Transport critical current of MgB2 wires: pulsed current of varying rate compared to direct current method

Abstract

The measurement of transport critical current (Ic) forMgB2 wires and tapes has been investigated with two different techniques, the conventionalfour-probe arrangement with direct current (DC) power source, and a tailored trianglepulse at different rates of current change. The DC method has been widely used andpracticed by various groups, but suffers from inevitable heating effects when highcurrents are used at low magnetic fields. The pulsed current method has no heatingeffects, but the critical current can depend on the rate of the current change (dI/dt) in the pulse. Our pulsed current measurements with varyingdI/dt show that thesame values of Ic are obtained as with the DC method, but without the artifacts of heating. Our method isparticularly useful at low field regions which are often inaccessible by DC methods. We alsoperformed a finite element method (FEM) analysis to obtain the time dependent heat distributionin MgB2 due to the electric potential produced at the current contacts to the superconductingsample and its gradient around the contacts. This gradient is defined as the currenttransfer length (CTL) of the samples and leads to Joule heating of the wire near thecontacts. The FEM results provide further evidence of the limitation of the DC method inobtaining high transport critical current.