Anders Van Der Aa Kühle

Electronic instability during vortex motion in cuprate superconductors regime of low and high magnetic fields

Abstract The critical velocity ν ϕ ∗ associated with the electronic instability predicted by Larkin and Ovchinnikov displays a cross-over effect from magnetic-field independent behavior at high fields to the proportionality ν ϕ ∗ ∼B − 1 2 at low fields. This low-field behavior results from the fact that ν ϕ ∗ multiplied with the inelastic quasiparticle scattering time τ in must reach at least the intervortex distance, thereby ensuring spatial homogeneity of the nonequilibrium quasiparticle distribution. At high magnetic fields the flux-flow instability becomes unobservable due to the onset of fluctuation effects contributing to the resistivity.

Alternating current losses of a 10 metre long low loss superconducting cable conductor determined from phase sensitive measurements

The ac loss of a superconducting cable conductor carrying an ac current is small. Therefore the ratio between the inductive (out-of-phase) and the resistive (in-phase) voltages over the conductor is correspondingly high. In vectorial representations this results in phase angles between the current and the voltage over the cable close to 90 degrees. This has the effect that the loss cannot be derived directly using most commercial lock-in amplifiers due to their limited absolute accuracy. However, by using two lock-in amplifiers and an appropriate correction scheme the high relative accuracy of such lock-in amplifiers can be exploited. In this paper we present the results from ac-loss measurements on a low loss 10 metre long high temperature superconducting cable conductor using such a correction scheme. Measurements were carried out with and without a compensation circuit that could reduce the inductive voltage. The 1 µV cm-1 critical current of the conductor was 3240 A at 77 K. At an rms current of 2 kA (50 Hz) the ac loss was derived to be 0.6±0.15 W m-1. This is, to the best of our knowledge, the lowest value of ac loss of a high temperature superconducting cable conductor reported so far at these high currents.

Measuring AC-loss in high temperature superconducting cable-conductors using four probe methods

Measuring the AC-loss of superconducting cable conductors have many aspects in common with measuring the AC-loss of single superconducting tapes. In a cable conductor all tapes are connected to each other and to the test circuit through normal metal joints at each end. This makes such measurements considerably more complex, especially for samples of laboratory scale (1-5 meters). Here we discuss different measurement configurations using four probe methods and lock-in detection. We conclude that the voltage should be picked up at end of the connecting joints, and we show how the resistive contribution from these joints can be identified and subtracted from the measured data. We also show measurements which indicate that the size of the loop constituted by the voltage leads has no influence on the measurements.

Microstructructural comparison of YBa2Cu3O7−x thin films laser deposited in O2 and O2/Ar ambient

Abstract The use of a diluted O 2 /Ar atmosphere for laser deposition of YBa 2 Cu 3 O 7−x thin films results in a strong decrease of the surface outgrowth density as compared to deposition in pure O 2 . The smoother films need a longer oxygenation period and show slightly lower critical current densities, though still in excess of 10 6 A cm −2 at 77 K. Electron microscopy revealed that the outgrowths mainly consist of a large copper-oxide grain connected to Y 2 O 3 grains. Y 2 O 3 nano-scale inclusions are present irrespective of the deposition atmosphere, however, at remarkably low densities compared to other literature data. We find that the twin plane density is lower and the twin structure more homogeneous in the case of films deposited in a mixture of O 2 /Ar. This we ascribe to the absence of surface outgrowths which seem to block regular twin structure formation. Possibly the differences in necessary post deposition oxygenation time and in the electrical properties should be found in the difference in twin structure.

Pulsed laser deposition of high-Tc superconducting YBa2Cu3Ox thin films in Ar/O2 atmosphere

Electrical and structural properties of superconducting YBa2Cu3Ox thin films, obtained with pulsed laser deposition in an Ar/O2 mixture at different oxygen partial pressures and constant total pressure, were studied. NdGaO3 (110) and LaAlO3 (001) substrates were used. The crystal quality of the films was found to correlate to the contents of inclusions with the axis oriented normal to the substrate plane. The increase of argon partial pressure resulted in decrease of particle density on the film surface to less than 106 cm−2, accompanied by an increase of a-oriented phase content and a rapid fall of the critical temperature from 91 K to 80 K

Operating a Cryogenic Test RIG for a 10 Meter Long Liquid Nitrogen Cooled Superconducting Power Cable

One way to cool a high temperature superconducting cable is to circulate liquid nitrogen (LN2) by means of a mechanical pump through a sub-cooler and through the core of the cable.

AC losses in circular arrangements of parallel superconducting tapes

The DC and AC properties of superconducting tapes connected in parallel and arranged in a single closed layer on two tubes (corresponding to power cable conductor models with infinite pitch) with different diameters are compared. We find that the DC properties, i.e., the critical currents of the two arrangements, scale with the number of tapes and hence appear to be independent of the diameter. However, the AC loss per tape (for a given current per tape) appears to decrease with increasing diameter of the circular arrangement. Compared to a model for the AC loss in a continuous superconducting layer (Monoblock model) the measured values are about half an order of magnitude higher than expected for the small diameter arrangement. When compared to the AC loss calculated for N individual superconducting tapes using a well known model (Norris elliptical) the difference is slightly smaller.

Measurements of AC losses in different former materials

A high temperature superconducting cable may be based on a centrally located cylindrical support, a so-called former. If electrically conductive, the former can contribute to the AC losses through eddy current losses caused by unbalanced axial and tangential magnetic fields. With these measurements we aim at investigating the eddy current losses of commonly used former materials. A one layer cable conductor was wound on a glass fibre reinforced polymer (GFRP) former. By inserting a variety of materials into this, it was possible to measure the eddy current losses of each of the former candidates separately; for example copper tubes, stainless steel braid, copper braid, corrugated stainless steel tubes, etc. The measured data are compared with the predictions of a theoretical model. Our results show that in most cases, the losses induced by eddy currents in the former are negligible. However, for materials with a low resistivity the eddy current losses may become significant, e.g., for high purity Cu or Al.

Electronic instability a high flux-flow velovities-Tc uperconducting films

Abstract The nonlinearity in the flux-flow process due to the finite energy relaxation rate of the quasiparticles has been studied in the mixed state of epitaxial c-axis oriented YBa 2 Cu 3 O 7−δ films. The inelastic quasipartcle scattering rate was found to decrease rapidly with decreasing temperature below T c .

Measuring ac losses in superconducting cables using a resonant circuit: resonant current experiment (RESCUE)

A simple way to obtain true ac losses with a resonant circuit containing a superconductor, using the decay of the circuit current, is described. For the measurement a capacitor is short circuited with a superconducting cable. Energy in the circuit is provided by either charging up the capacitors with a certain voltage, or letting a dc flow in the superconductor. When the oscillations are started - either by opening a switch in case a dc is flowing or by closing a switch to connect the charged capacitors with the superconductor - the current (via a Rogowski coil) or the voltage on the capacitor can be recorded using, for example, a digital oscilloscope. The amplitude decay of the periodic voltage or current accurately reflects the power loss in the system. It consists of two components - an ohmic purely exponential one (from leads, contacts, etc.), and a nonexponential component originating from the superconductor. The method has been successfully applied for the measurement of the ac loss in a 1 m long superconducting cable model.