Junichi Miyazawa

Design and development of high-temperature superconducting magnet system with joint-winding for the helical fusion reactor

An innovative winding method is developed by connecting high-temperature superconducting (HTS) conductors to enable efficient construction of a magnet system for the helical fusion reactor FFHR-d1. A large-current capacity HTS conductor, referred to as STARS, is being developed by the incorporation of several innovative ideas, such as the simple stacking of state-of-the-art yttrium barium copper oxide tapes embedded in a copper jacket, surrounded by electrical insulation inside a conductor, and an outer stainless-steel jacket cooled by helium gas. A prototype conductor sample was fabricated and reached a current of 100 kA at a bias magnetic field of 5.3 T with the temperature at 20 K. At 4.2 K, the maximum current reached was 120 kA, and a current of 100 kA was successfully sustained for 1 h. A low-resistance bridge-type mechanical lap joint was developed and a joint resistance of 2 nΩ was experimentally confirmed for the conductor sample.

Detachment Phenomena in LHD Compared to W7-AS

Abstract Self-sustained detachment, named the Serpens mode (Self-regulated plasma edge ’neath the last closed flux surface), has been observed in the Large Helical Device (LHD), which is equipped with an intrinsic open helical divertor, in spite of the low divertor neutral pressure of order 10–3 Pa. Other states of detachment are also observed; one is transient partial detachment, observed only in the gas puff port during massive gas puffing, and another is complete detachment, which takes place when the hot plasma boundary shrinks below the last closed flux surface. The Serpens mode is categorized as a subset of complete detachment, but with large fluctuations in the divertor flux and Hα signal, which result from a rotating radiation belt called the serpent. In this paper, these are compared with experimental observations of partial detachment and multifaceted asymmetric radiation from the edge (Marfe) in a modular stellarator, Wendelstein 7-AS (W7-AS). There are many similarities between transient partial detachment in LHD and partial detachment in W7-AS and, especially, between the Serpens mode in LHD and Marfe in W7-AS.

Dynamics of three-dimensional radiative structures during RMP assisted detached plasmas on the large helical device and its comparison with EMC3-EIRENE modeling

The resonant magnetic perturbation (RMP) island introduced in the stochastic edge of the large helical device (LHD) plasma plays an important role in the stabilization of the plasma detachment (Kobayashi et al 2013 Nucl. Fusion 53 093032). The plasma enters in the sustained detachment phase in the presence of an RMP once the line averaged density exceeds a critical value with a given input power. During detachment the enhanced radiation from the stochastic edge of the LHD undergoes several spatiotemporal changes which are studied quantitatively by an infrared imaging video bolometer (IRVB) diagnostic. The experimental results are compared qualitatively and quantitatively with the radiation predicted by the 3D transport simulation with fluid model, EMC3-EIRENE. A fair amount of qualitative agreement, before and after the detachment, is reported. The issue of overestimated radiation from the model is addressed by changing the free parameters in the EMC3-EIRENE code till the total radiation and the radiation profiles match closely, within a factor of two with the experimental observations. A better quantitative match between the model and the experiment is achieved at higher cross-field impurity diffusion coefficient and lower sputtering coefficient after the detachment. In this article a comparison, the first of its kind, is established between the quantified radiation from the experiments and the synthetic image obtained from the simulation code. This exercise is aimed towards validating the model assumptions against the experimentally measured radiation.

Refueling for Steady-State Plasma by Repetitive Pellet Injection in Large Helical Device

A repetitive pellet injector has been developed for investigation of refueling issues towards the steady-state operation in Large Helical Device (LHD). Continuous operation of more than 10000 pellet launching at 10 Hz has been demonstrated. The maximum repeating rate is 11 Hz. No technical constraint for longer operation has been found. The reliability of pellet launch has exceeded 99.9%. The initial application to the NBI-heated plasmas has been successful in the last experimental campaign of LHD. Although the pulse length is limited by the operational constraint of NBI, the plasma with a density of 8 × 1019 m-3 has been sustained for 2 s by the pellet injection at 10 Hz. A prospect for the future experiment is discussed on the basis of the initial result.

Possibility of Profile Control using Compact Toroid Injection on Large Helical Device

Compact toroid (CT) injection is an attractive method for central fueling into hot fusion plasmas. Induction of plasma rotation is expected by momentum input along the three-dimensional trajectories of the injected CT. The CT trajectories in the magnetic field of a large helical device (LHD) are calculated while changing the initial injection velocity and injection point, to determine the optimum conditions for CT injection into LHD. The possibility of central fueling is confirmed with a model CT with a diameter of 20 cm, 1022 m-3 electron density, and 300 km/s initial velocity, in the case of CT injection into an LHD magnetic field of 1.5 T. The input profiles of the density and the momentum obtained assuming a simple CT decay model show the potential of CT injection as a profile control method.

Difference in Electron Transport Between Co- and Counter-NBI-Heated Plasmas in the Inward-Shifted Configurations on LHD

Abstract In the low-density plasmas of the Large Helical Device, the shape of the electron temperature profile changes depending on the direction of the tangential neutral beam injection (NBI) when the magnetic axis position is inward-shifted at R = 3.50 m. Core flattening was observed in plasmas heated by counter-NBI. The electron thermal diffusivities in co-NBI and counter-NBI-heated plasmas are compared. The diffusivity becomes large at the central region in the case of counter-NBI. This result shows that the flattening in the electron temperature profile is not caused simply by a change in the power deposition only. Some magnetic fluctuations are seen during counter-NBI. On the other hand, it is a promising feature that the electron thermal diffusivity at the peripheral region does not increase with the heating power in co-NBI plasmas.