Th. Schneider

Investigation of n-values of composite superconductors

Knowledge of the voltage-current characteristics of composite superconductors is very important for the development of superconducting high-field magnets, especially NMR magnets. These curves are characterized by the n-values and were systematically investigated for NbTi, (NbX)/sub 3/Sn with additional components, X, and BSSCO-2223 composite superconductors by the standard four-point measurement technique. The influence on n-values of coating the NbTi filaments with Nb as well as the dependence of n-values on the internal structure of composite NbTi and (NbX)/sub 3/Sn superconductors will be presented. The n-values of Nb/sub 3/Sn are discussed under the aspect of the heat treatment parameters reaction time and reaction temperature. The n-values of various BSCCO-2223 superconductors will also be compared.

Investigation of Bi-HTS wires for high field insert coils

In the High Magnetic Field Laboratory of the Institute for Technical Physics, the test facility HOMER II is currently under construction. In a first step, a magnetic field of 20 T in a bore of 180 mm produced by advanced LTS materials is aspired. In a second step, insert coils built of HTS wires are planned to be added in order to obtain resulting fields up to 25 T. With these intentions in the background, the superconducting properties of different Bi-HTS wires were investigated in a bath cooled superconducting magnet system at 4.2 K and magnetic fields up to 10 T. The critical current I/sub c/ was examined resistively using a high resolution four-point measurement technique. In consideration of the determined current carrying capacity of the wires, two layouts for HTS insert coils are presented.

Investigation of critical current distribution in composite superconductors

For the design of superconducting magnets, the E-field vs. current curve (E(I)-curve) of the composite superconductor is an important property. We studied a model which describes the E(I)-curve by means of a Gaussian distribution of local critical currents. Therefore, resistive measurements in magnetic fields up to B=15 T were performed on several niobium-titanium, niobium-tin conductors and Bi-2223-conductors. We calculated the critical current distribution by differentiating E(I) twice with respect to the current. For metallic superconductors we got only the lower portion of the distribution because of sample quenches. That means, no complete distribution could be seen, but only a fraction of the curve. We developed a new numerical method to estimate the parameters of these fragmented critical current distributions. The knowledge of the parameters enabled us to calculate the whole curves and to compare them with the results of the measurements. This comparison clearly showed that for NbTi and Nb/sub 3/Sn composite superconductors, which are not additionally stabilised, the quench of the sample occurs far below the mean critical current.

Superconducting High Field Magnet Engineering at KIT

For over thirty years superconducting high field solenoid coils have been designed, constructed and operated in the High Magnetic Field Laboratory at the Karlsruhe Institute of Technology (KIT). During this time, three generations of experimental facilities have been built and commissioned to routine operation. Using metallic superconductors commercially available at the time, designs were created that enabled maximum field strengths to be achieved within specified boundary conditions. The latest facility is HOMER II, which is unique in its ability to generate a magnetic field strength of 20 T within a free bore of 185 mm. In this article various configurations of the facilities and their physical parameters are presented. Emphasis is placed on the experimental facility HOMER II including its cryogenic system, quench detection and analysis, as well as its protection concept.

Degradation of Bi-2223 Tape After Cooling With Superfluid Helium

Future superconducting magnets for fields of 25 T and above have to be composed of LTS-HTS hybrid coil systems. To obtain a higher field contribution and for reasons of stability, the outer low temperature superconducting (LTS) magnet section is cooled particularly with superfluid helium. In the classical set-up, the high temperature superconducting (HTS) insert is assembled together with the LTS outsert in a common bath, i.e. in our case it is cooled with superfluid helium. Our first 5 T Bi-2223 prototype insert coil was successfully operated and produced 5.4 T in a background field of 11.5 T. After warming up, ballooning was observed in the tape apparently caused by the penetration of superfluid helium. In this paper we investigate the impact of superfluid helium on the superconducting properties of the Bi-2223 tape used for our HTS insert. In particular, the voltage-current relation, U(I), is examined. It is shown that the resulting critical current and the n-value, which is a differential variable, are not adequate to describe the widely degraded U(I)- curves. In addition, we suggest the use of an integral method. The measurement results and the interpretation of the U(I)-curves are presented and discussed.

Microstructure and current-voltage characteristics of bronze processed niobium tin composites

Bronze route conductors are widely used for high field purposes. To shift their limits to even higher fields it is decisive to understand the role that microstructure plays in controlling the current-voltage characteristic. Microstructural properties of Nb/sub 3/Sn are crucially determined by the reaction heat treatment. Therefore, several heat treatments on samples of a commercial composite were performed. The reaction temperatures extend from 600/spl deg/C up to 750/spl deg/C. SEM micrographs were taken from filaments at different positions within the samples. These micrographs were used to carry out a quantitative analysis of grain size. To investigate the connection of microstructural and electrical properties, resistive measurements of the current-voltage characteristic at magnetic fields up to 15 T and 4.2 K were performed.

Ultrahigh-Speed Pulsed Laser Deposition of YBCO Layer in Processing of Long HTS Coated Conductors

Pulsed laser deposition (PLD) is classically considered as a very reliable process that provides correct stoichiometry of the deposited layer when the deposition speed is below 0.1 nm per deposition pulse. Initially, we found significant critical currents of 200-400 A/cm-width by very high deposition rates. Later, in this work, a study of reproducibility and feasibility of high- and ultrahigh-speed PLD is performed. With high speed, we denote a range from 0.1 to 0.3 nm/pulse, whereas ultrahigh speed corresponds to higher numbers, i.e., above 0.3 nm/pulse. At ultrahigh laser energy density, an optimum deposition rate is found that maximizes the critical current, i.e., Ic. The Ic at this maximum may reach 500-700 A/cm-width by relatively low (<; 1.5 MA/cm2) Jc. The optimal deposition rate and energy density may be employed in cost-feasible technological solutions because deposition time may be shortened by a factor of ~5. A suggested double-layer model with different critical currents in layers provides good fitting of the experimental dependence.

Characterization of Commercial MgB 2 Conductors for Magnet Application in SMES

Nowadays, long lengths of MgB2 conductors on the kilometer scale fabricated by the powder-in-tube process are commercially available. Due to its relatively low cost, high critical current density Jc, low anisotropy, and a critical temperature of about 40 K, MgB2 may be attractive for several applications. Examples of these are as MgB2 coils for superconductor transformers, as induction heaters, or as MRI magnets. MgB2 conductors also promise to be a usable option for superconducting coils in wind generators or as a superconducting magnetic energy storage (SMES) for the recently proposed LIQHYSMES approach. LIQHYSMES as a new hybrid energy concept combines the use of LIQuid HYdrogen as the primary high-density energy carrier with SMES for fast and efficient buffering. In this paper, we present the physical properties of MgB2 wires tested in several experimental configurations of a projected MgB2-based solenoid of a small LIQHYSMES test setup.

Heat treatment optimization of differently alloyed Nb/sub 3/Sn superconductors

For an upgrade of our HOMER II magnet facility from 20 T to 24 T one possibility is to build insert coils made of alloyed Nb/sub 3/Sn wires. These wires require a heat treatment (HT) to form the superconducting phase by solid state diffusion. The temperature and duration of the HT determines the size of the superconducting layers and the grain structure within these layers and hence physical properties like critical current and n-value. To optimize the HT with regards to the planned operation at very high magnetic fields, two differently alloyed conductors were heat treated between 600/spl deg/C and 750/spl deg/C for 50 h up to 350 h. Critical currents and n-values are determined by E(I)-measurements in magnetic fields up to 15 T. Special attention is drawn to the upper critical field and the maximum pinning force, which allow, in principle, an extrapolation of the measured data to higher magnetic fields. The results show differences of the optimum HT for different magnetic background fields, which will be discussed.

Optimization of the niobium-tin inserts for the 80 kA current lead to be used in the TOSKA facility for the ITER toroidal field model coil test

One essential tool in the construction of the forced-flow cooled 80 kA current lead to be used in the TOSKA facility at FZK for the ITER TFMC tests is the use of niobium-tin inserts in the lower temperature region of the heat exchanger allowing the operation of the lead with minimum He mass flow rate in a wide current range. Due to the fact that there is only limited space for the superconductor inserts in the 80 kA current lead, it was necessary to look for niobium-tin strands having a very high critical current at low magnetic fields and high temperatures. It was decided to use internally stabilized bronze routed strands because of the absence of an outer barrier which may hinder the current transfer from the copper of the current lead to the superconductor filaments. During the test measurements, it was found that the current transfer from the external copper to the superconductor was a second important criterion for the strand choice. A series of measurements were done using superconductor strands embedded in profiles made of different copper materials. The result of the test measurements are presented which were the base for the decision which strand material is appropriate for the inserts.