Probability of Premature Quenching of HTS Coil Due to Local Reduction of Critical Current

In the 2
^nd
generation of HTS wires prepared as “coated conductors” often the value of critical current,
I_c
, fluctuates along the length. Then in DC transport a disproportionate heating could take place in the “weak spots” with reduced critical current. Recently an analytical model has been developed for predicting the maximum current acceptable in the weak spot before turning into a “hot spot”. One of the findings is that shorter weak spots will sustain higher currents than longer weak spots. But, the testing of critical current is usually performed on conductor lengths substantially exceeding the millimeter size of a weak spot. Then, a small weak spot could easier escape attention during such inspection. The analysis allows to find the relation between the critical current, obtained in testing of the sample containing a weak spot, and the current causing its thermal runaway. It is then possible to draw recommendation for the distance of voltage taps in the critical current testing of a conductor intended for use in an HTS coil. Also, the benefit of utilizing more sensitive criterion for the
I_c
detection, i.e. 0.1 μV/cm instead of the usual 1 μV/cm, can be quantified.