Nobuhiro Saga

High-Power (over 100 mW) Green Laser Diodes on Semipolar

Continuous-wave operation of InGaN green laser diodes (LDs) on semipolar {2021} GaN substrates with output powers of over 100 mW in the spectral region beyond 530 nm is demonstrated. Wall plug efficiencies (WPEs) as high as 7.0–8.9% are realized in the wavelength range of 525–532 nm, which exceed those reported for c-plane LDs. The longest lasing wavelength has reached 536.6 nm under cw operation. These results suggest that the InGaN green LDs on the {2021} plane are better suited as light sources for applications requiring wavelengths over 525 nm.

\{20\bar{2}1\}

Green laser diodes (LDs) on the {2021} plane exhibit lower threshold current densities, nearly half of those on the c-plane in the green region between 520–530 nm. The threshold current of a typical {2021} green LD lasing at 525.5 nm under room temperature cw operation is 51.1 mA, which corresponds to a threshold current density of 4.3 kA/cm2. The threshold voltage is 6.38 V. The characteristics temperature T0 is measured to be 175 K. The perpendicular θ⊥ and parallel θ|| beam divergence angles at half power of the {2021} green LDs are 24 and 11°, respectively. From the viewpoint of the device characteristics, especially the threshold current density, we conclude that the green LDs on the {2021} plane GaN substrates have the essential advantage for obtaining efficient green LDs.

GaN Substrates Operating at Wavelengths beyond 530 nm

True green GaInN laser diodes with a lasing wavelength above 525 nm under continuous wave operation have been successfully fabricated on semipolar {2021} GaN substrates by improving both the diode structure and epitaxial growth conditions. At a case temperature of 55 °C, their lifetime was estimated to be over 5000 h for an optical output power of 50 mW and over 2000 h at 70 mW.

Low Threshold Current Density InGaN Based 520–530 nm Green Laser Diodes on Semi-Polar {2021} Free-Standing GaN Substrates

We are developing Bi-2223 superconducting tapes on a wide viewpoint from fundamental superconducting property to application usage. In basic characterization of superconductivity, the behavior of critical current density (J/sub c/) in terms of both temperature and magnetic field was studied in detail and a new scaling law which can estimate the value of J/sub c/ in multiple conditions is defined. There were significant improvements in the long length fabricating process. The 1 km order length tape having J/sub c/ at 77 K over 20 kA/cm/sup 2/ can be produced reproducibly. The distribution of J/sub c/ through the whole length is very uniform. These results in both basic characterization and production progress made it possible and easy to design a high field magnet. Then, we have successfully developed a magnet using a refrigerator.

Long-Lifetime True Green Laser Diodes with Output Power over 50 mW above 525 nm Grown on Semipolar \{20\bar{2}1\} GaN Substrates

The basic property of high-Tc superconducting cables (HTS cables) using Bi-2223-based Ag-sheathed multifilamentary wire (Ag-sheathed wire) have been investigated for the realization of large-scale and compact cables, these being replaceable with conventional cables in existing ducts or tunnels. The AC performance of multi-layer HTS conductors, and three-phase HTS cables with coaxial superconducting magnetic shielding structure was evaluated. The characteristics of the HTS conductor and cable models of long length was investigated on a 50 m scale.

Status of Bi-2223 tapes performance and development

Silver-sheathed BiPbSrCaCuO 2223 superconducting wires with long length and high Jc of over 10/sup 4/ A/cm/sup 2/ were developed by using the powder-in-tube method. Future possibilities to obtain much higher Jcs are discussed in relation to crystal alignment, connectivity between grains and flux pinning. High amperage wires and high strength wires were also developed for large scale magnet application. High I/sub c/ of over 300 A at 77 K were obtained for the wire with large cross sectional area. Good stress and strain tolerant characteristics were also obtained for silver alloy sheathed wires. In order to apply the HTS wires for AC use, it is necessary to reduce AC loss. So, the AC loss of the wires are also discussed. As a progress of wire technology, we have fabricated many types of magnets, such as pancake magnets and solenoidal magnets. In the case of 77 K application, it is promising to apply for AC use because of large heat capacity of HTS wires and liquid nitrogen. So, we are developing the 500 kVA transformer and pulse magnet for SMES. A refrigerator cooled magnet operated near 20K was fabricated. This magnet was operated at 21 K and generated 3 T inside a /spl phi/40 mm room temperature bore, stably generated 2.5 T continuously for over 150 hours and could be operated at high ramp rate of over 12 T/min. As for the 4.2 K operation, high field insert magnet for 1 GHz NMR application are developed. Highest magnetic field of 24 T was achieved using the hybrid magnet at MIT and persistent current mode operation was done by using the layer wound coil with persistent current switch.

HTS large scale application using BSCCO conductor

Abstract Silver-sheathed Bi(2223) superconducting wires with long length and high J c of over 10 4 A/cm 2 at 77.3 K in a zero magnetic field were developed by using the powder-in-tube method, and the performance of many application prototypes has progressed. For long wires, a multifilamentary structure is effective for good flexibility and it is easy to handle the long wire without degradation of superconducting properties. We have fabricated the prototype of a refrigerator-cooled magnet with the react and wind technique. This magnet has been operated at 21 K and generated 3.0 T in a 40φ mm room temperature bore (60 φ mm winding bore) and 2.5 T continuously for over 150 h.

Development of Ag-sheathed Bi2223 superconducting wires and their application to magnets

We have developed new type HTS conductors in which all strands are transposed to balance the current distribution. Ic of the transposed conductor assembling subcables which consists of stranded Bi-2223 round wires was up to 1.2kA AC loss of the conductor was one order lower than that of the non-transposed conductor which has the multi-layer structure, assembling Ag sheathed tapes.

Development of Ag-sheathed Bi(2223) superconducting wires and coils

A design to reduce ac losses of the cable conductors was investigated by using Bi-2223 Ag-sheathed strands with round shape. These new cable structures were designed and evaluated concerning with productivity based on the production experience of conventional metallic ac superconducting wires. Properties required for the conductors were also discussed regarding with characteristics of the strands. It was found that a design of practical cable conductors with low ac losses less than 1W/m was possible by realizing Bi-2223 Ag-sheathed round wires with Jc reaching 10,000A/cm2 at 77K.

AC Loss of Transposed HTS Cable Conductors

The current-limiting properties of high-Tc tapes were examined in order to investigate the possibility of using these tapes as superconducting fault current limiters. The short samples studied comprised two types of Bi-2223 tape having different sheath resistances, and two types of YBCO thin-film tape having different silver coating thicknesses on the YBCO surface. The relationships between the current- limiting performance of these samples and the characteristics of the tapes were clarified through experiments. Fault current limiter models were then fabricated with non-inductively wound high-Tc tapes, and their characteristics and current-limiting properties were evaluated.