H.-W. Neumüller

Bio: H.-W. Neumüller is an academic researcher from Siemens. The author has contributed to research in topics: Superconductivity & Flux pinning. The author has an hindex of 7, co-authored 8 publications receiving 156 citations.

Development and application of superconducting transformers

Abstract: Superconducting transformers are an important innovation for future power transmission and transportation systems. Powerful, lightweight, energy-saving and environmentally friendly they offer enormous benefits compared to their conventional counterparts. Siemens is developing a 1-MVA demonstrator transformer for laboratory testing, exhibiting innovative features like horizontal design, cabled-conductor windings and a closed cooling cycle with sub-cooled nitrogen. Being one of the most promising applications Siemens has started a programme towards the development of on-board transformers for electrical rail vehicles. This paper summarises world-wide efforts in the development of superconducting transformers and reports on the progress achieved at Siemens.

Magnetic properties of Ln2−xCexCuO4−δ(Ln=Nd, Pr, Sm) in the superconducting and normal-conducting state

Abstract: We report on magnetic properties of the electron-doped cuprates Ln 2− x Ce x CuO 4− δ with Ln=Nd, Pr, Sm in the superconducting and normal conducting state. The preparation process of Ln 2− x Ce x CuO 4− δ bulks (Ln=Nd, Pr, Sm), was in vestigated by means of TGA and X-ray analysis. The normal state magnetic susceptivility of Ce-doped Ln 2 CuO 4− δ (Ln=Nd, Pr) is described by taking into account crystal field effects on the Ln 3+ sites as well as the contribution of the Cu 2+ ions. Furthermore we present results for the upper and lower critical fields and calculate from these the superconducting parameters ξ, κ and λ in the framework of the anisotropic Ginzburg-Landau theory.

Pinning model derived from resistive measurements on melt processed Bi2Sr2CaCu2O8

Abstract: On melt processed samples of the 86 K superconductor Bi 2 Sr 2 CaCu 2 O 8 we have performed resistive measurements in the low field limit B ⩽013 T and 40 K⩽ T ⩽77 K The voltage drop is found to rise exponentially with current E ∝exp j / j 0 , which is interpreted in terms of thermal activation of pinned flux lines An activation energy U 0 ( T )⩽90 meV is derived from the transition width j 0 ( T ) and is related to a plausible core pinning interaction of flux lines with normal conducting precipitates This reproduces the measured j c ( B, T ) values in the whole regime investigated We conclude that pinning centers must have a minimum size in order to control flux creep Finally we demonstrate that conventional summation of the single site pinning forces cannot account for the observed macroscopic depinning current density

Enhancement of critical current in melt processed Bi2Sr2CaCu2O8 by irradiation with 400 MeV oxygen ions

Abstract: Dense melt processed Bi2S2CaCu2O8 superconductor was irradiated below Tc with 400 MeV oxygen ions up to a dose of 155 × 1015 cm−2 Nearly linear with the dose we found a drop in Tc of 7 K and an increase of resistivity of 37% which recovered partly during annealing at 300 K Irradiation and annealing did not alter the transition width After irradiation the irreversible magnetization showed a marked increase at low temperatures, while the adverse effect of Tc reduction seemed to prevail for T > 60 K On the other hand the logarithmic time decay 1/M dM/d ln t was nearly unchanged This indicates, that the irradiation induced defects act as additional pinning centers with an activation energy U close to that of the intrinsic defects In contrast to magnetization currents, irradiation was found to enhance the critical transport current density jc(B,T) mainly at higher temperatures In particular the marked decrease of jc with field, which was found before irradiation, was reduced

Roebel cables from REBCO coated conductors: a one-century-old concept for the superconductivity of the future

Abstract: Energy applications employing high-temperature superconductors (HTS), such as motors/generators, transformers, transmission lines and fault current limiters, are usually operated in the alternate current (AC) regime. In order to be efficient, the HTS devices need to have a sufficiently low value of AC loss, in addition to the necessary current-carrying capacity. Most applications are operated with currents beyond the current capacity of single conductors and consequently require cabled conductor solutions with much higher current carrying capacity, from a few kA to up to 20-30 kA for large hydro-generators. A century ago, in 1914, Ludwig Roebel invented a low-loss cable design for copper cables, which was successively named after him. The main idea behind Roebel cables is to separate the current in different strands and to provide a full transposition of the strands along the cable direction. Nowadays, these cables are commonly used in the stator of large generators. Based on the same design concept of their conventional material counterparts, HTS Roebel cables from REBCO coated conductors were first manufactured at the Karlsruhe Institute of Technology (KIT) and have been successively developed in a number of varieties that provide all the required technical features such as fully transposed strands, high transport currents and low AC losses, yet retaining enough flexibility for a specific cable design. In the past few years a large number of scientific papers have been published on the concept, manufacturing and characterization of such cables. Times are therefore mature for a review of those results. The goal is to provide an overview and a succinct and easy-to-consult guide for users, developers, and manufacturers of this kind of HTS cables.

Josephson Junction Model of Critical Current in Granular Y1Ba2Cu3O7-δ

Abstract: We calculate the transport critical-current density in a granular superconductor in magnetic fields below about 5 x 10/sup -3/ T. The field dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions. Various model calculations are fitted to transport critical-current data on bulk Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub delta/ ceramic superconductors, whose average grain size somewhat exceeds 10 ..mu..m. The results yield an average junction cross-sectional area (thickness x length) of 4--6 ..mu..m/sup 2/. If the junctions are at the grain boundaries, a London penetration depth of about 150--300 nm is inferred, consistent with other estimates. We conclude that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries. The parameters of the fit are not consistent with Josephson junctions at twinning boundaries.

Roebel cables from REBCO coated conductors: a one-century-old concept for the superconductivity of the future

Abstract: Energy applications employing high-temperature superconductors (HTS), such as motors/generators, transformers, transmission lines and fault current limiters, are usually operated in the alternate current (ac) regime. In order to be efficient, the HTS devices need to have a sufficiently low value of ac loss, in addition to the necessary current-carrying capacity. Most applications are operated with currents beyond the current capacity of single conductors and consequently require cabled conductor solutions with much higher current carrying capacity, from a few kA up to 20–30 kA for large hydro-generators. A century ago, in 1914, Ludwig Roebel invented a low-loss cable design for copper cables, which was successively named after him. The main idea behind Roebel cables is to separate the current in different strands and to provide a full transposition of the strands along the cable direction. Nowadays, these cables are commonly used in the stator of large generators. Based on the same design concept of their conventional material counterparts, HTS Roebel cables from REBCO coated conductors were first manufactured at the Karlsruhe Institute of Technology and have been successively developed in a number of varieties that provide all the required technical features such as fully transposed strands, high transport currents and low ac losses, yet retaining enough flexibility for a specific cable design. In the past few years a large number of scientific papers have been published on the concept, manufacturing and characterization of such cables. Therefore it is timely for a review of those results. The goal is to provide an overview and a succinct and easy-to-consult guide for users, developers, and manufacturers of this kind of HTS cable.

ROEBEL Assembled Coated Conductors (RACC): Preparation, Properties and Progress

Abstract: RBCO (R = Y or rare earth element) coated conductors (CC) are the most promising HTS materials for future high field coils operated at moderately high temperature (40-50 K). Coils are planned for the second generation of fusion reactors (DEMO, "DEMOnstrator") and beyond. A ROEBEL bar concept for a high current (kA-class) low AC loss cable is the most suitable assembling technique for conductors in magnet windings due to the flat rectangular cross section. The presented RACC-cable technique (RACC=ROEBEL assembled coated conductors) works with pre-shaping of tapes into strands with the ROEBEL specific meander geometry. The usually very good bending properties of the CC support the assembling procedure of the RACC-cable. We report on a 16 strand RACC-cable with 19 cm transposition length made from CC material from the commercial supplier SuperPower which reached 1020 A transport critical current at 77 K (Jeng = 11.3 kAcm-2). The basic properties of the virgin YBCO tapes and the shaped strands like orientation and field dependent transport currents, current homogeneity and bending effects, were investigated and correlated with the measured properties of the RACC-cable. Calculation of the self field effects by means of a model adapted to the specific RACC-cable geometry and in particular taking into account the current distribution in the cable, explained the 30% current reduction in the cable quantitatively.