Willy Boon

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.

Experimental techniques for pulsed magnetic fields

Abstract Recent developments at the K.U. Leuven pulsed field laboratory are reviewed. This includes new 60 T user magnets, pulse shaping, decentralized data recording for multiple experiments, low temperature cryogenics, vibration damping, rotatable sample holders and sensitive magnetization measurements. Experimental results (on semiconductors, super-conductors and organic conductors) are shown to illustrate the improvements in the experimental techniques.

Magnetoresistance and conductivity fluctuations in single crystalline bi1.7pb0.3sr1.9cacu1.9o8+x

Abstract The magnetoresistance of a Bi1.7Pb0.3Sr1.9CaCu1.9O8 + x single crystal with Tc=91.5 K has been investigated in pulsed magnetic fields up to 22 T and in the temperature range T=(67–100)K. The experimental data at T

Ceramic superconductors synthesized by sol-gel methods

A sol-gel method is described based on the use of organic complexing agents in liquid solution, such as citric acid, ethylene-diamine-tetra-acetic acid (EDTA) and others. It is shown how the procedures can be designed on a scientific basis, using pM′-pH diagrams for the respective cation and complexing agent. This technique yields powders and sintered materials with superior properties to the dry mix and calcine method. An important advantage of the method is its versatility due to the fact that stable complexes can be formed for a wide range of elements. Precise control of stoichiometry is possible, however the residual carbon level is more difficult to control.

Scaling properties of the anisotropic magnetoresistance in YBa2Cu3O7⧸PrBa2Cu3O7 superlattices

Abstract We report on measurements of the magnetoresistance as a function of the angle θ between magnetic field H and the basal plane in YBa2Cu3O7/PrBa2Cu3O7 superlattices with two different thicknesses of the separator layer (YBCO:PrBCO=1:1 and 1:3). The angular dependence of the magnetoresistance for different field intensities can be scaled into a single curve by introducing the reduced field, H(sin 2 θ+γ -2 cos 2 θ) 1 2 , with the anisotropy paramete γ. In the YBCO/PrBCO=1:3 superlattice, quite good scaling is achieved with only the perpendicular field component, which implies fully decoupled YBCO layers (γ=∞). In the YBCO/PrBCO=1:1 superlattice, however, a finite anisotropy (γ≈50) is necessary to obtain a convincing scaling. Measurements of the magnetoresistance for different directions between magnetic field and transport current in these superlattices have shown that the resistive transition broadening in the field is independent of the macroscopic Lorentz force. An interpretation of these data based on the two-dimensional fluctuation model is also presented.

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.

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.

Effect of dimensional crossover on the magnetoresistance of YBa2Cu3O7/PrBa2Cu3O7 superlattices

Abstract We report on magnetoresistance measurements of YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PrBCO) superlattices in fields up to 12 T at temperatures below, but not too far from, the superconducting transition temperature Tc. By varying the thickness of the PrBCO separator layer, the influence of the interlayer coupling on the superconducting properties is studied. An excellent fitting by the Larkin two-dimensional superconducting fluctuation theory with only two scaling parameters (A(T) and HΦ(T) for each R(T) curve shows that by taking thicker PrBCO layers it is possible to induce the dimensional crossover from the anisotropic 3D behavior in pure YBCO to the quasi-2D behavior in YBCO/PrBCO superlattices. The resistive transition broadening in a magnetic field can be related to giant conductivity fluctuations in the quasi-two-dimensional CuO2 superconducting double layers in YBCO. For Y:Pr=1:3 and 1:5 superlattices, the temperature dependence of the first scaling parameter A(T), determined by the fluctuation amplitude, follows quite well the Larkin β(T) function typical for 2D systems. For Y:Pr = 1:1 and 5:6 superlattices, the YBCO layers are not completely decoupled or too thick and A(T) deviates from β(T). The temperature dependence of the second scaling parameter HΦ(T), the phase coherence breaking field, is consistent with the recent theoretical prediction by Reizer for the quasi-two-dimensional electron-electron interactions. The resistive transition tail, where the characteristic resistance is much smaller than the normal state resistance, is not directly caused by these fluctuations but rather is related to thermally assisted flux motion.