Optical transmittance investigation of 1-keV ion-irradiated sapphire crystals as potential VUV to NIR window materials of fusion reactors

We investigate the optical transmittances of ion-irradiated sapphire crystals as potential vacuum ultraviolet (VUV) to near-infrared (NIR) window materials of fusion reactors. Under potential conditions in fusion reactors, sapphire crystals are irradiated with hydrogen (H), deuterium (D), and helium (He) ions with 1-keV energy and ∼ 1020-m-2 s-1 flux. Ion irradiation decreases the transmittances from 140 to 260 nm but hardly affects the transmittances from 300 to 1500 nm. H-ion and D-ion irradiation causes optical absorptions near 210 and 260 nm associated with an F-center and an F+-center, respectively. These F-type centers are classified as Schottky defects that can be removed through annealing above 1000 K. In contrast, He-ion irradiation does not cause optical absorptions above 200 nm because He-ions cannot be incorporated in the crystal lattice due to the large ionic radius of He-ions. Moreover, the significant decrease in transmittance of the ion-irradiated sapphire crystals from 140 to 180 nm is related to the light scattering on the crystal surface. Similar to diamond polishing, ion irradiation modifies the crystal surface thereby affecting the optical properties especially at shorter wavelengths. Although the transmittances in the VUV wavelengths decrease after ion irradiation, the transmittances can be improved through annealing above 1000 K. With an optical transmittance in the VUV region that can recover through simple annealing and with a high transparency from the ultraviolet (UV) to the NIR region, sapphire crystals can therefore be used as good optical windows inside modern fusion power reactors in terms of light particle loadings of hydrogen isotopes and helium.Iwano K., Yamanoi K., Iwasa Y., et al. Optical transmittance investigation of 1-keV ion-irradiated sapphire crystals as potential VUV to NIR window materials of fusion reactors. AIP Advances 6, 105108 (2016); https://doi.org/10.1063/1.4965927

Analysis of the frequency dependence of the AC loss in MgB2 based on the eddy current model

The frequency dependence of the AC loss peak in x (T) of MgB2 is analyzed using the eddy current model. In this model, eddy current loops are induced in the grain of a superconductor subject to small AC fields. This current, which generates lhe loss peak, is proportional to the area enclosed by the loops and inversely proportional to the resislance of the loops. The AC loss peak is a result of the competition between the rapidly diminishing resistance of the material below Tc and the decreasing area where the hannonic field can thread as lhe penetralion depth λ decreases in the superconducting state [1]. Utilizing equations for temperature dependent penetration depth and electrical resistance, a fitting function for x (T) which is proportional to i(T)/Bac is obtained. A circular grain of radius r is assumed. For a particular λ, the area where the field can penelrates is equal to A(T) =πr2 -(r -λ(T))2 (2) The temperature dependence of the penetration depth is chosen such that A(T) attains a zero value at low temperatures. The resulting eddy current equation is (2) At Tc, A.=lo and is very large compared to the grain radius; r. At temperatures here r is less than λ, the penetrated area is maintained at A (T) = πr2 . lc is a fitting parameter that describes the rate of decrease of λ(T). R0 is the value of the electrical resistance at Tc. R0 is constant for all measurement since the material is in a region where the magnetization varies linearly with field. An increase in R0 indicates a nonlinear magnetization. b is a fitting parameter that describes the rale of decrease of R(T). The superimposed plots of the experimental data and the resulting curve employing the eddy current model are shown in the figure. The corresponding model fit show satisfactory agreement with the experimental X (T) measurements. As the frequency is increased from 200 to 3200Hz, 1c decreases. This means that λ decreases faster at a higher frequency and the flux penetrated area attain the zero value faster. Thus. the penetration depth decreases with increasing frequency. On the other hand, the fitting parameter b, increases as the frequency is increased from 200 to 3200Hz. This equivalently means that the rate of change of R(T) is decreased with increased frequency. Thus, the resislance increases with frequency. This behavior is similar to the behavior of metals in the presence of AC field. Thus, indicating that MgB2 indeed a highly metallic material as previously observed

Synthesis of Iron Oxide Nanostructures via Carbothermal Reaction of Fe Microspheres Generated by Infrared Pulsed Laser Ablation

Iron oxide nanostructures were synthesized using the carbothermal reaction of Fe microspheres generated by infrared pulsed laser ablation. The Fe microspheres were successfully deposited on Si(100) substrates by laser ablation of the Fe metal target using Nd:YAG pulsed laser operating at λ = 1064 nm. By varying the deposition time (number of pulses), Fe microspheres can be prepared with sizes ranging from 400 nm to 10 µm. Carbothermal reaction of these microspheres at high temperatures results in the self-assembly of iron oxide nanostructures, which grow radially outward from the Fe surface. Nanoflakes appear to grow on small Fe microspheres, whereas nanowires with lengths up to 4.0 μm formed on the large Fe microspheres. Composition analyses indicate that the Fe microspheres were covered with an Fe3O4 thin layer, which converted into Fe2O3 nanowires under carbothermal reactions. The apparent radial or outward growth of Fe2O3 nanowires was attributed to the compressive stresses generated across the Fe/Fe3O4/Fe2O3 interfaces during the carbothermal heat treatment, which provides the chemical driving force for Fe diffusion. Based on these results, plausible thermodynamic and kinetic considerations of the driving force for the growth of Fe2O3 nanostructures were discussed

Optical transmittance investigation of 1-keV ion-irradiated sapphire crystals as potential VUV to NIR window materials of fusion reactors

We investigate the optical transmittances of ion-irradiated sapphire crystals as potential vacuum ultraviolet (VUV) to near-infrared (NIR) window materials of fusion reactors. Under potential conditions in fusion reactors, sapphire crystals are irradiated with hydrogen (H), deuterium (D), and helium (He) ions with 1-keV energy and ∼ 1020-m-2 s-1 flux. Ion irradiation decreases the transmittances from 140 to 260 nm but hardly affects the transmittances from 300 to 1500 nm. H-ion and D-ion irradiation causes optical absorptions near 210 and 260 nm associated with an F-center and an F+-center, respectively. These F-type centers are classified as Schottky defects that can be removed through annealing above 1000 K. In contrast, He-ion irradiation does not cause optical absorptions above 200 nm because He-ions cannot be incorporated in the crystal lattice due to the large ionic radius of He-ions. Moreover, the significant decrease in transmittance of the ion-irradiated sapphire crystals from 140 to 180 nm is related to the light scattering on the crystal surface. Similar to diamond polishing, ion irradiation modifies the crystal surface thereby affecting the optical properties especially at shorter wavelengths. Although the transmittances in the VUV wavelengths decrease after ion irradiation, the transmittances can be improved through annealing above 1000 K. With an optical transmittance in the VUV region that can recover through simple annealing and with a high transparency from the ultraviolet (UV) to the NIR region, sapphire crystals can therefore be used as good optical windows inside modern fusion power reactors in terms of light particle loadings of hydrogen isotopes and helium