Ryuichi Sakamoto

The Large Helical Device (LHD), the largest superconducting helical system in the world, is equipped with advanced heating and diagnostic tools, facilitating plasma control and physics research. Data assimilation was employed for electron temperature control using a real-time Thomson scattering system and real time prediction code. A virtual LHD environment enabled visualization of escaping high-energy tritium ions and demonstrated that these ions impact the rear side of the divertor plate. Pioneering results crucial to plasma control have also been achieved. Real-time wall conditioning using Lithium granule dropping improved bulk ion energy and particle transport while simultaneously enhancing the heavy impurity transport. Progress has also been made in the investigation of turbulence-driven transport. At the confinement bifurcation, ion-scale turbulence decreased, while electron-scale turbulence increased. A change in the anisotropy of turbulent eddies was also observed at the confinement bifurcation. Coexistence of local and non-local turbulence was identified in electron-scale turbulence. Non-local turbulence exhibited the rapid spatial propagation of perturbations throughout the plasma, while local turbulence followed the temperature gradient. A transition between drift-wave turbulence and magnetohydrodynamics (MHD) turbulence was observed with the turbulence minimized at the transition condition. Machine learning analysis was employed to evaluate the temperate and density conditions of this turbulence transition. Then, real-time control of fueling and heating was applied to maintain the turbulence transition condition, improving the energy confinement enhancement factor by 20%. In addition, evidence was obtained for collisionless ion heating by energetic-ion-driven geodesic acoustic modes and MHD bursts. These achievements represent unique contributions to the development of fusion reactors.

Experiments on physics issues in burning plasma were conducted in the deuterium campaign of Large Helical Devices (LHD). One is an isotope mixing experiment, and the other is the collisionless energy transfer from energetic particles to bulk ions.The important finding for ion mixing in the LHD experiment is that the ITG turbulence (but not TEM turbulence) contributes to the isotope and ion mixing, which is beneficial for controlling the isotope ratio (deuterium and tritium) and helium ash exhaust. The experimental identification of energy transfer from energetic particles to bulk ions through Landau damping and transit-time damping suggests a possible ion-heating process for the ion-ITB plasma with (T _i /T _e > 1) through collisionless energy transfer from alpha particles to bulk ions through energetic particle-driven instability, so-called alpha channeling.

Understanding helium (He) plasma-induced tungsten (W) surface modifications and the effect of irradiation defects on He plasma-induced W surface modifications under a real divertor environment are important for the operation of fusion reactors. In this study, two iron (Fe) ions pre-irradiated in W samples with dislocation loops and voids, and two unirradiated W samples were exposed to He plasma at the divertor leg position of the Large Helical Device. The gross erosion rate is 1.0 × 10²⁰ atoms m⁻² s⁻¹, 1.0 × 10²⁰ atoms m⁻² s⁻¹, 9.3 × 10¹⁹ atoms m⁻² s⁻¹ and 7.4 × 10¹⁹ atoms m⁻² s⁻¹ for W9, W10, W11 and W12, respectively. The surface of each sample after the exposure was different at the strike point and the two regions on either side of the strike point, the scrape-off layer (SOL) region and the private region. The typical He plasma-induced structures in the SOL region are stripe structure, sawtooth structure and non-undulating structure, which are collectively called He-structures in the present study. At the strike point, the typical He plasma-induced structures are dense W protrusions. In the private flux region, the typical He plasma-induced structures are semi-formed He-structures. The formation of these structures is dependent on the grain orientation. Pinholes were observed in these structures. No significant difference in He plasma-induced structures is found between the pre-irradiated W and the unirradiated W. The formation of non-undulating structure and stripe structure is discussed based on the observed semi-formed He-structures.

Recently, experiments on basic plasma physics issues for solving future problems in fusion energy have been performed on a Large Helical Device. There are several problems to be solved in future devices for fusion energy. Emerging issues in burning plasma are: alpha-channeling (ion heating by alpha particles), turbulence and transport in electron dominant heating helium ash exhaust, reduction of the divertor heat load. To solve these problems, understanding the basic plasma physics of (1) wave–particle interaction through (inverse) Landau damping, (2) characteristics of electron-scale (high-k) turbulence, (3) ion mixing and the isotope effect, and (4) turbulence spreading and detachment, is necessary. This overview discusses the experimental studies on these issues and turbulent transport in multi-ion plasma and other issues in the appendix.

The experimental reproduction of the conditions of pellet injection expected in future large devices being not possible in present day machines, it is mandatory to validate as thoroughly as possible the available ablation models. Among the different points still under discussion, there is the relation between the spectroscopic measurement of the ablation clouds and the local ablation rate. This relation is investigated by coupling an emission model to the time-dependent simulation of ablation clouds with the HPI2 pellet ablation/deposition code. The simulated quantities are the time-evolution of the cloud visible spectrum (λ = 400–700 nm) and images in different wavelength domains (e.g. H α , H β or the continuum centered at λ = 576 nm ). It is found that the cloud emission is anisotropic, this is particularly the case for H _α and H _β lines, and that the relation between the cloud emission and the ablation rate depends not only on the conditions of pellet injection, but also on the direction of observation. It follows that, in general, it is not possible to estimate the ablation profile from that of an emission line ( H α or H β ) . The code predictions are compared with corresponding measurements for a welldocumented pellet injected in LHD, showing a good agreement for global values and main trends. The reasons for observed discrepancies are discussed.

The impact of helium plasma exposure on the tungsten surface damage structure development and erosion has been investigated by comparing the impact of hydrogen plasma exposure. Crystal orientation dependence of the undulating surface structure formation and erosion rate is observed on the plasma-exposed tungsten surface independently from the plasma species. The top surface of the plasma exposed tungsten has a tendency to {100} plane independently from the initial surface orientation. Although hydrogen and/or helium cause no erosion in tungsten under incident ion energy exposure conditions below the sputtering threshold, inevitable minute impurities, like oxygen, play an essential role in erosion, and significant erosion can be observed even at 30 eV.

We have recently incorporated the occupation probability formalism (OPF) in the simulation model [C. Stehlé and S. Jacquemot, Astron. Astrophys. 271, 348 (1993)] to have a smooth transition from discrete lines to continuum spectrum in the wavelength range near the Balmer series limit. We have analyzed spectra measured for the hydrogen pellet ablation cloud in the Large Helical Device with the revised model, and have found that the electron density in the ablation cloud has a close correlation with the electron temperature of the background plasma. This type of correlation is first confirmed in the present analysis and should give a new insight in the simulation studies of pellet ablation for the magnetically confined fusion plasma.

Pellet injection is currently the primary candidate for achieving efficient plasma fuelling, one of the key issues for steady-state operation in large fusion devices. In this paper, pellet injection experiments are performed for several magnetic configurations of the TJ-II stellarator. The aim of this study is to increase the understanding of the role played by rational surfaces in plasmoid drift and deposition profiles in stellarators. The analysis of experimentally observed plasmoid drifts is supported by simulations of such cases made with the HPI2 code. Plasmoid drift is found to be significantly reduced, as in tokamaks, in the vicinity of rational surfaces. This is attributed to the fact that plasmoid external charge reconnection lengths are shorter near rational surfaces, resulting in a more effective damping of the plasmoid drift. Although the effect of plasmoid external currents on the drift is expected to be negligible in stellarators, compared with those caused by plasmoid internal currents, the effect observed in TJ-II is clearly measurable. In addition, simulations show that enhanced drift reductions near rational surfaces lead to significantly different deposition profiles for the magnetic configurations included in this study. This implies that it should be possible to select the magnetic configurations to obtain more efficient pellet fuelling.