High-flux Helium Plasma Research Articles

Evolution of liquid lithium corrosion behavior for CLF-1 steels induced by high-flux helium irradiation

The influence of high-flux helium plasma irradiation on compatibility between liquid lithium and CLF-1 steel, a reduced activation ferritic/martensitic steel, was investigated since CLF-1 shows potential application as substrate material for liquid lithium plasma facing components. After exposure to helium plasma with flux of 5 × 1022 m−2s−1, porous fuzz nanostructures formed on the surface of CLF-1 steel, which size increases with the increasing plasma fluence. The plasma-induced weight loss rate of CLF-1 steel decreased gradually with the increasing He fluence. At 350 °C, the contact angle of lithium droplet on smooth CLF-1 surface was about 60°.After irradiation, the fuzz structure improved the wettability of liquid lithium on the CLF-1 surface due to the increasing surface roughness. After the static corrosion of CLF-1 in liquid lithium at 350 °C for 500 h, the significant chromium depletion, decarbonization and the phase transform from martensite to ferrite were found in both non-irradiated and irradiated samples. The porous fuzz structure significantly aggravates the corrosion behavior. The corrosion occurred preferentially at the grain boundaries for the non-irradiated sample, while the most severe corrosion location was on the top of fuzz protrusion for the irradiated samples.

Evolution of vacancy defects in heavy ion irradiated tungsten exposed to helium plasma

Evolution of vacancy-type defects has been investigated in undamaged and copper ion pre-damaged tungsten exposed to low-energy and high-flux helium plasma (60 eV, 1 × 1022 He/m2s). The results measured by Doppler broadening positron annihilation spectroscopy (DB-PAS) indicate that helium-vacancy complexes generate due to intense self-trapping in the undamaged tungsten after helium plasma exposure. In contrast, the occupation of pre-existing vacancies and vacancy clusters, caused by the presence helium atoms, plays a dominant role in the pre-damaged tungsten, but the density of vacancy-type defects in the pre-damaged case is still higher than that in the undamaged case. This means that the dominating process of helium-vacancy complexes formation in tungsten changes from self-trapping to vacancy-trapping in the case of high vacancy density. Meanwhile, the elastic recoil detection analysis (ERDA) and transmission electron microscopy (TEM) results reveal that these pre-existing defects can increase helium retention and helium nano-bubble size. However, it is surprising that in the case of pre-damaged sample, the density/volume of vacancy-type defects also decreased even outside of the helium distribution depth. We attribute this phenomenon to the interstitial dislocation loops punched by helium clusters preferentially diffusing into the pre-damage regions of tungsten far beyond the helium distribution depth, resulting in the significant recombination with vacancies or vacancy clusters. This intrinsic mechanism is further verified by TEM observations and molecular dynamics (MD) simulations.

Formation of Nanostructured Tungsten with Arborescent Shape due to Helium Plasma Irradiation

Deeply nanostructured tungsten with an arborescent shape was found for the first time to be formed on tungsten-coated graphite by a high-flux helium plasma irradiation at surface temperatures of 1250 and 1600 K, an incident ion energy of 12 eV (well below the physical sputtering threshold) and a helium ion fluence of 3.5 × 1027 m-2.

Micron-Bubble Formation on Polycrystal Tungsten due to Low-Energy and High-Flux Helium Plasma Exposure

Surface modification of polycrystal tungsten due to low-energy and high-flux helium plasma exposure was investigated. Micron-sized holes and bubbles were formed on the surface at a surface temperature of 2200 K even when the incident ion energy of helium was 10 eV. Hole formation was significantly reduced on the surface when the incident ion energy of helium was around 5 eV and no surface modification was seen at less than 5 eV. Coalescence of micron-sized helium bubbles was observed in the cross section of a W sample.

Formation mechanism of bubbles and holes on tungsten surface with low-energy and high-flux helium plasma irradiation in NAGDIS-II

A systematic study on the formation mechanism of micron-sized He bubbles and holes in powder metallurgy tungsten due to helium ion irradiation with an ion energy below 30 eV and a particle flux above 10 22 m −2 s −1 has been performed in the linear divertor plasma simulator NAGDIS-II. Holes are formed with incident helium ion energy above 5 eV, which could be related to the surface barrier potential energy for He penetrating into tungsten. Tungsten surface temperature strongly influences the number and size of hole. Above 1600 K, bubbles and/or holes with several hundreds nano-meter diameter appear on the tungsten surface. Single crystal tungsten, which has much fewer intrinsic defects than powder metallurgy tungsten, was also irradiated by He plasmas. There is no qualitative difference in the hole formation between the two grades of tungsten. Bubble and hole formation mechanisms are discussed based on the experimental results.

Surface morphology and helium retention on tungsten exposed to low energy and high flux helium plasma

Surface modification and helium retention on powder metallurgy tungsten and plasma sprayed tungsten coated CFC exposed by low energy (100 eV) high flux (3.83×10 21–1.20×10 22 m −2 s −1) helium plasma have been investigated. Fluence is about 10 26 He m −2. It is found that fine surface morphology change occurs by the helium exposure. There is little difference of the surface modification between the powder metallurgy tungsten and the plasma sprayed tungsten. In the case that the surface morphology do not changed, two helium desorption peak appeared. The amount of desorption of helium is larger than that of deuterium from tungsten under the same exposure condition by heating up to 1273 K by TDS. In the case that fine surface morphology change occurs, desorption peak at high temperature newly appears. Surface modification such as blister is not formed on the surface exposed to helium under the same exposure condition which the blister is formed by deuterium exposure.

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

Tungsten (W) specimens were irradiated by low energy (<65 eV) and high flux (∼ 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.