H. van Weeren

A device to investigate the axial strain dependence of the critical current density in superconductors

We have developed an instrument to study the behavior of the critical current density (Jc) in superconducting wires and tapes as a function of field (μ0H), temperature (T), and axial applied strain (ea). The apparatus is an improvement of similar devices that have been successfully used in our institute for over a decade. It encompasses specific advantages such as a simple sample layout, a well defined and homogeneous strain application, the possibility of investigating large compressive strains and the option of simple temperature variation, while improving the main drawback in our previous systems by increasing the investigated sample length by approximately a factor of 10. The increase in length is achieved via a design change from a straight beam section to an initially curved beam, placed perpendicular to the applied field axis in the limited diameter of a high field magnet bore. This article describes in detail the mechanical design of the device and its calibrations. Additionally initial Jc(ea) dat...

Scaling the reversible strain response of MgB2-conductors

The reversible axial strain dependence of the critical current of MgB2 conductors is shown to vary with the temperature and magnetic field. The measured critical temperature and irreversibility field are also found to change reversibly with the axial strain. Combining these effects, we show empirically how the strain dependence of the whole critical surface can be scaled with just three parameters: the strain dependences of its three corner points.

Adiabatic normal zone development in MgB/sub 2/ Superconductors

A-priori knowledge of the normal zone development in MgB/sub 2/ conductors is essential for quench protection of applications. Therefore the normal zone propagation in a monofilament MgB/sub 2//Fe conductor under near-adiabatic conditions at 4.2 K has been measured and simulated. The results show normal zone propagation velocities up to several meters per second. In addition, by including the voltage-current relation into the computational model, the influence of the n-value on the normal zone propagation is determined. The simulations show that lower n-values suppress the normal zone propagation velocity due to lower heat generation in the MgB/sub 2/ filaments.

Conductor related design considerations for a 1 meter 10 T Nb/sub 3/Sn dipole magnet

A single-bore 1 meter long 10 T Nb/sub 3/Sn dipole magnet with a 88 mm bore is being developed. Based on powder-in-tube (PIT) Nb/sub 3/Sn strands exhibiting 22 /spl mu/m thick filaments, a Rutherford-type cable with a stainless steel core of 25 micron is employed. All properties relevant for magnet design and operation like critical current, filament magnetization and coupling current control have been investigated experimentally. Their impact on magnet operation is discussed. The effectiveness and reliability of a quench protection system for Nb/sub 3/Sn accelerator magnets relies mainly on the normal zone propagation properties and properly designed and positioned protection heaters. We present measurements and calculations of propagation properties and discuss the consequences for protection heater design.

Normal zone propagation in high-current density Nb/sub 3/Sn conductors for accelerator magnets

Self-absorbing quench protection schemes for accelerator magnets mainly rely on longitudinal and turn-to-turn normal zone propagation (NZP) immediately after the occurrence of a quench and subsequently on the effectiveness of protection heaters. Especially for impregnated Nb/sub 3/Sn coils the protection should not only aim at limitation of the hot spot temperature and internal voltages but also at avoidance of large temperature gradients and local stress accumulation. Considering Rutherford types of cable based on present high current density Nb/sub 3/Sn wires with a relatively low stabilizer content, a priori knowledge about their NZP properties is mandatory. Especially the medium and low-field properties appear to be critical for coil protection. The longitudinal NZP velocity of PIT-type Nb/sub 3/Sn conductors are investigated both experimentally and numerically in nearly adiabatic conditions typical for impregnated coils. Numerical simulations are extended to extremely high current density Nb/sub 3/Sn conductors and protection heater performance.