Self field effects on the superconducting to normal transition of S/C cables

The onset of the transition of a superconducting (S/C) cable to the normal state is described by the relation V=V_c (the critical voltage, iⁿ⁺¹) where V is the voltage drop per unit length, V_c the critical voltage, i the reduced current I/I_c (I_c is the critical current), and n an integer ranging from 10 to 40 depending on the cable quality. A correct evaluation of the critical current and n value must take into account the effects due to the self-field at the cable. For high-current cables the self-field can affect the measurement by about 15%. A theoretical model is developed and it is applied to the results obtained in the critical current measurements of the S/C cables for HERA dipoles. It is found that the critical current and n are strongly dependent on the self-field