Challenges in Developing Materials for Fusion Technology - Past, Present and Future

Characterization of plasma parameters and neutral particles in microwave and radio frequency discharges in the Toroidal Magnetized System

A characterization of plasma parameters and neutral particle energies and fluxes has been performed for radio frequency and microwave discharges in the Toroidal Magnetized System (TOMAS). A movable triple Langmuir probe was used to study the electron densities and temperatures, and a time-of-flight neutral particle analyzer was used to measure the energy and fluxes of neutral particles, as a function of the total injected power and the antenna frequency used to generate the plasma. The experimental results can provide information on the behavior of neutral particles at low energies in wall conditioning plasmas.

Microstructure and High-Temperature Performance of High K-Doped Tungsten Fibers Used as Reinforcement of Tungsten Matrix

Tungsten (W) fiber-reinforced tungsten (Wf/W) composite with ultra-high strength and high-temperature resistance is considered an attractive candidate material for plasma-facing materials (PFM) in future fusion reactors. The main component of Wf/W composite is tungsten wire, which is obtained through powder metallurgy and the drawing process. In this paper, high potassium (K)-doped tungsten wires with 98 ppm of K and 61 ppm of impurities are prepared using traditional and optimized processing technologies, respectively, and a comparative study with conventional K-doped tungsten wires with 83 ppm of K and 80 ppm of impurities is conducted. The high-temperature mechanical properties as well as the microstructure’s evolution of the prepared tungsten wires are investigated. The results show that the high-temperature performance of K-doped tungsten wires is improved by increasing the K content and by simultaneously reducing the impurities. By adopting small compression deformation and low-temperature processing technology, the high-temperature performance of high K-doped tungsten wires can be further improved. A microstructure analysis indicates that the excellent high-temperature performance is attributed to a combination of the small K bubble size, high K bubble number density, and long K bubble string, which are produced through optimization of the processing technology. A study on the processing technology and the performance of tungsten wires with a high K content and a high purity can provide important information regarding Wf/W composites.

Effect of Nano-Y2O3 Content on Microstructure and Mechanical Properties of Fe18Cr Films Fabricated by RF Magnetron Sputtering

In this work, FeCr-based films with different Y₂O₃ contents were fabricated using radio frequency (RF) magnetron sputtering. The effects of Y₂O₃ content on their microstructure and mechanical properties were investigated through scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), inductive coupled plasma emission spectrometer (ICP) and a nanoindenter. It was found that the Y₂O₃-doped FeCr films exhibited a nanocomposite structure with nanosized Y₂O₃ particles uniformly distributed into a FeCr matrix. With the increase of Y₂O₃ content from 0 to 1.97 wt.%, the average grain size of the FeCr films decreased from 12.65 nm to 7.34 nm, demonstrating a grain refining effect of Y₂O₃. Furthermore, the hardness of the Y₂O₃-doped FeCr films showed an increasing trend with Y₂O₃ concentration, owing to the synergetic effect of dispersion strengthening and grain refinement strengthening. This work provides a beneficial guidance on the development and research of composite materials of nanocrystalline metal with a rare earth oxide dispersion phase.

Theoretical Model of Helium Bubble Growth and Density in Plasma-Facing Metals

We present a theoretically-motivated model of helium bubble density as a function of volume for high-pressure helium bubbles in plasma-facing tungsten. The model is a good match to the empirical correlation we published previously [Hammond et al., Acta Mater. 144, 561–578 (2018)] for small bubbles, but the current model uses no adjustable parameters. The model is likely applicable to significantly larger bubbles than the ones examined here, and its assumptions can be extended trivially to other metals and gases. We expect the model to be broadly applicable and useful in coarse-grained models of gas transport in metals.

Evolution of Dislocation Loops Induced by Different Hydrogen Irradiation Conditions in Reduced-Activation Martensitic Steel

Hydrogen can be induced in various ways into reduced-activation ferritic/martensitic (RAFM) steels when they are used as structural materials for advanced nuclear systems. However, because of the fast diffusion of hydrogen in metals, the effect of hydrogen on the evolution of irradiation-induced defects was almost neglected. In the present work, the effect of hydrogen on the evolution of dislocation loops was investigated using a transmission electron microscope. Specimens of reduced-activation ferritic/martensitic (RAFM) steels were irradiated with hydrogen ions to 5 × 10²⁰ H⁺ • m⁻² at 523–823 K, and to 1 × 10²⁰ H⁺ • m⁻² − 5 × 10²⁰ H⁺ • m⁻² at 723 K. The experimental results reveal that there is an optimum temperature for dislocation loop growth, which is ~723 K, and it is greater than the reported values for neutron irradiations. Surprisingly, the sizes of the loops produced by hydrogen ions, namely, 93 nm and 286 nm for the mean and maximum value, respectively, at the peak dose of 0.16 dpa under 723 K, are much larger than that produced by neutrons and heavy ions at the same damage level and temperature. The results indicate that hydrogen could enhance the growth of loops. Moreover, 47.3% 12 a₀ <111> and 52.7% a₀ <100> loops were observed at 523 K, but 12 a₀ <111> loops disappeared and only a₀ <100> loops existed above 623 K. Compared with the neutron and ion irradiations, the presence of hydrogen promoted the formation of a₀ <100> loops.

Investigation of Helium Behavior in RAFM Steel by Positron Annihilation Doppler Broadening and Thermal Desorption Spectroscopy

The behavior of helium in reduced-activation ferritic/martensitic steels was investigated systematically with positron annihilation Doppler broadening measurement and thermal desorption spectroscopy. Specimens were irradiated with helium ions with different energies to various fluences at different temperatures. A threshold fluence was observed above which the rate of formation and growth of helium bubbles dramatically increased. Irradiation at higher temperature could suppress the formation and growth of He_nV_m clusters with low binding energies and enhance that of helium bubbles and He_nV_m clusters with high binding energies. Different changes of S parameters were observed in various depth after the irradiation temperature was increased from 523 K to 723 K. Irradiation of 18 keV-He⁺ enhanced the growth of He_nV_m clusters and helium bubbles compared with 100 keV-He⁺ irradiation. A possible mechanism is discussed.