogen at 77 K with no external magnetic field. Then the sample was tested in gaseous helium at 20 - 40 K under a magnetic field of 6 - 8 T, and the results show that the basic requirements were satisfied., source:https://doi.org/10.1585/pfr.17.2405076, identifier:0000-0002-4489-8241

Edge MHD instabilities with pressure collapse are found in relatively high beta and low magnetic Reynolds number regions with a magnetic axis torus outward-shifted configuration of the large helical device (LHD), and characteristics and onset conditions of the instability are investigated. The instability has a radial structure with an odd parity around the resonant surface, which is different from that of the interchange instability typically observed in the LHD. The onset condition dependence on the magnetic axis location shows that the onset beta increases as the magnetic axis location moves more torus inwardly, and the instability appears only in limited configurations where the magnetic axis is located between 3.65 and 3.775 m. In such configurations, the resonant surface location is close to an index of the plasma boundary. This fact suggests that the distance between the resonant surface location and the plasma boundary plays an important role in the onset, and a possibility that the instability is driven by an external mode., source:https://doi.org/10.1063/5.0111360, identifier:0000-0003-3754-897X

The rotation of resistive interchange instabilities excited in the periphery of plasmas compared to plasma flows under various plasma conditions in the Large Helical Device. The observed magnetohydrodynamic modes exhibit nominal rotation in the electron diamagnetic direction in the laboratory frame. We assumed that the ion (electron) flows consist of the E × B and ion (electron) diamagnetic drift flows. These drift flows were individually evaluated and geometrically compensated using a finite beta equilibrium. Experiments with different neutral beam injection conditions and different electron densities show that the frequencies of the observed modes were quantitatively consistent with those of the electron flow., source:https://doi.org/10.1585/pfr.8.1402123, identifier:0000-0003-3754-897X

We report the first observation of the formation of a magnetic island before the occurrence of mode locking in helical plasma. New analysis and observation techniques applied to the ECE signal and poloidal flow in LHD experiments yield the following results. (i) A magnetic island structure is present, rotating at the end of the rotating phase. (ii) The rotation speed of the island is not uniform in space and time. The rotation of the island changes significantly at the end of the rotating phase, and the deformation increases until the mode is locked.

Optimization of the helically-winding structure required for the helical fusion reactor utilizing the high temperaturesuperconductor (HTS) tape is studied by numerical calculation. When the HTS tapes are wound to bethe helical structure without the torsion, a significant edgewise strain appears. This might lead to the degradationof the performance of the HTS tape. Therefore, the edgewise strain must be minimized. It is found that the edgewise strain can be suppressed by the proper determination of the torsion by the optimization utilizing the Fourier decomposition of the distribution of the edgewise strain. The HTS tapes with edgewise strain are intrinsically deformed due to the torsion to reduce the edgewise strain so to be an “edgewise-strain-free” helical shape.

The magnetic island in the large helical device (LHD) shows the dynamic behaviour of the healing/growth transition withthe hysteretic behaviour. The thresholds of plasma beta and poloidal flow for island healing are larger than that for growth.The threshold of resonant magnetic perturbation (RMP) for healing is smaller than that for growth. Furthermore, thresholds of the amplitude of RMP depend on the magnetic axis position Rax in the LHD. The RMP threshold increases as the magnetic axis position Rax increases. The poloidal viscosity may be considered as a candidate to explain the experimental observation from the viewpoint of the relationship between the electromagnetic torque and the viscous torque., source:https://iopscience.iop.org/article/10.1088/0029-5515/55/7/073004

The effects of resonant magnetic perturbation (RMP) field on impurity radiation, divertor footprint distribution, and core plasma transport are investigated in the detachment discharges of the Large Helical Device (LHD). The RMP with m/n = 1/1 mode creates an edge magnetic island in the stochastic layer, which enhances the impurity emission from low charge states, C2+ and C3+, and then triggers a detachment transition. Emission from the higher charge states, C4+ and C5+, implies enhanced penetration of impurities during the detachment phase with RMP. The toroidal divertor particle flux distribution exhibits n = 1 mode structure in both the attached and detached phases, but with a large toroidal phase shift between the two phases. The distribution in the attached phase is well correlated with the magnetic footprint of field line connection length calculated by the vacuum approximation. During the detached phase, however, the phase shift is not well explained by the vacuum approximation, where a significant plasma response to the external RMP is observed. The energy confinement time becomes systematically shorter with RMP application due to the shrinkage of plasma volume caused by the edge magnetic island. On the other hand, the pressure profile during detachment with RMP is found to be more peaked than without RMP. The analysis using the core transport code TASK3D, considering the heating profiles of neutral beam injection, shows no significant transport degradation during detachment with RMP application, even with the enhanced radiation, reduced divertor flux, and possible impurity penetration., source:M. Kobayashi et al 2019 Nucl. Fusion 59 096009, source:https://doi.org/10.1088/1741-4326/ab26e6

The deuterium (D) operation was initiated in the LHD in 2017. In the first campaign of the D experiments, we successfully extended the high temperature regime in the LHD. The new record of the ion temperature (Ti) of 10 keV associated with the ion internal transport barrier (ITB) was achieved due to several operational optimization. The thermal confinement characteristics of ITB plasmas were compared between hydrogen and D discharges. The effective ion thermal diffusivity of the ion-ITB plasmas was found to be smaller in the D discharges than that in the H discharges. The profiles of the Ti, the electron density, and the impurity of the high Ti plasmas strongly depended on the magnetic configuration and these profiles tended to peaked in the inward-shifted configuration. It was also found that the electron thermal confinement of the electron-ITB plasmas was clearly improved in the deuterium case. The GKV simulation showed the linear growth rate of TEM/ITG reduced in the plasmas with D both for the ion ITB and the electron ITB plasmas and qualitatively agreed with the tendency of the change in the thermal diffusivity obtained from the power balance analysis., source:H. Takahashi et al 2018 Nucl. Fusion 58 106028, source:https://doi.org/10.1088/1741-4326/aad87e

The mode slowing down mechanism of the locked-mode-like instability without a large magnetic island is investigated, based on the LHD experimental analysis. The mode frequency coincides with E × B rotation frequency at the resonant surface and the slowing down is caused by two processes. One is the resonant surface moving to the small E × B rotation frequency region and the other is the slowing down of the E × B rotation frequency near the resonant surface. Both processes are almost the same as those of the locked-mode-like instability with a large magnetic island. The results suggest that the slowing down occurs even though the precursor does not have a large magnetic island. However, when the external RMP is imposed, the mode frequency in the slowing down phase sometimes does not coincide with the E × B rotation frequency. Moreover, the mode amplitude during the slowing down phase increases with the decrease of the mode frequency both with and without the imposed external RMP, which suggests that the instability growth in the slowing down phase is more strongly related to the mode frequency than the E × B rotation frequency because the mode sometimes does not rotate with the E × B rotation., source:Y. Takemura et al 2019 Nucl. Fusion 59 066036, source:https://doi.org/10.1088/1741-4326/ab169f, identifier:0000-0003-3754-897X

The slowing-down mechanism of the locked-mode-like instabilities with and without an island structure is investigated through the effects of an external RMP (resonant magnetic perturbation) on the instabilities. For both instabilities, the slowing-down duration decreases with the increase in the external RMP, and the RMP dependence is consistent with the braking model of the j × B force due to the interaction between the instabilities and the external RMP. Moreover, the relationship between the amplitude and the frequency of both locked-mode-like instabilities during the slowing down is consistent with the force balance model between the j × B force due to the external RMP and a viscous force. These results suggest that the slowing down of both locked-mode-like instabilities with finite external RMP occurs due to the j × B force driven by the external RMP., source:Y. Takemura et al 2021 Nucl. Fusion 61 026011, source:https://doi.org/10.1088/1741-4326/abc935, identifier:0000-0003-3754-897X

We investigate the penetration threshold of resonant magnetic perturbation (RMP) by the external coils in the Large Helical Device (LHD) for various plasma aspect ratio configurations. The qualitative dependence on the collisionality is opposite to that in a high plasma aspect configuration; this is a quite unique property first found in the LHD. We also investigate the threshold dependence on the ion species, and find that the threshold in deuterium discharges is much smaller than that in hydrogen discharges. In all cases the thresholds are higher as the poloidal rotation becomes faster, which shows that poloidal rotation is the dominant driver to the RMP shielding. This is qualitatively consistent with the torque balance model between the electromagnetic and poloidal viscous torques. In a configuration of the LHD, the dependence of the threshold on the density is qualitatively similar to that in Ohmic tokamak plasmas, but the beta dependence is opposite to that of tokamaks. The difference arises from the cause of the viscous torque., source:K. Y. Watanabe et al 2019 Nucl. Fusion 59 086049, source:https://doi.org/10.1088/1741-4326/ab2281, identifier:0000-0002-5653-5579

0000-0003-3754-897X, The rotation of resistive interchange instabilities excited in the periphery of plasmas compared to plasma flows under various plasma conditions in the Large Helical Device. The observed magnetohydrodynamic modes exhibit nominal rotation in the electron diamagnetic direction in the laboratory frame. We assumed that the ion (electron) flows consist of the E × B and ion (electron) diamagnetic drift flows. These drift flows were individually evaluated and geometrically compensated using a finite beta equilibrium. Experiments with different neutral beam injection conditions and different electron densities show that the frequencies of the observed modes were quantitatively consistent with those of the electron flow.

0000-0002-3541-6298, The characteristics of low-n ideal external modes in the Large-Helical-Device are studied. The low-n analysis is carried out with the TERPSICHORE code [W.A. Cooper, Plasma Phys. Control. Fusion 34, 1011 (1992)]. Under fixed boundary conditions, the low-n instability region is limited within the Mercier unstable region and the external mode does not appear. On the other hand, with free boundary conditions, the low-n unstable region spreads and the external modes become unstable. The effects of a conducting wall similar in shape to that of the plasma show that stability to external modes of higher growth rate can be obtained even with the wall further away from the plasma.