Shuichi Takamura

Sub-ms laser pulse irradiation on tungsten target damaged by exposure to helium plasma

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.

Incident ion energy dependence of bubble formation on tungsten surface with low energy and high flux helium plasma irradiation

Abstract Tungsten (W) specimens were irradiated by low energy ( 10 22–23 m −2 s −1 ) helium plasma to investigate the incident ion energy dependence of helium bubble formation. Experimental results indicate the existence of the incident ion energy threshold for the bubble formation. The threshold energy around 15 eV could associate with the surface potential for He ions entering to the W surface.

Prompt ignition of a unipolar arc on helium irradiated tungsten

A fibreform nanostructured layer is formed on a tungsten surface by helium plasma bombardment. The helium fluence was of the order of 1026 m−2, and the surface temperature and incident ion energy during helium irradiation were, respectively, 1900 K and 75 eV. By irradiating a laser pulse to the surface in the plasma, a unipolar arc, which many people have tried to verify in well-defined experiments, is promptly initiated and continued for a much longer time than the laser pulse width. The laser pulse width (~0.6 ms) and power (~5 MJ m−2) are similar to the heat load accompanied by type-I edge localized modes (ELMs) in ITER. The unipolar arc is verified from an increase in the floating potential, a moving arc spot detected by a fast camera and arcing traces on the surface. This result suggests that the nanostructure on the tungsten surface formed by the bombardment of helium, which is a fusion product, could significantly change the ignition property of arcing, and ELMs become a trigger of unipolar arcing, which would be a great impurity source in fusion devices.

Helium I line intensity ratios in a plasma for the diagnostics of fusion edge plasmas

Helium I line intensity ratios obtained by a collisional radiative model, including new recommended excitation rate coefficients and the effects of hot electrons, enable us to measure electron density (ne) and temperature (Te) in high density plasma. Measured ne and Te using 492.2 nm (4 1D→2 1P)/471.3 nm (4 3S→2 3P), 504.8 nm (4 1S→2 1P)/471.3 nm, and 492.2 nm/504.8 nm line intensity ratios are in good agreement with the Langmuir probe results in the helium discharge plasma in the NAGDIS‐I linear device (Nagoya University Divertor Simulator) for ne and Te regions of 1011–4×1012 cm−3 and 5–20 eV. Hot electrons in the plasma are important for the He I line emissions when Te is below the excitation energy ≊20 eV. A resonance scattering effect included in the calculation accounts for the experimental result of the enhanced 501.6 nm (3 1P→2 1S) line emission.

Phillips–Tikhonov regularization of plasma image reconstruction with the generalized cross validation

The use of the Phillips–Tikhonov regularization is proposed for numerically stabilizing the ill‐conditioned plasma image reconstructions. An objective function to be minimized leads to a linear estimator of the image intensity distribution and, with the aid of the singular value decomposition, makes it possible to use the generalized cross validation for optimizing a regularization parameter. An excellent behavior of the estimator with computational facility is obtained on the Hα emission computerized tomography of a toroidal plasma.

Comparison of He I line intensity ratio method and electrostatic probe for electron density and temperature measurements in NAGDIS-II

The electron density and temperature obtained from the line intensity ratio method of HeI (λ=667.8, 706.5, and 728.1nm) are compared to the probe method in a divertor simulator. When a collisional radiative model that does not include the effect of the radiation transport was used for the analysis, ne obtained from the spectroscopic method was significantly higher than that from the electrostatic probe method. The discrepancy between the two methods increases with the gas pressure; in other words, it increases with the optical thickness. In the case that the effect of the radiation trapping is taken into consideration using optical escape factor, the discrepancy becomes moderate. And then, the parameters obtained from the line intensity ratio method agree with the probe method within a factor of 2 in the case that the radiation trapping was introduced with R=0.05m, which corresponds to the column radius of the spatial profile of the excited population density. In recombining plasmas, however, it was shown...

Effects of Ion Flow by E×B Drift on Dust Particle Behavior in Magnetized Cylindrical Electron Cyclotron Resonance Plasmas

Interactions between poloidal ion flow associated with E×B drift and dust particles have been investigated in magnetized cylindrical electron cyclotron resonance plasmas, in which radial electric field is controlled by a biased ring-shaped electrode. The dust particles are spun out and removed from the plasma due to poloidal ion drag force, even when the inward electric force on the dust particles is larger than the gravitational force. This indicates the possibility of using a new method to remove the dust particles from the processing plasma. On the other hand, in order to sustain the dust particles in the plasma, the effects of this poloidal ion flow should be substantially reduced by the friction between the dust particles and neutral gas.

Suppression of blister formation and deuterium retention on tungsten surface due to mechanical polishing and helium pre-exposure

Low-energy deuterium (D) plasma exposure on tungsten (W), which is an important material for ITER, results in blister formation on the surface. Blister formation increases both micron-sized dust production and D retention. Blister formation depends greatly on surface pre-treatment. Deuterium plasma exposure on mirror-finished powder metallurgy W at 500 K for 3 h forms a blister with a diameter of a few hundred micrometres on the surface. Blister formations on the mechanically-polished and helium-pre-exposed surfaces are drastically suppressed. Deuterium retention is also reduced on both the mechanically-polished surface and the helium-pre-exposed surface compared with that on the mirror-finished surface. The suppressive effect of blister formation on the mechanically-polished surface is maintained for 50 h. The size of blisters and D retention on mirror-finished surface exposed for 50 h increases by some degrees (500 µm, 7 × 1020 m−2) compared with that on mirror-finished surface exposed for 3 h (200 µm, 5 × 1020 m−2), but is not proportional to the exposure time. The saturation level of D retention even on blister-rich surfaces seems to be lower than an order of 1021 D2 m−2.

Molecular dynamics and Monte Carlo hybrid simulation for fuzzy tungsten nanostructure formation

For the purposes of long-term use of tungsten divertor walls, the formation process of the fuzzy tungsten nanostructure induced by exposure to the helium plasma was studied. In the present paper, the fuzzy nanostructures formation has been successfully reproduced by the new hybrid simulation method in which the deformation of the tungsten material due to pressure of the helium bubbles was simulated by the molecular dynamics and the diffusion of the helium atoms was simulated by the random walk based on the Monte Carlo method. By the simulation results, the surface height of the fuzzy nanostructure increased only when helium retention was under the steady state. It was proven that the growth of the fuzzy nanostructure was brought about by bursting of the helium bubbles. Moreover, we suggest the following key formation mechanisms of the fuzzy nanostructure: (1) lifting in which the surface lifted up by the helium bubble changes into a convexity, (2) bursting by which the region of the helium bubble changes into a concavity, and (3) the difference of the probability of helium retention by which the helium bubbles tend to appear under the concavity. Consequently, the convex-concave surface structure was enhanced and grew to create the fuzzy nanostructure.

Reduction of laser power threshold for melting tungsten due to subsurface helium holes

Interactions between nanosecond laser pulses and a tungsten substrate having submicron holes near the surface formed by exposure to helium plasmas are investigated experimentally and numerically. After tungsten surface having helium holes was irradiated by nanosecond laser pulses in helium plasmas, scanning electron microscope micrographs of the tungsten surface show that the roughness of the surface is significantly enhanced under certain experimental conditions. For an understanding of the physical mechanisms to arise the phenomena, heat conduction in the substrate having holes is modeled by solving a three-dimensional heat conduction equation. The model calculations show that the surface reaches a melting point locally even if the pulse energy is low enough to lead to the melting of a virgin substrate. On the basis of surface temperature calculations and from an evaluation of the tensile stress put on the lid of the hole, repetitive explosions of the helium holes caused by heating the lids are consider...