Current-voltage characteristics of an HTS coil having a hot spot

Due to a slanted electric field-current density characteristics some power dissipation exists in HTS magnets even at operation currents below a critical current determined by using some electric field criterion. Thus, the temperature inside an HTS magnet does not remain constant during a single voltage (V)-current (I) measurement. Furthermore, a sufficiently large operation current will produce a hot spot in the turn of the maximum electric field inside the coil. In this paper the effect of the temperature increase during a V(I)-measurement on the V(I)-characteristics of a Bi-2223/Ag coil is studied at 20, 40 and 56 K. The V(I)-calculations take into account both the material inhomogeneity and anisotropy and are based on the short sample data measured with the Bi-2223/Ag tape. The investigation is concentrated at overcritical currents, where the effect of temperature increase on the V(I) -characteristics of the coil is significant.