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

The onset of the transition of a S/C cable to the normal state is well described by the relation V=V/sub c/ i/sup (n+1)/ where V is the voltage drop per unit length, V/sub c/ the critical one, i the reduced current I/I/sub c/ (I/sub c/ is the critical current), n is 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 the high current cables (High Energy Physics applications) the self field can affect the measurement for about 15%. In the present work 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, actually performed at I.N.F.N. laboratory in Genoa. It is found that the critical current and the n-value are strongly dependent on the self field