Recently, we successfully developed a new Cu-Ag microcomposite alloy with a promising combination of high mechanical strength and high electrical conductivity. When a Cu-16 at% Ag alloy ingot was cold-worked into a wire or a sheet with several times of intermediate annealing at 350-450°C, it shows high conductivity of 75-83% IACS and a high tensile strength of 0.7-1.1GPa at room temperature. These values are superior to those of Cu-Nb microcomposite alloy. The Cu-Ag microcomposite alloy shows excellent mechanical strength with cold work of over 93% areal reduction ratio, while a very heavy cold work of more than 99.97% areal reduction ratio is necessary for realizing such mechanical strength in the Cu-Nb microcomposite. A further advantage of the Cu-Ag microcomposite is easy casting of the alloy ingot, resulting in excellent homogeneity of the microstructure and, therefore, the properties in the alloy wire and sheet. We wound the Cu-Ag microcomposite alloy wire into several pulsed magnets. One of them generated non-destructively 73.4T with duration time of 5msec in a 10mm bore. An other one generated non-destructively 65.3T with duration time of 100msec in a 16mm bore. The feasibility study of the Cu-Ag microcomposite alloy sheet as Bitter magnet sheets is progressing now in collaboration with the Francis Bitter National Magnet Laboratory and the National High Magnetic Field Laboratory. A steady field of 35.2T could be generated by inserting the Cu-Ag microcomposite alloy Bitter sheets into the highest-field region in Hybrid III of FBNML. These pulsed fields and the steady one are world records as of this writing.
The purpose of this study is to develop the NbTi superconducting magnet system directly cooled by a 4-K two-stage G-M (Gifford-McMahon) refrigerator. The performance test has been done by the system which had the following characteristics. 1) The coil has been made by multifilamentary NbTi conductor of 0.249mm in diameter. The coil constant is 0.233T/A at the center field. Therefore a high magnetic field can be generated by a small excitation current. 2) The Bismuth oxide superconductor has been used as a portion of current lead to decrease the Joule-heating along the lead. 3) The 2nd-regenerator material of Er3Ni has been used for the G-M refrigerator to give a very low operating temperature. The cool-down time from room temperature to 3.9K was about 6 hours. The NbTi magnet was operated at 3.9K. The central magnetic field of 7.3T and the maximum field of 7.6T was generated at a quench current of 31.1A. This data agrees well with the critical state of the NbTi conductor. The continuous operation at 30.8A has been obtained reproducibly.
This paper presents an experimental study of optical observations on normal zone propagation in an immersion-cooled superconducting coil and pressure wave propagation in liquid helium induced by quenching. The normal zone was visualized as a dark zone on the illuminated coil surface. Transient behavior of the dark zone was recorded with a high-speed video system under various transport currents. The results obtained were compared with the measured time histories of tap voltages and temperatures for a test coil. Maddock's stability criterion, or cooled end stability, was also successfully observed with a reduced transport current. Finally, the pressure wave propagation was found to be induced by the thermal shock of quenching.