The mission of the physics team of the STC is:
The physical measurements laboratory performs most of the physical measurements on HTS materials and conductors used to assess their performance. The laboratory employs a SQUID magnetometer to measure superconducting transition temperatures, Tcs, magnetic hysteresis loops to determine critical current densities on small samples and magnetic relaxation measurements to examine "flux creep." Measurement of current-voltage characteristics to determine the critical current density, Ic, of HTS conductors is our primary diagnostic for conductor performance and can be carried out at currents up to 350A, Variable temperature cryostats incorporating superconducting solenoids allow IC versus temperature and magnetic field curves to be measured over the ranges 2-120K and 0-9T. A 7-Tesla horizontal bore solenoid and a 1-Tesla horizontal iron-core magnet system facilitate measurements of IC versus field orientation. Long tape conductors up to 1-m length can be measured in a probe containing 100 voltage tapes allowing IC determination as a function of position with 1-cm resolution. A scanning Hall probe sensor system allows transport current profiling with 0.1-mm resolution. Information obtained from these measurements is fed back to the materials development team to guide improved processing.
The applied physics program has employed the facilities of the STC physical measurements laboratory, the National High Magnetic Field Laboratory, the Argonne National Laboratory Heavy Ion Accelerator, and the LANL 0.8 GeV Proton Accelerator to carry out a number of investigations concerning the mechanisms determining high critical current densities in HTS conductors. In particular we have focussed on studies of flux pinning of quantized vortices by microstructural defects either naturally occurring in the materials or introduced artificially. Strong flux pinning is required to stabilize vortices in the presence of Lorentz forces applied by transport currents, as flux motion results in ohmic resistance. We have studied flux pinning enhancement by introduction of "columnar defects" formed by irradiation by fast heavy ions and by means of fission tracks resulting from irradiation by 0.8 GeV protons. In addition we have studied the phenomenon of "giant flux creep" caused by thermal activation of vortices out of potential wells formed by defect pinning. These studies are presently carried out on YBCO coated conductors and are aimed at determining the optimum defect microstructure to maximize critical current density.
The applications measurements are primarily focussed on studies of ac losses in prototype HTS power transmission cables. We have developed a calorimetric measurement system that determines the ac losses developed in a 1-m-length of transmission cable by measuring the parabolic temperature profile resulting when ac currents flow through it. The cable is isolated from radial heat flow by a vacuum enclosure and thermally fixed at the ends by contact with a nitrogen bath.
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