Motoi Wada

In the context of ion sources for Neutral Beam Injection (NBI) in magnetically confined nuclear fusion plasmas, negative ion production on two different surface materials exposed to low-temperature, low-pressure deuterium plasma is investigated. A novel electride material, promising for its potential to enable highly efficient negative ion production, is compared to gadolinium, a low work function metal, with the aim of highlighting both similarities and differences in surface-driven negative ion formation. For both materials, negative ion yields and surface work functions are measured. The lowest work function obtained for the electride material is 2.7 eV, higher than the previously reported value of 2.4 eV. No clear correlation is observed between the negative ion yield and the work function for the electride highlighting a behavior with respect to negative-ion production which is not the one of a metal . In contrast, gadolinium exhibits the expected behavior with a clear correlation between yield and work function. The lowest work function measured for gadolinium is 2.2 eV, a value comparable to that of caesium, the material currently used in NBI sources, and significantly lower than the tabulated value for pure gadolinium (2.9 eV), likely reflecting the influence of surface impurities.

Negative carbon (C−) and oxygen (O−) ions are expected to find wide applications for ion implantation processes. However, systematic experimental data required for their efficient generation and the elucidation of their underlying atomic processes are lacking. In this study, we systematically measured the angular and incident beam energy dependencies of the reflected carbon and oxygen ion properties from Mo, W, and low-work function material 12CaO⋅7Al2O3 electride targets, for low-energy (<1 keV) carbon (C+) and oxygen (O+) ion injections. For all targets, both C+ and C− ions were reflected for the C+ ion injection, whereas only the reflected O− ions were observed for O+ ion injection. The energies of C+, C−, and O− ions reflected from the electride and Mo decreased significantly with increasing reflection angle. The effect of the lower work function was an increase in the intensity of the O− ions reflected at larger-incident and smaller-reflection angles by comparing the reflected ions from Mo and electride targets. On the other hand, the effect of the work function on the production of C− ions was small. When 1 keV-O+ ions were incident on W, the fraction of the reflected O− ions was 10%–30%, and the rest were neutral oxygen particles. Good agreement between the measured and calculated incident angular dependency of reflected O neutral particles was observed.

Titanium dioxide (TiO2) has been extensively investigated for applications in photocatalysis due to its stability, nontoxicity, and robust photocatalytic properties. Although thin films of TiO2 are effective in wastewater treatment, their dense structure limits surface accessibility. The oblique angle deposition (OAD) technique can be used to create high surface area nanostructured thin films essential for photocatalytic applications. In this study, OAD was used to grow TiO2 thin films. To extend the activity to the visible light region, plasma-reduced silver (Ag) particles were coupled to the TiO2 surface. The Ag-decorated TiO2 (Ag-TiO2) nanostructured films were fabricated using a custom-built magnetron sputter deposition system with OAD technique capability. Different substrate angles of 0°, 35°, and 70° with respect to the target normal were used. The films were characterized for their structural, compositional, and optical properties. The photodegradation performance was evaluated using methylene blue (MB) as the test analyte under visible light irradiation. The results showed a significant improvement in degradation efficiency using plasma-reduced Ag-TiO2. Substrates tilted at 70° achieved the highest MB degradation efficiency of 89.84% after 5 h exposure. This tilt angle also revealed high surface roughness and high surface area. By tilting the substrate with respect to the target, high surface area thin films can be fabricated. This technique is ideal for applications that depend on the area of contact, such as photocatalysis.

To efficiently generate H− ions from positive atomic or molecular hydrogen beams injected onto a solid surface, it has been suggested to use a material with a low-work function as the target material. However, it is not clear under what conditions the most efficient H− production is realized for incident beam parameters or reflection angles. Therefore, we studied the interaction between low-energy atomic and molecular hydrogen beams (less than 1 keV/nucleon) with a low-work function electride 12CaO⋅7Al2O3 (C12A7). The production ratio of H− to H+ ions from the C12A7 electride was much higher than Mo targets with higher work functions, especially at smaller incident and smaller reflection angles. The H− to H+ production ratio slightly increased as the incident energies were decreased, but there was no significant difference between the electride and Mo targets. These results indicate that smaller incident angles and lower beam energies of the incident hydrogen beam are favorable for the enhancement of the production ratio of H− to H+ ions in C12A7. The higher production ratio appeared at the vertical beam energies less than on the order of 100 eV, where quantum mechanical processes may become important.

A high-speed emittance measurement system installed at the J-PARC RF negative hydrogen (H^-) ion source test stand can construct emittance diagrams of the beam of a specific frequency component. The driving 2 MHz RF power produces small amplitude oscillating components at 2 MHz and at higher harmonic frequencies in the total H^- ion beam detected by a Faraday installed downstream of the emittance measurement system. Magnitudes of oscillating component at 2 MHz and 4 MHz frequencies become appreciable as the slits of the emittance measurement system separate the H^- ion beam into the position/angle resolved phase space elements. The reconstructed emittance diagrams of H^- ion beams for the operation condition with continuous supply of Cs realizing large H^- ion current to total extraction current ratios indicate that 2 MHz and 4 MHz frequency oscillating beam components propagate surrounding the center DC beam. Both 2 MHz and 4 MHz oscillating beam components comprise two to three beam particle groups in the phase space. The DC component of the H^- ion beam exhibit a single peak in the angular distribution. Meanwhile, oscillating components often take the minima where the DC H^- ion beam takes the maximum in the angular distribution diagram. Thus, the regions that produce large amplitude oscillations of the H^- ion beam are considered located at the circumference of the beam extraction aperture. One possible explanation for observing larger amplitude beam oscillation can be the enhanced H^- ion surface production at the plasma facing the wall of the beam extraction aperture. The operation without the Cs delivery to the source increased electron current coextracted with H^- ions and changed the structure of the beam emittance diagram. In the direction perpendicular to the magnetic fields created by a magnetic filter and electron suppression magnets, the peak of the 4 MHz oscillating beam components appeared at the center of the DC beam. This observation suggests the extraction of H^- ions formed in the central region by the volume production process near the internal RF antenna. Mechanisms that can excite and diminish the beam fluctuation amplitudes of the extracted H^- ion beam are discussed.

The effect of varying the water vapor content in a DC magnetron sputtering process was investigated for zinc oxide film formation. The plasma parameters near the substrate surface were measured using a single Langmuir probe, and the deposited films were characterized using X-ray diffraction, X-ray reflectivity, optical transmittance, and 4-point probe methods. In the region near the substrate surface, the addition of water changes the plasma properties, and the measured plasma parameters showed the changes corresponding to the transition in the film growth mechanism from Zn to ZnO. Depositing at 40% water content resulted to a highly transparent film with a ρ of 1.20 Ω cm. The band gap of films deposited at 40%–100% water content ranged from 3.36 to 3.34 eV, which matches the expected shallow hydrogen donor doping in ZnO.

The photocatalytic activity of titanium dioxide (TiO2) was improved by forming a matrix containing silver (Ag) nanoparticles (NPs) prepared by a plasma process. The plasma-assisted reduction process successfully formed metallic Ag on the surface of the TiO2 NP matrix to create the Ag-TiO2 Janus heterostructure. The synthesized NPs were characterized using Brunauer, Emmett, and Teller surface analysis, transmission electron microscopy, x-ray diffraction spectroscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, and UV-vis spectroscopy. Photocatalytic dye decomposition experiments revealed up to 96% methylene blue removal after 4 h of visible light irradiation, a fivefold increase in removal efficiency compared to using TiO2 NPs only.