Function Of Transport Current Research Articles

Transport relaxation measurements and glassy state effects in superconducting MgB 2

Time dependent effects in superconducting MgB 2 have been studied systematically for the first time by transport relaxation measurements ( V– t curves) as a function of transport current ( I), temperature ( T) and external magnetic field ( H). At very low dissipation levels (below ∼1 μV), it was observed that the sample voltage grows up smoothly in time by exhibiting the details of initial stage of relaxation process. At high dissipation levels, steady state corresponding to constant flow rate is maintained within a very short time and monitoring of details of flux dynamic evolving along sample becomes difficult on long time scales. Another interesting behavior is the appearance of voltage peak when the transport current was reduced to a finite value. After peak, it was observed that the sample voltage relaxes smoothly by leveling off within a very short time. The evolution of V– t curves suggests that formation of resistive flow channels along sample develops easily, which is quite similar to that of obtained for the superconducting ceramic samples whose grain boundaries are improved. Time dependent effects were also observed in magnetovoltage measurements ( V– H curves) as the field sweep rate (d H/d t) varies. The observations were interpreted mainly in terms of flux trapping in grains.

Current transfer in MgB2 wires with different sheath materials

The current transfer length in compositeMgB2 wires with different metallic sheaths and filament numbers was analysed. Theelectric potential gradient along the wire in the vicinity of the current contact wasmonitored as a function of transport current in various external magnetic fields up to14 T. In all samples the characteristic current transfer length is of the order of amillimetre, varying with detailed sheath composition in the range 0.35–2 mm.For the wires with a clear reaction layer at the filament/matrix interface, theelectrical resistivity of the material constituting such a barrier was estimated.

Loss and inductance investigations in a 4-layer superconducting prototype cable conductor

One important issue in the design and optimization of a superconducting cable conductor is the control of the current distribution between single tapes and layers. This presentation is based on a number of experiments performed on a 4-layer three meter long prototype superconducting cable conductor. The self and mutual inductances of the layers are studied theoretically. The current distribution between the superconducting layers is monitored as a function of transport current. The results are compared with the expected current distribution given by the authors' equivalent electrical circuit model. The AC losses are measured as a function of transport current and a given current distribution and compared with the monoblock model. Recommendations for design of future cable conductor prototypes are given.

Tip heater for minimum quench energy measurements on superconducting strands

Superconducting strands can be characterized by their minimum quench energy (MQE), i.e. the minimum heat pulse needed to trigger a quench in operation conditions (field, temperature, current), in the limit of a (temporally and spatially) /spl delta/-shaped disturbance. The sub-mm//spl mu/s range of perturbation space has only been achieved using the electrical graphite-paste heater technique. The present work has put this technique into practice for the strands of the LHC main magnets, which are designed to operate at 1.9 K in peak fields of up to 9 T. No way has been found yet to calibrate MQE measurements. To make relative statements on the MQE of different samples possible, the reproducibility of the measurements was emphasized. First heater prototypes did not come up to this stipulation. Finally the tip-heater configuration was found to meet the requirements. It generates a heat pulse in a thin resistive graphite paste deposit on top of a small tip that is pressed against the sample with a clamp. The clamp guarantees a maximum of exposure of the sample to the surrounding cryogen. The most striking aspect of repeated measurements on a reference sample is that in open bath conditions the MQE as a function of transport current in subcooled helium can reach hundred times the corresponding value in adiabatic conditions (i.e. with the sample potted in a low conductivity medium). This extraordinary cooling performance of superfluid helium, predicted by many has rarely been shown in superconductor stability experiments.

Transport current-activated flux creep in high-Tc superconductor thin films

This paper discusses the relaxation of a magnetic flux trapped in a superconducting film under the action of a transport current. The evolution of the magnetization after the first jump, at the moment the Lorentz force appears, has been found to obey the creep laws. The effective pinning potential has been obtained as a function of transport current. The distribution of the pinning forces is presented and its statistical properties considered. The standard deviation of the pinning forces is the same for films prepared using the same technique, while the expectation values characterize the attainable critical current density. A criterion for estimating the quality of thin films from the distributions of the pinning forces is formulated.

Comparison of dynamic behaviours of superconducting NbTi, V 3Ga and Nb 3Sn composites

Stability and current sharing in three kinds of superconducting composites with different critical temperatures were investigated by observing terminal voltages and temperatures as a function of transport current after transition to the normal state. The superconducting composites were made by embedding a NbTi monolithic wire, a V 3Ga strand wire, or a Nb 3Sn strand wire in copper stabilizers with relatively large cross-sections. The quenching performance of the superconducting composites was realized by flowing transport current larger than their critical currents through them. Depending strongly on boiling heat transfer characteristics of liquid helium, the dynamic behaviours of the superconducting composites were similar to each other in spite of differences in their critical temperatures. The propriety of parameters, which are introduced into the cryogenic stabilization theories, i.e. critical temperatures, current-sharing threshold temperatures, and fraction of current sharing, is discussed.