Walter H. Fietz

Experimental results of a 70 kA high temperature superconductor current lead demonstrator for the ITER magnet system

In the frame of the European Fusion Technology Programme, the Forschungszentrum Karlsruhe and the CRPP Villigen have designed and built a 70 kA current lead for the ITER TF Coils using High Temperature Superconductors (HTS). At the beginning of 2004 the HTS current lead was installed and tested in the TOSKA facility of the Forschungszentrum Karlsruhe. The scope of the experiment was to characterize the current lead in steady state conditions and to explore the operation limits as well. For this, the temperature profile, the contact resistances, the heat load at 4.5 K, the required 50 K He mass flow rate, and the temperature margin were evaluated. The safety margin in case of a loss of He mass flow was studied, too. The paper describes the experimental results as well as the thermal and electrical models developed.

X-ray investigations of Y1Ba2Cu4O8 under high pressure

Abstract The lattice parameters of the high temperature superconductor (HTSc) Y1Ba2Cu4O8 with Tc=80 K have been investigated in a pressure range of 0 to 25 GPa and at temperatures between 30 and 301 K. The linear compressibility in the b-axis direction of 1.0x10-3 GPa-1 is found to be quite low, compared to the very high linear compressibilities in the a-axis direction of 2.8x10-3 GPa-1 and in the c-axis direction of 4.5x10-3 GPa-1. The resulting bulk moduli of B=112 GPa at 301 K and B=122 GPa at 30 K are quite low compared with other HTSc materials. This strong anisotropy in all three lattice directions has not been observed in other HTSc materials so far. Within the investigated pressure and temperature range no further anomalies could be observed.

High Temperature Superconductor Current Leads for WENDELSTEIN 7-X and JT-60SA

Forschungszentrum Karlsruhe has taken over the responsibility for the design, construction and testing of the High Temperature Superconductor (HTS) current leads for two fusion experiments, i.e. the stellarator WENDELSTEIN 7-X (W7-X) and the satellite tokamak JT-60SA. W7-X is presently under construction at the Greifswald branch of the Max-Planck-Institute for Plasma Physics and consists of 50 non-planar and 20 planar coils with a maximum conductor current of 17.6 kA. In total 14 current leads are required with a nominal current of 14 kA that are mounted upside down with the warm end at the bottom. In the frame of the Broader Approach Agreement between Japan and the EU and concomitantly to the ITER project, the satellite tokamak project JT-60SA has been agreed in 2006. The magnet system of JT-60SA consists of 18 toroidal field coils, 4 central solenoid modules and 7 poloidal field coils. In total 26 leads mounted in vertical, normal position are required. For W7-X and JT-60SA a common basic design will be used which will be adapted to the special needs of the machines. All current leads will be of the Cu-HTS binary type. The HTS part covers the range between 4.5 K and 60 K and is cooled by heat conduction from the 4.5 K end, only. The Cu heat exchanger is cooled with 50 K He and covers the range between 60 K and room temperature. The paper describes the status of the HTS current lead development for W7-X and JT-60SA.

Design and fabrication of a 70 kA current lead using Ag/Au stabilized Bi-2223 tapes as a demonstrator for the ITER TF-coil system

In the frame of the European Fusion Technology Programme, the Forschungszentrum Karlsruhe and the Centre de Recherches en Physique des Plasmas, CRPP, Villigen, develop a 70 kA current lead to demonstrate the feasibility of the usage of High Temperature Superconductors (HTS) for the current leads of the ITER TF coils. Design and fabrication of the conventional heat exchanger that covers the temperature range from 65 K to room temperature is done at the Forschungszentrum Karlsruhe. The HTS module connects the current feeder terminal at 4.5 K with the conventional heat exchanger and is cooled only by heat conduction from the cold end. Its design and fabrication has been performed by American Superconductor, AMSC. This module consists of Bi-2223 Ag/Au tapes embedded in stainless steel carriers with copper end caps. It is highly instrumented with voltage taps, temperature sensors and Hall probes. The integration with the heat exchanger and the current feeder terminal is carried out at the Forschungszentrum Karlsruhe. The current lead will be tested in the TOSKA facility in the first months of 2004. The paper describes the design, fabrication and results of pretests carried out at AMSC and Forschungszentrum Karlsruhe.

Giant pressure effect in oxygen deficient YBa2Cu3Ox

Abstract The pressure effect on T c of polycrystalline and single crystalline YBa 2 Cu 3 O x investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x > 6.93 the single crystals show a negative d T c /d p , as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find d T c /d p = 0.4 K/GPa which points to pressure effects on T c aside from charge transfer. In the underdoped region x T c /d p values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, d T c /d p increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the d T c /d p drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa 2 Cu 3 O x structure and can be explained by considering the anisotropic structure of YBa 2 Cu 3 O x . The decrease of the c -axis lattice parameter results in a charge transfer to the CuO 2 -planes mainly [1], whereas the compression of the a - and b -axis lattice parameter is known to produce different pressure effects which are responsible for the peak in d T c /d p at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of T c to the equilibrium value T c ( p ) at this pressure with a time constant depending on the oxygen content x . A decrease x results in a peak effect in d T c /d p at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, d T c /d p first drops to 5 K/GPa at x = 6.6, but the increases to values of more than 20 K/GPa for x T c increase (d T c /d p ) O due to pressure induced oxygen ordering via oxygen motion between unit cells.

Possible transformation of YBa2Cu3O7 under pressure

Crystal structure measurements on the high-Tc material YBa2Cu3O7 were carried out under pressure. Several X-ray reflections do not reveal a uniform shift with increasing pressure. Two models are presented, which show the experimental data to be consistent with an extensivec-axis orb-axis decrease, respectively.

The effect of high hydrostatic pressure on Tc of YBa2Cu3Ox as a function of the oxygen content

Abstract The AC-susceptibility χ′, χ′ of YBa 2 Cu 3 O x polycrystals has been determined under hydrostatic He-pressures up to 0.8 GPa. The experimental data allow a separation to be made of the pressure effect on the superconducting transition temperature T c into two parts, the pressure effect from the weak link structure of the polycrystalline material and the true pressure effect of the bulk material. These investigations have been performed on a series of polycrystalline samples of YBa 2 Cu 3 O x with various oxygen content. Each sample has been cut from the same pellet followed by a careful adjustment of the oxygen content by a heat treatment in air. The true pressure effect of the bulk material d T c /d p shows an increase from ≈ 0.4 K/GPa to ≈ 4 K/GPa for an oxygen content decreasing from x ≈ 6.93 to x ≈ 6.77. If the oxygen content is reduced further, d T c /d p is almost constant, except for a narrow maximum of ≈ 7 K/GPa at x ≈ 6.72. This maximum can be related to the oxygen content, where the transition temperature of YBa 2 Cu 3 O x changes from the 90 K to the 60 K plateau.

Separation of the intrinsic pressure effect on Tc of YBa2Cu3O6.7 from a Tc enhancement caused by pressure-induced oxygen ordering

Abstract The superconducting transition temperature Tc of polycrystalline YBa2Cu3O6.7 has been investigated under purely hydrostatic He-gas pressure. In a series of experiments, it has been shown that the degree of oxygen ordering depends on the applied pressure and the storage temperature TS of the sample. At temperatures below ≈240 K, there is no indication for oxygen ordering. When pressure is applied at such low temperatures, the Tc values show no relaxation effects, and (dTc/dp)i=7.4 K/GPa is observed which we believe is intrinsic. At temperatures above 240 K, the oxygen sublattice reacts reversibly to the applied temperature and pressure. Changes of the storage temperature or pressure cause Tc relaxations with time constants that are temperature dependent. When the relaxations were completed the effects of pressure and storage temperature on Tc are independently determined to be: (1) a pressure-independent effect of the storage temperature Tc with dTc/dTS=−0.02, (2) a temperature-independent effect of pressure-induced oxygen ordering with (dTc/dp)o=4.1 K/GPa. For temperatures above 240 K, the intrinsic and the oxygen ordering effect add to dTc/dp=11.5 K/GPa.

C60 under pressure-bulk modulus and equation of state

C60 has been investigated under pressure up to 13 GPa using angular dispersive X-ray scattering and a diamond anvil cell. The resolution of the experimental setup allows to examine the volume decrease dV/dp under pressure even for pressures of a tenth of a GPa. The obtained data of numerous experimental runs result in a bulk modulus of 13.4 GPa, which is much smaller than the value reported by Duclos et al. [1]. At 170 K and 70 K a bulk modulus of 14.2 GPa and 14.7 GPa was obtained, respectively. The pressure induced fcc-sc transition at 300 K was clearly visible at approx. 0.3 GPa with a jump in the lattice parameter of 0.05 Å. With increasing pressure we found an extreme change in dV/dp, which disables the usage of common equations of state (EOS), like the Murnaghan [2] or Birch [3] equation. Considering the small compressibility of the fullerence molecules we suggest a modified EOS to describe the experimental data.

70 kA High Temperature Superconductor Current Lead Operation at 80 K

For the superconducting magnet system of the International Thermonuclear Experimental Reactor, ITER, 60 current leads for a total current of more than 2500 kA are needed. To reduce the resultant large refrigerator load at 4.5 K, High Temperature Superconductor current leads (HTS-CL) could be used. Therefore, EFDA CSU Garching had launched a development program for a 70 kA HTS-CL demonstrator. The Forschungszentrum Karlsruhe and CRPP developed and built this CL optimized for 50 K Helium operation. In 2004, the CL was successfully tested in the TOSKA facility at the Forschungszentrum Karlsruhe. The very encouraging results lead to testing this CL with 80 K Helium because ITER provides a large 80 K Helium cooling capacity for the thermal shields. At the end of last year, the test could be successfully performed demonstrating that high current capacity current leads can be stably operated at about 80-85 K. Recently, the CL was retested using liquid nitrogen which would be an interesting alternative option. In this paper, the test results for the 80 K He cooling operation as well as for the operation with LN2 are presented and compared to the results obtained before for nominal conditions