R. C. Duckworth

Measurements of temperature dependence of the stability and quench propagation of a 20-cm-long RABiTS Y-Ba-Cu-O tape

Thermal stability and quench propagation in a composite tape made of YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) superconductor were studied experimentally. Quench propagation in each test was initiated by applying a sequence of a short overcurrent pulse followed by a longer pulse at a typical operating current for the tape. The resulting change in resistivity due to internal heating was measured through voltage taps across different zones of the tape. Measurements were performed as a function of both initial overcurrent and operating current for several operating temperatures between 45 and 80 K. These experimental results provided the thermal stability margin, the minimum propagation current, and the quench propagation velocity for the tape. Experimentally obtained temperature dependence of normal zone propagation velocity was compared with the adiabatic theory taking into account minimum propagation current. It was noted that the measured normal zone propagation velocity compared favorably with the theory at each operating temperature.

Tests of tri-axial HTS cables

The Ultera/ORNL team have built and tested 3-m and 5-m triaxial cables rated at 3 and 1.3 kA-rms, respectively. The three concentric superconducting phases are made of BSCCO-2223 HTS tapes, separated by layers of cold-dielectric tapes. A copper braid is added as the grounding shield on the outside of the three active phases. Tests of these cables were performed at temperatures ranging from 70 to 84 K. AC loss data reconfirmed the previous result on a 1.5-m prototype cable that the total 3-phase ac loss is about the sum of the calculated ac losses of the three concentric phases. These and other test results of the 1.3 and 3 kA cables will be used to construct a second 5-m triaxial cable rated at 3 kA-rms, 15 kV. Preliminary test results supporting this new cable and the associated termination are summarized.

Substrate and stabilization effects on the transport AC losses in YBCO coated conductors

In support of second generation HTS conductor development for ac applications, transport ac loss measurements were conducted on a series of RABiTS-processed YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) coated conductors with different nickel alloy substrates and copper stabilization at 77 K. Each 1-cm wide sample had a critical current density between 1.0 and 2.0 MA/cm/sup 2/ and had either a 75 /spl mu/m Ni-5at%W substrate or a 75 /spl mu/m Ni-10%Cr-2%W substrate with 2-/spl mu/m nickel overlayer. Samples with copper stabilization had a 50-/spl mu/m strip of 1 cm wide copper laminated to a 3-/spl mu/m thick silver coated YBCO sample. Using thermal and electrical measurement techniques, the ac losses were measured as a function of the peak current ratio at 60 Hz. Experimental results were compared to the Norris thin strip and elliptical models to determine the influence of the ferromagnetic loss of the substrate and the copper lamination on the total ac loss.

Nanodielectric system for cryogenic applications: Barium titanate filled polyvinyl alcohol

In the current study we focus on dielectric properties (as a function of frequency and temperature) of a polymeric composite system composed of polyvinyl alcohol and barium titanate nano powder. In the investigations, the temperature range is between 50-295 K, and the frequency range is between 20 Hz-1 MHz. Polarization and conduction processes are investigated in the linear regime. Dielectric breakdown strengths of samples are also reported. The materials presented have potential to be implemented in cryogenic capacitor or field grading applications.

Over-current testing of HTS tapes

High-temperature superconducting (HTS) transmission cables are subjected to short-circuit fault currents 10 to 30 times the normal operating current and lasting up to 15 cycles. These over-currents will drive the HTS conductor normal and generate heat during the fault. A concern is whether the fault current will either electromechanically or thermally damage the HTS conductor and degrade it or burn-out the tape altogether. Electromechanical and thermal limitations of over-current pulses were measured on BSCCO and YBCO tapes in a liquid nitrogen bath. With pulse lengths as short as 35 ms, it is found that single BSCCO and YBCO tapes can be pulsed to at least 1 to 1.2 kA without being damaged electromechanically. Longer pulses at moderate (450-750 A) over-currents indicated that HTS tapes can be heated transiently to over 400 K without suffering degradation. Thus, it is likely that other considerations of the cable rather than the HTS tape itself would set the limit for short-circuit fault protection.

Large Scale Superconducting Wind Turbine Cooling

General Electric proposes to apply transformational technology in the form of low-temperature superconductivity to the design of direct-drive wind turbine generators of the 10-MW power level and greater. Generally, optimal steady state 4 K cryogenic cooling of a large thermal mass (> 10 000 kg) and its dimensions (> 4 m diameter and 2.5 m length) with minimum levelized cost of energy is difficult to achieve. A cooling strategy has been found that turns this size disadvantage to ones favor, and furthermore enables the design scalability of the field winding cooling assembly towards 15 to 20 MW. In this design study, we show that size and efficiency are not mutually exclusive and that it is indeed possible to minimize cryogenic complexity and reduce cost. The cryogenic push-button closed loop circulating system is invisible within the nacelle of a wind turbine and requires no handling of cryogenic liquids. Besides the occasional cryocooler service requirement, the proposed solution is maintenance-free in all operating states and allows the system health to be monitored remotely. The design solutions proposed could potentially make large superconducting generators a reality for off-shore wind turbine deployment.

AC Losses in YBCO Coated Conductor With Inkjet Filaments

To achieve low ac losses in applied ac fields, YBa2Cu3Ox (YBCO) filaments were created on a RABiTS buffered substrate through solution inkjet deposition. A metal organic decomposition (MOD) solution was placed into an inkjet dispenser and filaments of widths of 100 mum and 0.8 mm were deposited on the substrate at a spacing of 50 to 100 mum. Each sample, which had a width of 1 cm and a nominal length of 4 cm, was placed in a perpendicular ac field and the ac losses were measured thermally as a function of the field strengths up to 100 mT and at frequencies between 60 Hz and 120 Hz. Samples with inkjet filaments had a high coupling loss. This coupling between filaments may extend along the entire sample length because removal of the conductor ends did not reduce the coupling loss contribution. Reduction in ac loss was observed in samples with laser-scribed filaments that were made from the same MOD solution.

The Development of the Material Plasma Exposure Experiment

The availability of future fusion devices, such as a fusion nuclear science facility or demonstration fusion power station, greatly depends on long operating lifetimes of plasma facing components in their divertors. ORNL is designing the Material Plasma Exposure eXperiment (MPEX), a superconducting magnet, steady-state device to address the plasma material interactions of fusion reactors. MPEX will utilize a new highintensity plasma source concept based on RF technology. This source concept will allow the experiment to cover the entire expected plasma conditions in the divertor of a future fusion reactor. It will be able to study erosion and redeposition for relevant geometries with relevant electric and magnetic fields in-front of the target. MPEX is being designed to allow for the exposure of a priori neutron-irradiated samples. The target exchange chamber has been designed to undock from the linear plasma generator such that it can be transferred to diagnostics stations for more detailed surface analysis. MPEX is being developed in a staged approach with successively increased capabilities. After the initial development step of the helicon source and electron cyclotron heating system, the source concept is being tested in the Proto-MPEX device. Proto-MPEX has achieved electron densities of more than 4×1019 m-3 with a large diameter (13 cm) helicon antenna at 100 kW power. First heating with microwaves resulted in a higher ionization represented by higher electron densities on axis, when compared with the helicon plasma only without microwave heating.

Cooling Configuration Design Considerations for Long-Length HTS Cables

Recent successes in demonstrating high temperature superconducting (HTS) cable systems hundreds of meters in length have inspired even longer length projects. A compact and energy efficient cooling configuration can be achieved using a counterflow-cooling arrangement. This is particularly attractive when all three phases are contained in a single cryostat because of the elimination of the space and thermal requirements of a separate liquid nitrogen return line. Future cable projects will utilize second generation (2G) wire which is expected to become lower in cost but may have different thermal requirements than first generation (1G) BSCCO wire due to the lower critical temperature and to a lesser extent, the lower thermal conductivity of the wire. HTS cable configurations are studied with a numerical model to assess thermal hydraulic performance with AC and thermal losses; a summary of the results from the analysis will be presented. An analysis of the cable thermal-hydraulic response to over-current faults will be presented.

An investigation of the current distribution in the triaxial cable and its operational impacts on a power system

An investigation of the current distribution in a three-phase triaxial superconducting cable is underway to study phase imbalances under steady-state operation and to assist in the construction of a transient model to study operational impacts of the cable in a power grid. The triaxial cable consists of three superconducting concentric phases inside a copper shield, with each phase composed of multiple layers of BSCCO tape wound helically in opposite directions. Current distribution among the phases of the cable is determined by using an electric circuit (EC) model containing the self and mutual inductances resulting from both axial and tangential fields. An ac loss term is also included in the model. Building on the EC model, a lumped cable model is used to investigate the effects of the triaxial cable on a power grid when faults are applied to the system. Cable lengths practical for future applications (/spl sim/10 km) are considered.