F. Hornung

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.

High field magnet facilities and projects at the Forschungszentrum Karlsruhe

Three experimental facilities JUMBO, HOMER I, and MTA exist in the high field magnet group of the Institute for Technical Physics for investigations in high magnetic fields up to 20 T. All setups are based on advanced superconducting magnets. The facility Homer II is presently under construction in order to achieve magnetic fields up to 26 T. Our current projects focus on the development of solenoids for NMR (nuclear magnetic resonance) spectrometers. Together with our industrial partner Bruker Analytik GmbH, we introduced in 1991 the worlds first 750 MHz (17.6 T) and in 1995 the worlds first 800 MHz NMR spectrometer (18.8 T) on the market. At present a prototype magnet system for a 900 MHz spectrometer (21.1 T) is in progress. To further improve the resolution of the NMR spectrometers, a national project for the development of a 1000 MHz spectrometer (23.5 T) has been started together with Bruker Analytik GmbH and Vacuumschmelze GmbH.

Manufacture and test of a 5 T bi-2223 insert coil

With the goal of obtaining a magnetic field of 25 T in our facility HOMER II with a superconducting LTS-HTS hybrid magnet, a first prototype 5 T high temperature superconducting (HTS) insert coil has been constructed and tested. The HTS insert consists of 16 double pancakes made of stainless steel reinforced Bi-2223 tapes manufactured by American Superconductor. The HTS coil was operated at 1.8 K and produced 5.4 T at a current of 151.2 A. In a background field of 11.5 T provided by our facility HOMER I, a total field of 16.9 T was obtained several times. No training or quench of the coil was observed during the test, but after warming up a defect in the winding of one double pancake was detected, presumably due to a ballooning of the tape. The design of the coil and the results of the test are presented and discussed.

Quench Considerations and Protection Scheme of a High Field HTS Dipole Insert Coil

The large scale particle accelerators of the future in the 20 T regime are enabled by high temperature superconducting magnets. The dipole magnets needed in new high-field accelerators can be constructed with an YBCO insert and a Nb3Sn outsert. Such a configuration makes the quench analysis and magnet protection challenging because the quench behavior in both of these coils is different and there is very strong inductive coupling between the coils. The Nb3Sn coil is characterized by high energy and current and relatively fast quench propagation velocity. However, quench propagates slowly in YBCO coils because of typically wide spread large temperature margin. Currently, in the EuCARD project, a European collaboration is targeting to construct a small-scale high field YBCO-Nb3Sn hybrid magnet. In this paper, we scrutinize quench in the YBCO insert. We utilized an approach based on a solution of the heat diffusion equation with the finite element method. Additionally, we present a protection scheme for the coil.

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.

The compressibility of Rb3C60 derived by X-ray experiments under high pressure

Abstract The pressure dependence of the lattice parameter a ( p ) of Rb 3 C 60 has been investigated at 300 K and pressures up to 6 GPa using a diamond anvil cell and angular dispersive X-ray scattering. Rb 3 C 60 shows an anomalous increase of the bulk modulus under pressure, similar to the C 60 fullerite [1]. This behaviour is caused by the extremely small compressibility of the C 60 molecules, which can be considered by a modification of a standard EOS. The experimental a ( p ) data of Rb 3 C 60 are in an excellent agreement with a modified Birch equation, resulting in a bulk modulus of 20.5 GPa. Starting with the obtained modified EOS of Rb 3 C 60 we can calculate the T c ( a ( p )) behaviour of this material by using the experimental T c ( p ) data of Sparn et al. [2]. In contrast to Zhou et al. [3] the obtained T c ( a ( p )) dependence of Rb 3 C 60 is linear in the entire investigated pressure range and agrees with the T c ( a (M)) values from several superconducting M 3 C 60 compounds.

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.

Development of superconducting and cryogenic technology in the Institute for Technical Physics (ITP) of the Research Center Karlsruhe

Since the early 1970s the Institute for Technical Physics of the Research Center Karlsruhe has been involved in the development of superconductivity for research and industrial applications. A broad program with a focus on the superconducting magnet technology was established to include large magnets for nuclear fusion, high-field magnets for nuclear magnetic resonance spectrometers, ore separation and energy storage magnets. Research and development work was performed in collaborative projects with other national as well as international institutions and industry. The success of these projects has been supported by a broad foundation of engineering science in superconductor development, electrical and cryogenic engineering. Several well known test facilities like TOSKA, STAR, HOMER, MTA along with well equipped laboratories for conductor development, materials at cryogenic temperatures, cryogenic high-voltage engineering have made substantial contributions to in-house, national and international projects. A strong cryogenic infrastructure with two refrigerators and sophisticated cooling circuits from about 4.5 K down to 1.8 K assure the reliable operation of these large facilities. Last but not least, cryogenic research, including vacuum pumps for International Thermonuclear Experimental Reactor, improvements in thermal insulation, cryogenic instrumentation and small on board refrigerators has supported progress in this field. High-temperature superconductivity projects for low AC loss conductors, a 70 kA current lead and a fault current limiter are currently in progress.

Usage of Bi-HTS in high field magnets

At present, superconducting high field magnets built up of metallic low temperature superconductors (LTS) like NbTi and ternary/quaternary Nb/sub 3/Sn is near to the upper limit of achievable field strength. Fields above approx. 23 T seem to be only reachable with LTS-HTS hybrid configurations consisting of an outer LTS section and a high temperature superconductor (HTS) insert. Commercially available Bi-HTS wires were investigated for their application in high field facilities like the HOMER II system with the goal of 25 T and in new generations of NMR magnets of 1000 MHz and above. Therefore the superconducting properties of the HTS wires were examined at 4.2 K in magnetic fields up to 10 T. The voltage-current relation was examined resistively using a high resolution four-point measurement technique. The dependence of the critical current and the n-value on the winding diameter, on the field alteration (increasing/decreasing), and on the field orientation to the wire is presented and discussed.