Ravi Sawh

The role of uranium, with and without irradiation, in the achievement of Jc 300000 A cm-2 at 77 K in large grain melt-textured Y123

Abstract The addition of uranium to large single grains of Y123 improves J c prior to irradiation, by over 80%. Observations on the U chemistry and microstructure studies are presented. Irradiation with thermal neutrons, to fission the U235 component, results in further increases in J c . Total increases of J c by factors of, e.g. > 30 at 77 K, 0.25 Tesla, and > 20 at 50 K, 10.0 Tesla, result. Values of J c of, e.g. 290000 A cm −2 at 77 K, 0.25 Tesla, and 1 million A cm −2 at 50 K, 0.25 Tesla, result. T c is reduced by 0.5 K and creep rate is increased by 1/5. The chemical and irradiation effects are multiplicative and neither is yet optimized.

The role of uranium chemistry and uranium fission in obtaining ultra-high in textured Y123

When uranium (U) is added to Y123 powders, after texturing the deposits containing U are found to be nm. These deposits act as pinning centres, and increase by 90% per 0.8 wt% U. Neutron irradiation further increases . Total increases of by factors of e.g. (at 77 K, 0.25 Tesla), and (at 50 K, 10 Tesla) result. Magnitudes of are (at 77 K, 0.25 T) and (at 50 K, 0.25 T). The U/n process is not yet optimized. Chemistry, microstructure, radiation effects and cost are discussed.

Activation of trapped field magnets by flux pumping

High temperature superconducting trapped field magnets (TFM) offer great potential as an alternative to second generation YBCO wire, both in cost and performance. The difficulty of activating this material is the primary problem hindering their incorporation into electromechanical devices. An alternate method called flux pumping is discussed in which the magnetic field trapped within a precooled TFM is increased, i.e., pumped up incrementally. A boundary element formulation is employed to illustrate what happens to the current density within the TFM during this pumping procedure. This is followed by a rigorous analytical formulation to simulate flux pumping within a 2-cm diameter TFM; the simulation is compared to an experiment using a NdFeB magnet as the pumping source.

Effect of uranium doping and thermal neutron irradiation on the flux-pinning of silver-clad Bi-Sr-Ca-Cu-O tapes

Ag/Bi-2223 tapes doped with small quantities of /sup 235/UO/sub 4/ powder were prepared by the powder-in-tube process and irradiated in a thermal neutron environment. Substantial improvements in critical current density (J/sub c/)-applied field (H) performance and anisotropy have been previously reported. However, the radioactivity of the silver sheath is a limiting factor for commercial and industrial applications of this technique. Here we report the performance of the technique using various doping levels (from 0.15 to 2 percent by weight (wt.%) UO/sub 4/) and thermal neutron fluences (/spl Phi//sub n/), in order to further reduce the silver radioactivity. Optimum fluence levels are identified and an optimum combination in terms of J/sub c/ - H performance is discussed. At a doping level of 2% /sup 235/UO/sub 4/, a normalised J/sub c/ enhancement of 250 times is observed for an 0.8 T field aligned along the c-axis, and 25 times at 3 T along the ab-plane at 77 K, compared to pre-irradiation values. At 0.6%, these figures are 500 and 10 times, respectively. The effects of the uranium doping and thermal neutron irradiation on the flux pinning strengths are also directly probed using dynamic relaxation techniques. The results show an increase in the effective pinning potential after doping and irradiation.

Simulating the Trapped B Field in Bulk Superconductors Using a Mutual Inductance Coupling Technique

The field and current induced in a trapped field magnet during a transient current excitation is complicated by the fact that the conductivity of the material changes wildly depending on both the magnitude of the induced current and the local B field. A mutual inductance approach is presented for solving this coupled problem; it involves discretizing the bulk superconductor into cells and precomputing the inductance coupling matrix of all cells with one another and with the excitation coils. The benefits of this approach are that the numerical simulation is rapid and flexible. The latter is important because the conductivity of the individual cells is dependent on the current density, the magnetic field density magnitude, and the temperature. The proposed approach allows the modeling of a very sharp J-E relationship which has proved problematic using conventional finite element approaches. In addition, a phenomenological model relating the critical current density to the field is introduced for these tests in place of the Kim model. The field simulation is tested with a challenging dwell time experiment in which the trapped field above a bulk superconductor is predicted for various excitation current hold times.

Development and Analysis of YBCO Trapped Field Magnets in Electromechanical Devices

High-temperature superconducting trapped field magnets (TFMs) offer great potential as an alternative to second-generation YBCO wire, both in cost and performance. Attention is given to the calculation of current distribution within YBCO disks at partial and full activation and comparing this to experimental values. The best results are obtained by treating the current as a sequence of nested current rings. The fields are computed by integrating the elliptic integrals representing the fields from these rings and using variable metric optimization to choose the ring radii to best match the activation field over the unactivated material. A technique for treating the subregions of the TFM as voltage fed coils appears most expeditious for computing forces.

How and why discontinuous multiple-in-line-damage results in much higher Jc than continuous columnar pinning centres

We review a recent experiment in which pinning centres, composed of discontinuous multiple-in-line damage (MILD), were compared to continuous columnar pinning centres (CCPCs). The methods used, results of the experiment, and the initial steps taken toward a phenomenological theory explaining the unusual results, are discussed. Experimental results show that MILD pinning centres, despite their reduced pinning potential, are far superior to CCPCs. Using MILD pinning, a new record Jc has been achieved in large grain YBCO. The value of Hirr (MILD) is comparable to, or larger, than Hirr (CCPCs). Pinning effects alone, with no modification of texture, weak links or oxygenation of the YBCO, increase Jc by as much as a factor of 17. Splay is expected to further increase this factor. The experimental dependence of Jc on the energy lost by the ion per unit length is approximately reproduced by a phenomenological theory, including only the effects of diminished percolation, critical temperature, and pinning potential.

On the limiting mechanism of irradiation enhancement of I/sub c/

Irradiation may significantly increase I/sub c/ in HTS. A systematic pattern occurs: R=I/sub c/(afterirr.)/I/sub c/(beforeirr.) increases at low defect density, d. It reaches a peak, and then it falls below 1 at high d. The pinning center mechanism, which causes R to increase, has been extensively studied. The falloff in R has not. It has been considered a secondary effect. Here, we will show that the fall-off plays an important role in determining the maximum I/sub c/ enhancement achievable. A phenomenological model to describe the R-vs.-d curve, over the entire d range, is proposed. The idea is that R is the product of two competing effects. (i) Irradiation damage acts as pinning centers, hence increases critical current density, J/sub c/. (ii) Damage reduces the flow-area. Hence, it decreases the net critical current. Data on U/n processed Bi-2223 tapes are fitted to this model. The fitting indicates: (1) the reduction of the flow-area accounts for the majority of the R falloff; and (2) It is sufficient to describe J/sub c/ enhancement as linear with d, and it depending on field and temperature only through the ratio b=B/B/sub irr/, where B/sub irr/ is the irreversible field before irradiation.

Rotating Cylinder Planetary Gear Motor

This paper introduces a new electromechanical motor which incorporates free spinning magnetized cylinders in the air gap. The motor has some of the features of a magnetic planetary gear in that it amplifies torque but it does so to a significantly higher degree than a conventional planetary gear with passive ferromagnetic elements in the air gap (220% higher). The cylinder motor offers the additional advantage that it eliminates cogging torque. The resulting machine constitutes a new class electromechanical device with the introduction of a freely rotating source of MMF in the air gap that both couples and boosts the torque interaction of the rotor and the stator.

Magnetophoretic-Dielectrophoretic Field-Flow Fractionation

We present a novel embodiment of DEP-FFF that has been enhanced through the addition of a magnetophoretic component. A proof of principle demonstration of the discrimation of magnetically labeled and unlabeled particles is described. This new technique should further extend the applicability of DEP-FFF to the fractionation and isolation of target species in integrated microsystems.