Measurement of DC critical current in superconducting cable with non-uniformities

We investigated the effect of nonuniform current distribution on the critical current experimentally determined on a superconducting cable. In theoretical model, the cable is considered as a set of parallel current paths, each containing one single tape. Critical currents, n-factors and contact resistances between each tape and current termination are the input data of the model. Spread in these parameters results in a nonuniform distribution of DC current among tapes. Then, the voltages on different tapes would vary, and the determination of cable's critical current by a standard procedure becomes difficult. This was shown on a single-layer cable model, manufactured by placing 16 straight Bi-2223/Ag tapes in parallel on a cylindrical epoxy fiberglass mandrel. With the help of brass shunts connected in series with each tape, the currents in all the tapes can be measured simultaneously. Experimental signals registered on different tapes were in excellent agreement with theoretical predictions. We found the spread in contact resistances to be rather critical issue for our 1 m long model.