Dogyun Hwangbo

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.

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.

Nano-tendril bundles (NTBs) are clusters of fuzzy nanostructures that form on tungsten surfaces when they are exposed to helium plasmas under specific conditions, reaching lengths of over 100 µm. Tungsten samples with NTBs and fuzz were exposed to high-density hydrogen plasmas in the Upgraded Pilot-PSI linear plasma device to investigate their response, with a particular focus on unipolar arc ignition and hot spot formation. Although NTBs are prone to arcing due to their high field electron emission characteristics, unipolar arcs have never occurred on NTBs. Post-mortem analysis showed that the heights of the NTBs changed significantly during irradiation mainly due to rapid heating, and that these changes strongly influenced their interaction with high-density plasmas.

Nano-tendril bundles (NTBs) were formed on tungsten via helium (He) plasma exposure with various additional impurity gases, such as neon (Ne), nitrogen (N₂), and argon (Ar). The sizes of the NTBs showed different distributions with different additional impurity gases. The field emission property of the NTBs formed with various additional impurity gases was measured. The field-emission property was significantly affected by the morphology of the NTBs, especially the tips of the fibers. In the Ne- and Ar-seeded cases, the NTBs were formed with sharp tips, and the onset electric field for field emission was ∼1 kV mm⁻¹ for all the NTB samples. The Ne-seeded samples showed the most rapid increase in the emission current. In the N₂-seeded case, two types of NTBs were formed. The NTBs were formed with sharp tips when the ratio of N₂ impurity gas was 2.1%. With an increase in the ratio to 3.0% or higher, the fibers of the NTBs became thicker and the tips became rounder. In the Fowler–Nordheim (F-N) plot analysis, the field enhancement factors were approximately 6000–7000 without significant differences, for all NTBs with sharp tips. NTBs with round tips showed totally different field emission property, as the emission current was only several μA, which is one-tenth of that for the other samples. This suggests that the local morphology of NTBs, especially the geometrical shape of the tips, and not the general size of NTBs, is the main factor in determining the field emission property of NTBs.

Arc spot motions on nanostructrued “fuzz” tungsten surfaces are investigated on the basis of a two-dimensional Monte Carlo random walk simulation. To compare with previous experimental results, the effects of self-avoiding, bifurcation and extinction of arc spots, as well as the fuzz thickness dependence, are included in the simulation. Under a given temporal arc current profile which is set as close as possible to the experiment, the results derived from the simulation reveal that the motion and the number of arc spots depend significantly on the fuzz thickness.