Kris Rosseel

Pulsed magnet design software

In order to design pulsed field magnets with internal reinforcement, the construction parameters (material selection, thickness of reinforcements, etc.) must be optimized. Therefore we have combined in a single, user-friendly package the different aspects of pulsed magnet design by means of a VISUAL BASIC shell. At the basis of the program are the three FORTRAN codes, which calculate the field distribution and basic coil parameters, the discharge of a capacitor bank into the coil, and the stress distribution in the mid-plane of the coil (including winding pre-stress, thermal expansion and plastic deformation). Stress and strain are calculated both at peak field and after the pulse.

Critical currents, pinning forces and irreversibility fields in (YxTm1−x)Ba2Cu3O7 single crystals with columnar defects in fields up to 50 T

Abstract We have studied the influence of columnar defects, created by heavy-ion (Kr) irradiation with doses up to 6×1011 Kr-ions/cm2, on the superconducting critical parameters of single crystalline (YxTm1−x)Ba2Cu3O7. Magnetisation measurements in pulsed fields up to 50 T in the temperature range 4.2–90 K revealed that: (i) in fields up to μ0H≈20 T the critical current Jc(H,T) is considerably enhanced and (ii) down to temperatures T∼40 K the irreversibility field Hirr(T) is strongly increased. The field range and magnitude of the Jc(H,T) and Hirr(T) enhancement increase with increasing irradiation dose. To interpret these observations, an effective matching field Bφ· was defined. Moreover, introducing columnar defects also changes the pinning force fp qualitatively. Due to stronger pinning of flux lines by the amorphous defects, the superconducting critical parameters largely exceed those associated with the defect structures in the unirradiated as-grown material: J c,6×10 11 Kr-ions/cm 2 (77 K, 5 T)≥10Jc,ref (77 K, 5 T).

Multi-composite wires for pulsed field coils

Abstract “Multi-composite wire” is a new type of strong wire for winding coils to generate very strong-pulsed magnetic fields. The basic concept is filling the free spaces in a bundle of thin conducting wires with reinforcing fibres, and enclosing this composite wire in an insulating sleeve. In this concept, any desired combination of conductor and reinforcement can be used. To insulate the wire and keep the bundle together, a sleeve consisting of a strong insulating fibre is braided around the core. As a proof of principle, multi-composite wires have been made consisting of soft copper in combination with carbon fibres. Glass fibre was employed for the sleeve. The mechanical properties of the wires were determined from tensile and “explo-vessel” tests. The latter allows the investigation of the mechanical properties under conditions as they prevail in a coil. The first experimental wire had up to 88% of the theoretically calculated UTS. Further development is discussed.

Flux jumps driven by a pulsed magnetic field

The understanding of flux jumps in the high temperature superconductors is of importance since the occurrence of these jumps may limit the perspectives of the practical use of these materials. In this work, we present the experimental study of the role of heavy ion irradiation in stabilizing the HTSC against flux jumps by comparing un-irradiated and 7.5 × 10 10 Kr ion/cm 2 irradiated (Y x Tm 1- x )Ba 2 Cu 3 O 7 single crystals. Using pulsed field magnetization measurements, we have applied a broad range of field sweep rates from 0.1 to 1800 T/s to investigate the behavior of the flux jumps. The observed flux jumps, which may be attributed to thermal instabilities, are incomplete and have different amplitudes. The flux jumps strongly depend on the magnetic field, on the magneto-thermal history of the sample, on the magnetic field sweep rate, on the critical current density j c , on the temperature and on the thermal contact with the bath in which the sample is immersed.

Stress release at Y2BaCuO5 inclusions in fast melt processed YBa2Cu3O7-x observed by micro-Raman spectroscopy

We have studied the interface region between the superconducting YBa2Cu3O7−x (123) matrix and the embedded nonsuperconducting Y2BaCuO5 (211) inclusions in fast melt processed samples, by recording micro-Raman spectra along lines crossing these inclusions. The observed relative shifts of the 123 Raman modes indicate the accumulation of compressive stress in the bulk of the 123 material and a release of this stress around the 211 inclusions. These results show that for practical applications of the fast melt processed materials, beside strong pinning, proper mechanical properties around the pinning centers are important.

Magnet and wire technology at the K.U.Leuven Pulsed Field laboratory

The K.U.Leuven Pulsed Field Facility is well equipped to accommodate a large variety of experiments in solid-state physics under extreme conditions of high magnetic fields (>50 T) and low temperatures (down to 50 mK). This is supported by an extensive research program focused on the development of advanced concepts and materials for increasing the attainable magnetic fields as well as the reliability and service life of pulsed field coils. Pulsed field magnets with optimized reinforcement as well as multi-composite (MC) wire are under development. The combination of Zylon/spl reg/ for internal reinforcement and strong axial compression has resulted in the development of 75 T class pulsed magnets using soft copper as the conductor material. The MC wire is a new concept; it consists of a glass fiber sleeve braided around a core containing thin conductor wires and reinforcement fibers. The core composition and geometry can be varied to obtain the required properties of strength and electrical conductivity. For a proof of principle, an MC wire was developed with an ultimate tensile strength of 1 GPa and a conductivity of 38% IACS at 300 K.

Simulation and calibration of an open inductive sensor for pulsed field magnetization measurements

The relation between the voltage induced in an open sensor and the magnetic moment of a sample during pulsed field magnetization measurements has been calculated. The calculation takes into account the size of a sample, its internal flux distribution, and its position with respect to the center of the sensor. It has been found that the behavior of the response signal with respect to the sample position is the same for homogeneously magnetized Ni samples and high temperature superconductors. Because of this universality, it is possible to calibrate the open sensor even in the case of an unfavorable sample geometry. The calculations have been experimentally verified by using Ni samples with different geometries. Our results can easily be extended to samples with an arbitrary local field distribution.

Pulse shape modification for capacitor driven pulsed magnets

Two special design features of capacitor banks for driving pulsed magnets are proposed; these are dimensioned by Laplace transforms. The stray capacitance and the inductance of the wiring can result in sharp voltage pulses with amplitude of up to twice the initial charging voltage. A simple RC filter is designed which efficiently suppresses these overvoltages. The second feature is a circuit that modifies the pulse shape in order to facilitate magnetization measurements on superconducting samples where the magnetic response not only depends on the field but also on the rate of the field sweep. This is achieved by adding a suitable combination of inductors and capacitors to the capacitor bank circuit.

Frontiers of pulsed magnet design

At the K.U. Leuven Pulsed Field Laboratory, high performance coils with optimized internal reinforcement by fiber composites have been pioneered and continuously further developed. Coils made with soft copper wire and Zylon-epoxy composite have sustained fields up to 75 T. For extended service life, 75 T user coils are made with an inner section of strong micro-composite wire and an outer section of soft copper, with additional outer reinforcement. A system of axial compression has been designed that increased peak field significantly. Based on the results obtained with these coils, an outlook to possible future developments are given. This includes a discussion of new materials such as the multi-composite wire and high strength M5 fibers developed by Magellan.

Vortex dynamics studies in pulsed magnetic fields

Abstract We have studied the vortex dynamics of High-Temperature Superconductors (HTS) in pulsed magnetic fields up to 50 T. Due to the very high sweep rates involved ( μ 0 d H a /d t ≈10 4 T/s), the induced current density j approaches the non-relaxed critical current density j c0 . Turning the sweep rate during a high-field pulse makes it possible to vary the voltage criterion at which j is determined. Furthermore, the use of pulsed magnetic fields substantially increases the experimentally accessible region in the H – T phase diagram.