Hirohiko Tanaka

Transient heat loads like Edge Localized Modes (ELMs) can degrade detached plasmas that are essential for reducing divertor heat fluxes in future fusion reactors. To investigate the response of detached plasmas to transient electron energy deposition, a pulsed electron beam was injected into a detached helium plasma in the linear plasma device NAGDIS-II. Spatiotemporal evolutions of electron temperature and density were measured using laser Thomson scattering synchronized with the beam injection with high-temporal resolution. Despite the small beam source diameter (2 mm), significant changes in plasma parameters extended over a wide radial range (> 15 mm). The electron beam rapidly increased the electron temperature near the plasma center, resulting in a local transition from a recombining plasma to an ionizing plasma. In contrast, the electron density exhibited a slower temporal evolution, suggesting different underlying mechanisms governing temperature and density responses. The measured spatiotemporal plasma parameters were further analyzed using a collisional-radiative model and reproduced previously observed line-integrated emission dynamics. These results demonstrate that localized electron energy deposition can induce a global response in detached plasmas and provide new insight into the stability and dynamic behavior of detached plasmas subjected to transient energy loads.

We developed an advanced diagnostic system by adding an image intensifier and wavelength filters to the conventional high-speed camera setup to analyze rotating plasma ejection events in detached plasma which can be realized in the liner magnetized plasma device NAGDIS-II. The gating capability of the image intensifier achieved temporal resolution exceeding the camera frame rate, while conditional averaging and tomography processing revealed detailed structures of high-speed phenomena previously undetectable. Multi-wavelength measurements enabled spatial mapping of electron temperature distribution through intensity ratio analysis. This enhanced technique provides the first quantitative correlation between electron temperature evolution and spatial development of rotating ejection events.

A multi-ion-temperature model for hydrogen plasmas has been newly developed and implemented in the linear divertor plasma simulation code LINDA-NU to investigate detachment mechanisms involving atomic and molecular processes. In this model, H+, H2+, and H3+ ions are treated as separate fluids with independent temperatures, enabling a detailed assessment of ion-species-dependent thermal and momentum dynamics in detached plasmas. Plasma detachment transitions were studied by scanning atomic and molecular neutral densities. An increase in atomic hydrogen density leads to electron–ion recombination-dominated detachment, where charge exchange with atomic neutrals governs momentum loss. In contrast, increasing molecular hydrogen density results in molecular-activated recombination (MAR)-dominated detachment, in which molecular ions become quantitatively significant and strongly influence particle, momentum, and energy balances. A comparison with a simplified single-ion-temperature model demonstrates that the multi-ion-temperature treatment is essential for accurately describing the MAR-dominated regime, where multi-ion temperatures significantly modify molecular reaction rates, leading to qualitative differences in plasma profiles.

Conventional modeling of two-stage wave-heating magnetoplasma thrusters faces fundamental limitations in capturing the intrinsically interconnected processes including plasma generation, transport, and wave heating. Thus, there is a huge gap between theoriodical studies and applications.Presented here is an integrated self-consistent model for the two-stage wave-heating magnetoplasma thruster by developing a multi-module framework iteratively linking specialized solvers for various processes important for thrust and specific impulse, which are crucial for space missions. This approach achieves the first self-consistent whole-device simulation of the 50-kW STAR thruster, resolving the complete process chain from helicon plasma generation to ion cyclotron acceleration.The results provide the three-dimensional distribution characteristics of plasma/neutral-gas density and electron/ion temperature, which are key parameters for aerospace thrusters. Model benchmark shows good agreement against STAR experiments. Furthermore, the correlations of plasma generation, transport, wave energy deposition, and their influences on plasma parameters are revealed, providing a foundation for thrust/specific-impulse throttling in deep-space missions.

Tungsten co-deposited layers in fusion devices are a significant potential source of plasma-contaminating dust. This study investigates the mechanisms of dust release from helium-tungsten (He-W) and deuterium-tungsten (D-W) layers under high-density steady-state plasma and ELM-like plasma pulse superposition. High-speed imaging revealed different emission behaviors: the D-W layer, featuring pre-existing blisters, released dust immediately upon plasma exposure, while the He-W layer showed a delayed emission requiring damage accumulation from several pulses. Post-mortem SEM analysis confirmed distinct surface exfoliation corresponding to these behaviors. The immediate D-W dust release was identified as the rupture of inherently fragile blisters. In contrast, the delayed release of He-W dusts resulted from subsurface flaking, initiated by horizontal cracks forming from the interconnection of internal nanocavities. Both layers produced substantial dust, leading to much higher erosion rates than that of pristine tungsten. These results demonstrate that the trapped gas species fundamentally dictates the co-deposited layer's microstructure and subsequent dust emission pathway, establishing these layers as a critical and rapid dust source under transient plasma loads.

The dependence of the metastable state (23S) helium (He) density and temperature on the neutral gas pressure, Pn, in detached plasmas, was investigated in the NAGDIS-II linear plasma device using laser absorption spectroscopy and supporting diagnostics. Particular attention was paid to two operation conditions with different neutral gas residence times: (i) a low puffing and low pumping rates condition (long residence time) and (ii) a high puffing and high pumping rates condition (short residence time). Under condition (i), the metastable density exhibited a gradual increase, and the temperature remained nearly constant with increasing Pn. In contrast, under condition (ii), the metastable density increased by nearly an order of magnitude with increasing Pn, accompanied by an increase in the metastable temperature. This difference indicates that strong electron–ion volume recombination occurs under the short-residence condition (ii), leading to the generation of high-density and high-temperature metastable atoms. Optical emission and target ion flux measurements support the enhanced contribution of volume recombination processes under condition (ii). Simulations with the DISCOVER code reproduced these behaviors, showing that metastable atoms are mainly supplied by direct excitation in the upstream region and by recombination in the downstream region, with their temperature reflecting the local ion temperature. These findings highlight that the neutral gas residence time, in addition to Pn, provides an independent and effective control parameter for plasma detachment and metastable atom dynamics.

The permeation and retention of hydrogen isotopes in nuclear materials is critical to issues such as hydrogen embrittlement, tritium self-sufficiency, and radioactive safety in fusion reactors. Tungsten, the primary candidate for plasma-facing materials, undergoes high-flux helium plasma irradiation in fusion devices that may alter hydrogen isotope behaviors. However, few works have focused on the effects of helium plasma–induced surface modification on hydrogen permeation. This study investigates the effects of helium-induced surface fuzz structure on deuterium permeation through tungsten. Our findings indicate that the surface fuzz structure can significantly enhance deuterium permeation through tungsten, with the degree of enhancement positively correlated to both the thickness of the fuzz structure and the permeation temperature. In addition, as the temperature decreases, the contribution of the tungsten oxide layer covering the nanotendrils that hinder deuterium diffusion gradually becomes stronger, resulting in the weakening of the overall permeation enhancement. The influence of the fuzz characteristics on deuterium transport properties is discussed. This work implies that helium plasma–induced surface modification can considerably the deuterium transport behaviors in tungsten.

Pulsed plasma experiments simulating the transient divertor heat loads caused by edge-localized modes (ELMs) were conducted in Magnum-PSI, a linear plasma device capable of generating ITER-relevant conditions. The dynamic responses of detached recombining helium (He) plasmas to the pulsed plasma were observed through time-resolved laser Thomson scattering measurements and optical emission spectroscopy. Experimental results indicate that He atom depletion occurs in high-density pulsed plasmas due to a reduced mean-free-path and increased plasma pressure. This limits plasma-neutral interactions, suppressing energy dissipation via direct excitation and electron–ion recombination processes. The He II and He I emissions were found to peak sequentially, suggesting that doubly-ionized He ions recombine first, followed by the recombination of singly-ionized He ions. These findings suggest that the neutral penetration and recombination efficiency are significantly reduced in pulsed plasmas, challenging heat flux mitigation in fusion divertors. The results provide critical insights into the behavior of He ash transported by ELMs and plasma detachment under high-density pulsed conditions, highlighting the need for further studies on neutral fueling strategies in ITER-relevant environments.

The core-degradation effect due to tungsten (W) - a proposed plasma-facing material for the operation of future nuclear fusion reactors such as ITER and DEMO - calls for the study of sputtered W. In this study, sputtered W from a point source in the linear divertor plasma simulator, NAGDIS-II, was investigated. A hyperspectral imaging (HSI) camera was used to image the spatial profile of the sputtered W in helium and argon mixture plasma with different incident ion energies. By applying the Abel transform and fittings with several functions and comparing with the theoretical value, it is found that the ionization effect is not obvious but the geometrical spreading effect is dominant for the axial decay of the local W emission profile.