Tomoyoshi Motohiro

Cr content χ of 0.4 at% for a Cr doped Nd (1 at%): YAG laser rod (LR) gave a higher laser output (I_output) than that of 0.0, 0.7, and 1.0 at% in a specially designed compact solar-pumped laser (SPL) outdoors. I_outputs were measured as a function of an 808 nm pumping laser’s power indoors, changing the transmittance of the output coupler. From the obtained slope efficiencies, round-trip resonator losses Ls for the four χs were estimated, and the best-fit function L(χ) was derived. From the experimentally estimated Cr-to-Nd effective energy transfer efficiency η_Cr→Nd at the four χs, the best-fit function η_Cr→Nd(χ) was derived. Using L(χ), η_Cr→Nd(χ), and a wavelength λ- and χ-dependent absorption coefficient α(λ, χ), inferred from the literature, the power conversion efficiency η_power(χ) under 1 Sun was estimated. The estimated η_power(0.4) and η_power(0.7) were reproduced in experimentally deduced factors at the mode-matching efficiency η_mode = 0.19. The estimated maximum η_power(χ) appeared around χ = 0.2 at%, being 20% higher than that at χ = 0.4 at%. In addition to this, a composite LR (Cr, Nd: YAG core/Gd: YAG cladding) was found to achieve η_mode = 0.68 and η_power = 0.064, ranking among the highest-class SPL η_powers.

A 100 m-level spiral trench over 4-inch wafer is fabricated for the coil of the superconducting magnetic energy storage. 3-stepped structure can reduce the disconnection risk caused by the random defects. Si etching scallop is decreased and smoothened by the oxidization and the grown SiO₂ etching. This improves the superconducting film quality deposited later. The fabricated coils are 102 m-long single spiral for series coil and 89 m-long 7-parallel spirals for parallel coil.

A compact superconducting magnetic energy storage system (SMES) produced by Si micro fabrication technologies has been proposed to improve electricity storage volume density, w, in the sub-Wh/L range of conventional SMESs and to produce them at a low cost by mass production. In parallel with the experimental development reported previously, a series of trials was performed to estimate a feasible value of w based on the calculation of the magnetic field generated by the compact SMES by improving the calculation models step by step. In this work, the experimentally obtained magnetic flux density dependence of superconductive critical current density was taken into consideration for the first time in this series of trials, together with the additional improvement of the calculation models. The results of the estimation indicated that a compact SMES produced by the proposed concept can attain a w in the Wh/L range or more, ranking with or surpassing that of presently used capacitors.

A method to estimate the electricity storage capacity of our compact superconducting magnetic energy storage system composed of stacks of Si-wafers loaded with superconducting thin film coils in spiral trenches formed by MEMS process was developed and tested for initial 6 designs of wafer coils. The results show energy storage density as low as around 6 Wh/ℓ. This is not caused by the limitation of the hoop stress but by the limitation of the maximum magnetic flux density. Further calculations for optimum wafer coil designs will be continued in which the effect of the maximum magnetic flux energy and the effect of the maximum hoop stress on the limit of the amount of the electrical energy storage come into a good balance.

The performance of solar-pumped lasers (SPLs) employing transparent Cr-doped Nd:YAG ceramic laser media (LMs) has not been directly compared to that of Nd:YAG ceramic LMs with the same instrumentation. Using an original compact SPL system with a ∅ 76 mm caliber off-axis paraboloidal concentrating mirror, outdoor oscillation experiments using direct sunlight were performed to elucidate the effect of the Cr content on the performance. LMs of 1 mm × 1 mm × 10 mm (or 20 mm) with Cr contents of 0.0, 0.4, 0.7, or 1.0 at% and a fixed Nd content of 1.0 at% were tested. The SPL output increased more than eightfold between Cr contents of 0.0 and 0.4 at%, and then decreased gradually beyond 0.4 at%, where a significant increase in the round-trip loss due to scattering by Cr dopants was observed. Cr doping is thought to be a useful way of further improving the SPL efficiency.

To improve the energy storage volume density of our concept of a compact superconducting magnetic energy storage system, replacement of superconducting material NbN for the thin film coil in the spiral trench on a Si wafer by a high temperature superconductor YBa₂Cu₃O_7-δ was addressed. By using the successful result previously reported to grow a c-axis oriented film on a steel substrate, a superconducting film of Tc=45.5 K was obtained using similar buffer layers including a (100) oriented CeO₂ layer over a (100) oriented YSZ layer. The similar result reported on a SrTiO₃ single crystal substrate without any buffer layers indicates c-axis oriented film of Tc=90K can be obtained by further reduction of oxygen partial pressure.

We have proposed a light trapping concept for crystalline silicon photovoltaic (PV) cells used for power transmission from solar-pumped lasers (SPLs) emitting at 1064 nm. The underlying mechanism is multiple reflection between a multilayered angle-selective filter on the front surface and a diffuse reflector on the rear surface of the cell. For the stationary use and optical fiber connection of the SPL-PV combination in which the incident angles of the laser light to the PV cells (ϕ) change within 5–10°, double-cavity bandpass filters realize the required angle selection function. When the SPLs illuminate moving electric vehicles equipped with the PV cells at larger ϕ values, shortpass filters are appropriate. These specially designed angle-selective filters provide significant light trapping performance and consequently absorbances higher than the Yablonovitch limit up to ϕ = 45°. The fact that the present light trapping configuration requires no microfabrication processes is also a great advantage over two- and three-dimensional photonic crystal structures.

We have designed crystalline silicon photovoltaic (PV) cells used for power transmission from solar-pumped lasers (SPLs) emitting at 1064 nm. The practical light-trapping performance of the combination of a multilayered angle-selective filter on the front surface and a diffuse reflector on the rear surface was evaluated by ray-trace simulation, which was taken into account in device simulation. When the SPLs illuminate stationary PV cells or are connected using optical fibers and hence the incident angles to the PV cells are within 10°, a high laser-to-electricity conversion efficiency of around 50% would be feasible under 100 W/cm ² laser illumination using the 50-µm-thick cells. For power transmission to moving objects such as electric vehicles, in which the incident angles change up to 30°, the efficiency of the 75-µm-thick cells is slightly lower because of the less significant light-trapping effect. To realize these high efficiencies, reduction of contact resistance and surface recombination velocity is required.