Katsumi Miyashita

Impact of Cable Twist Pitch on

The performance of four Nb3Sn conductors for the ITER central solenoids was tested. The current sharing temperatures (Tcs) were measured over approximately 9000 electromagnetic cycles, including two or three thermal cycles between 4.2 K and room temperature. Tcs increased and became almost constant through the cycling. The gradient of the electric field against the temperature gradually decreased against cycling. The degradations caused by the electromagnetic force of the short twist pitch conductors were smaller than that of the original twist pitch conductor. The ac losses of short twist pitch conductors were several times higher than that of original twist pitch conductor. The dents and the removals of the Cr plating on the strands, which were formed during cabling, decreased the electric resistance between strands, which may cause the observed high ac loss. Inspection of the cable showed neither a clear bias of cable in the cross-sectional surface nor distorted strands in the lateral face. The high rigidity of the short twist pitch cable could prevent these plastic deformations, caused by the Lorentz force.

T_{cs}

Submicron-filament bronze-processed multifilamentary Nb/sub 3/Sn wires with Cu-5at%Sn-X (X: Mn, Ni, Si, Ge, B, P, Zn, Al) ternary alloy matrix and Nb-1at.%Ta alloy cores were fabricated. The cross-sectional structure of wires was the central-Cu-stabilizer type to facilitate, by external Sn diffusion, improvements in critical current density and hysteresis losses. The Nb-1Ta core decreased the hysteresis loss in spite of the increase of the critical current densities. By the addition of Zn, Al, the critical current densities and the hysteresis losses were improved. Although hysteresis losses were not decreased by Sn plating, the critical current densities were apparently improved at relatively low temperature heat treatment.<<ETX>>

-Degradation and AC Loss in

Abstract The simultaneous addition of 0.5 at%Ge to the matrix and 1 at%Ta to the coreenhances the Jc of Nb 3 Sn at 12T by more than one order of magnitude. The increase in the Jc seems to be attributed to the grain refinement of Nb 3 Sn by the Ge addition. The formation of thin layer richer in Ge is observed at the interface between the Nb 3 Sn and matrix, which may be effective for reducing the proximity effect among Nb 3 Sn filaments in the AC use. Ultrafine filamentary Nb 3 Sn wires with Ge addition to the matrix have been successfully fabricated. The addition of 0.25 at%Ge to the matrix and 1 at%Ta to the core improves the Qh/Jc ratio at 0.5 T by a factor of nearly four.

\hbox{Nb}_{3}\hbox{Sn}

A new type of rapid-heating, quenching and transformation-processed Nb3Al wire incorporating Ag-barrier structure have been developed to realize both the interfilament decoupling and the low wire breakage risk during the drawing. The barrier has a three-layered structure of Nb/Ag/Nb. Ag does not react with Nb at all even during the rapid-heating and quenching (RHQ) treatment and not penetrate into the Nb3Al filaments, which is necessary for maintaining the critical current density (Jc) characteristics. Drawability of the Ag-barrier Nb/Al precursor composite was excellent; a prototype 200 m wire with Ø1.35 mm was successfully drawn without any breakage. Ag intermediate layer allocated between the Nb layers suppresses flux jumps at low fields even at 1.8 K. It is notable that the Ag intermediate layer acts as a role of cushioning, when the wire is subject to applied strain, mitigating stress concentration inside the wire and leading to enhancement of the irreversible strain up to about 0.6%: this would be a key factor especially for the “React & Wind” magnet applications. Cu stabilizer can be easily attached through a “tube-method” after RHQ. The filament size can be reduced down to about 25 μm. The hysteresis loss (±3 T@4.2 K) per nonstabilizer volume was reduced down to 614 mJ/cm3.

Conductors for ITER Central Solenoids

Stability tests employing inductive heaters are useful to evaluate stability margins of cable in conduit conductors (CICCs). The inductive heater, which consists of the resistive induction-coil and the AC or pulse power supply, can introduce eddy currents in the CICC. The frequency is more than one hundred Hz and it can introduce the heat pulse with the duration of several ms. The stability test using the inductive heaters had been reported. However, the ratio of the deposited energies to the components; superconducting strands, conduit and induction-coil, have not been discussed exactly. By the inductive heater, the coupling and eddy current losses are introduced in the strand. In addition, inter-strand coupling loss, eddy current loss in the conduit and joule and/or eddy current losses in the induction-coil itself are introduced. We evaluated these kinds of losses by both experimental and numerical methods. Finally, we evaluated the quench energy margin of the cable in conduit conductor quantitatively.

Critical current densities and magnetic hysteresis losses in submicron filament bronze-processed Nb/sub 3/Sn wires

R&D on NbTi and Nb/sub 3/Sn superconductors has been carried out for application to AC power apparatus in the Super-GM project. 5 kA class NbTi stranded cables have already been tested and then the R&D aiming at the final target of 10 kA class conductors is in progress. However there is no testing apparatus which makes it possible to measure over 5 kArms AC quench current (Iq) and AC loss in AC magnetic field (50-60 Hz). The testing apparatus was developed under the specifications required for evaluating the AC characteristics of 10 kA class conductors. The AC magnet generating the maximum magnetic field of 1 Tpeak in the bore of 150 mm was manufactured. For the Iq measurement, the AC current is directly applied to the specimen in the field of 0.5 Tpeak (50-60 Hz), 10 kArms AC quench current was obtained for the developed large NbTi conductors. As regards the AC loss measurement, the magnetization method using pick-up coils was adopted and its sensitivity has been improved by reducing unnecessary output signal.

Effects of the germanium addition to the matrix of bronze-processed Nb3Sn wires

A bronze processed 0.275 mm-dia. Nb/sub 3/Sn strand composed of Nb-0.5at%Ta cores and a Cu-2.7at%Sn-1.0at%Ge matrix was fabricated into cables of 6 and 36 strands. Each strand contained 75990 filaments 0.28 /spl mu/m in diameter. The AC quench current for the 36-strand cable reached 2500 A/sub peak/ at 50 Hz and 0.5 T DC field. The 6-strand cable showed a significantly improved irreversible bending strain limit of over 10% due to the fine filament size. This result enabled building of a magnet through the react and wind method. A 50 mm-bore 2T-class magnet wound by the 6 strand cable was successfully operated at 52.7 Hz and 4.2 K. In order to reduce AC losses another bronze processed 0.218 mm-dia. Nb/sub 3/Sn strand with 147510 0.15 /spl mu/m-dia. filaments was fabricated from Nb05%Ta/Cu-4.9%Sn-2.0%Ge/Cu-2.7%Sn-2%Ni-1%Mn double matrix composite. The strand showed a hysteresis loss of 447J/m/sup 3/ at /spl plusmn/0.5 T/cycle and /spl lambda/J/sub c/ of 857A/mm/sup 2/ at 0.5 T after the heat treatment at 500/spl deg/C for 100 h.

Development of Ag-Barrier RHQT Nb 3 Al Wires

Submicron filament bronze-processed multifilamentary Nb3Sn wires with Cu-5at%Sn-X matrix and Nb-lat%Ta cores have been fabricated. The Ge content in bronze matrix is changed in the range from 0% to 1% for optimizing Ge content. The cross-sectional structure of wires was the central-Cu-stabilizer type to facilitate, by external Sn diffusion, improvement in critical current density,Jc, and hysteresis losses,Qh. The simultaneous addition of 0.25at%Ge to the bronze matrix and lat%Ta to the Nb core enhances the non Cu Je by 60%, and decrease the Qh by approximately 60%. By Sn plating, the non Cu Jc was improved by 10%, and the Qh was reduce by 5%. The formation of thin layer richer in Ge is observed in the Nb3Sn layer and the matrix adjacent to the Nb3Sn layer, which may be effective for the improvements in Jc and Qh characteristics.

Calibration of inductive heater for stability test of cable in conduit conductor

The internal-tin technique is an excellent method of fabricating Nb3Sn superconducting wires with high critical current densities. We have developed an internal-tin Nb3Sn wire in which the multi-filamentary wire is fabricated by assembling mono-filamentary Nb elements and mono-filamentary Sn elements. The simple fabrication process allows a reduction in the fabrication cost of the wires. By optimizing the spacing of the Nb3Sn filaments, a 600 m long wire with a high critical current density of 400 A/mm2 at 18 T and no flux jumping was fabricated.

Development of testing apparatus for 10 kA class AC superconductors

Critical current density and stability of sub-micron filament Nb-Ti superconducting wires with Cu-Ni-Mn alloy matrix are described. Characteristics of critical current density Jc depend on filament diameter df, kinds of matrix and ratio of filament spacing dn to df. In the case of large dn/df(=0.65-1) wires, increasing Jc with decreasing df is slight due to low pinning force. Residual Resistance Ratio (RRR) of the wires which were heat treated (100-250°C×1hour) at final size increased to 70-80. These values are three times those of the as drawn wires. The final heat treatment at more than 300°C degraded RRR and Jc, because of Ni diffusion. AC (50Hz) quench current at high field (>2T) of the secondary twisted cable with 36 strands (copper ratio 1.9) is higher than DC critical current of the secondary twisted cable.