Ken'ichi Kikuchi

A 0.2–0.5THz single-band heterodyne receiver based on a photonic local oscillator and a superconductor-insulator-superconductor mixer

We have demonstrated that a superconductor-insulator-superconductor (SIS) mixer pumped by a photonic local oscillator (LO) covers the whole frequency range of 0.2–0.5THz. In the bandwidth of 74% of the center frequency, this single-band receiver exhibits noise temperature of TRX⩽20hf∕kB, where h is Planck’s constant, f is the frequency, and kB is Boltzmann’s constant. Resultant TRX is almost equal to TRX of the identical SIS mixer pumped by three conventional frequency-multiplier-based LOs which share the 0.2–0.5THz band. This technique will contribute to simple, wide-band, and low-noise heterodyne receivers in the terahertz region.

Development of bronze-processed Nb/sub 3/Sn superconducting wires for high field magnets

High performance bronze-processed Nb/sub 3/Sn superconducting wires are desired to realize a compact, easy to operate high field magnet system. To meet the requirements, we carried out an investigation to increase Sn concentration in the bronze to improve the critical current density, and performed a trial manufacture of a newly designed wire to raise the tensile strength. Our findings indicated that a high critical current density of 135 A/mm/sup 2/ at 20 T, 4.2 K could be obtained by increasing the Sn concentration in the bronze to 16 wt%. In addition, a high strength bronze-processed Nb/sub 3/Sn wire reinforced with composite filaments incorporating Th exhibiting a high 0.2% proof stress of near 300 MPa and adequate overall J/sub cs/ at 4.2 K, was developed.

Absorption Spectra of Smoke Emitted from Heated Nylon Fabric Measured with a Continuous-Wave Sub-Terahertz Spectrometer

The absorption spectra of smoke emitted from heated nylon fabric are studied in the sub-terahertz band. The transmission loss of 1.5-µm-wavelength light in the smoke is 50 dB/m or more. Even in such dense smoke, the loss for waves with the frequency ranging from 200 to 500 GHz is so small that absorption lines of gas molecules in the smoke can be detected in this frequency range. Spectral analysis indicates that the smoke contains molecules of toxic gas, such as HCN and CH3CN, generated due to incomplete combustion of the nylon fabric when heated to 500 °C with air.

Production of a 11 km long jelly roll processed Nb/sub 3/Al strand with high copper ratio of 4 for fusion magnets

When building high field, large scale magnets such as those used in fusion applications where large electromagnetic forces are involved in the superconductor, Nb/sub 3/Al conductor provide a great advantage, because the strain sensitivity of the critical current for Nb/sub 3/Al is substantially smaller than that of Nb/sub 3/Sn. A co-project carried out between the Japan Atomic Energy Research Institute (JAERI) and Hitachi Cable, Ltd., has established a procedure for finishing with a long one piece length of Nb/sub 3/Al strands with the high Cu ratio of 4 through the Jelly Roll process. The hydrostatically extruded rod shows excellent workability, and a 0.74 mm dia Nb/sub 3/Al strand with the high Cu ratio of 4 has been successfully drawn in one continuous piece length of 11 km without wire breakage. The strand shows a Jc and n value of 590 A/mm/sup 2/ and 45 at 12 T and 1984 A/mm/sup 2/ and 70 at 7.4 T which is the maximum field experienced in the innermost conductor of the toloidal field coil winding for the modification of JT-60 fusion device being planned in JAERI. A 30 m long Nb/sub 3/Al cable with a cabling scheme 3 /spl times/ 3 /spl times/ 3 /spl times/ 3 /spl times/ 4 was fabricated from the strand.

Remote gas sensing in full-scale fire with sub-terahertz waves

A remote gas sensor based on a continuous-wave sub-terahertz spectrometer with an operation bandwidth between 200 and 500 GHz has been developed. The performance of the developed system was investigated in a full-scale fire. The measured transmission spectra of smoke show the existence of hydrogen cyanide (HCN) and water molecules, which were generated due to the combustion of a foam urethane block. The HCN content estimated from the absorption intensity agrees quite well with that obtained from the chemical analysis of gases sampled from the fire site. These results demonstrate the potential of the developed sensing system as a remote toxic gas sensor at disaster sites.

Heterodyne THz-wave receiver with a superconducting tunneling mixer driven by a high sweeping-speed photonics-based THz-wave local oscillator

We developed a broadband, highly sensitive heterodyne THz-wave receiver with a superconductor-insulator-superconductor (SIS) mixer. To fully utilize wide-bandwidth characteristics of the SIS mixer, we pump the SIS mixer with a photonics-based local oscillator (PLO) with fast frequency-sweeping speed. The received THz-wave signal down-converted to an intermediated frequency band is rapidly acquired by a high-speed analog-to-digital converter. The noise level of the developed receiver is equal to or smaller than twenty times the quantum-limited noise level between 225 and 425GHz. With this receiver, the frequency sweep from 225 to 475GHz with frequency resolution of 0.5GHz takes only about 2.5sec. Emission spectra of N2O gases measured with this receiver show its excellent performance.

Optically synchronized dual-channel terahertz signals for high-performance transmitter/receiver system

We developed a high-sweeping-speed optically synchronized dual-channel terahertz signal generator, in which the frequency difference between the two terahertz signals is independent of the frequency of the terahertz signals themselves. This feature is essential for heterodyne detection of terahertz signals with various frequencies. With this generator, a frequency-sweepable terahertz transmitter (Tx)/receiver (Rx) system with a wide dynamic range can be realized without sacrificing the high frequency-sweeping speed. Absorption line measurements for water vapor and nitrous oxide show that the developed Tx/Rx system can detect gas absorption with the optical depth of 0.04 or less. This result indicates the potential of the system as a remote gas sensor and gas analyzer.

SIS Mixers as a Noise Detector for Optimization of Photonic Local Oscillators in Terahertz Range

We have demonstrated that a superconductor-insulator-superconductor (SIS) mixer is applicable for a power detector to evaluate the sideband noise radiated from a submillimeter-wave local oscillator (LO) under development. A simple analysis shows SIS mixers provide better resolution of LO noise than Schottky barrier diode mixers when the available LO power is less than ~ 0.4 mW. Systematic measurement of dependence of receiver noise temperature T RX on components and operational conditions of a photonic LO (Ph-LO) is carried out, which provides a guide to their optimization, e.g., source-laser power, photodiode drive current, use of an optical filter, and selection of an optical amplifier. Noise temperature of the Ph-LO estimated from experimental results ranges from ~ 1 times 103 K to ~ 5 times 104 K as a function of operational conditions and parameters.

Development of high copper ratio Nb/sub 3/Al strands for fusion magnets

A joint project was carried out between the Japan Atomic Energy Research institute (JAERI) and Hitachi Cable, Ltd, to develop long length Nb/sub 3/Al strands with a Cu ratio of 4 using the Jelly Roll process with the intention to fabricate a CICC (cable-in-conduit-conductor). The workability of the hydrostatically extruded multifilament rod proved to be very good, comparable that the formerly developed Jelly Roll processed Nb/sub 3/Al strands with Cu ratio 2. The strands with Cu ratio 4 show the same Jc 605 A/mm/sup 2/ at 12 T as that for the strands with Cu ratio 2. A CICC is formed through a drawing process. The inserted cables have the pattern of 3/spl times/3/spl times/3/spl times/3/spl times/4 with a first triplet composed of 2 Nb/sub 3/Al strands and a Cu strand. The finished 5th stage cable is wrapped with SS304 tape, after which it if drawn inside a 1.9 mm thick SS316L tube to a void fraction of 38% as a demonstration of conductor fabrication feasibility.