Nuclear Fusion · 2007 · 190 citations · 27 references
Natural SciencesMaterials SciencePhysicsMetallurgyPlasma PhysicsSurface ScienceFusion PlasmasLaser-plasma InteractionsPlasma-surface InteractionsPlasma-wall InteractionsPowder MetallurgyRadiation Damage In MaterialsHelium Bubble FormationPlasma BubblesRadiation-induced Surface EffectsTransient Heat FlowTransient Heat LoadsTungsten Divertor MaterialsTungsten Divertor TechnologyTungsten-based Materials
The pulse width of the ruby laser (~0.6 ms) matches the expected heat load from type‑I edge localized modes in ITER. The study used a long‑pulse ruby laser in the NAGDIS‑II divertor simulator to examine the effects of transient heat loads on tungsten damaged by helium plasma irradiation. Laser pulses with sufficient energy removed helium‑induced holes and bubbles from tungsten, demonstrating that transient heat loads similar to ITER ELMs can mitigate such damage, while also producing melting traces and nanostructuring that markedly reduces surface thermal conductivity. Technol.
The effects of a transient heat load on tungsten damaged by helium plasma irradiation have been investigated using a ruby laser with long pulse duration in the divertor simulator NAGDIS-II (Takamura et al 2002 Plasma Sources Sci. Technol. 11 A42 ). The pulse width of the ruby laser was ∼0.6 ms, which is close to that of the expected heat load accompanied by type-I edge localized modes (ELMs) in ITER operation. Helium holes/bubbles, which were formed in the surface region of powder metallurgy tungsten due to the exposure to the helium plasma, disappeared after the laser pulse irradiation to the tungsten surface with sufficient pulse energy. The results indicated that the transient heat loads similar to those expected by ELMs will mitigate damages such as bubbles and holes produced by helium irradiation. When a vacuum plasma sprayed tungsten coating on graphite was exposed to the helium plasma, the surface was covered with arborescent nanostructured tungsten containing many helium bubbles inside the structure. Melting traces were found on the surface after the laser pulses irradiated the surface even though the pulse energy was lower than that for melting bulk tungsten. A numerical temperature calculation of the sample suggested that the effective thermal conductivity near the surface dramatically decreased by several orders of magnitude due to the formation of nanostructured tungsten.
27