Index Loss Effect in n-Value Measurements of Commercial Superconductors

n-value of the superconductor is key parameter in the persistent magnet design, affecting cost and performance. Analysis is presented that shows that a few mK overheat from the nonlinear index loss during standard Ic measurement of NbTi and Nb₃Sn conductors in the helium bath can significantly affect the measurements and produce artificially high n-values. The error increases in conductors with higher n. Effect of low heat transfer, E_ref criterion and index stability crisis are discussed. Analytical correlation is presented between the apparent and actual n-values.