Current Redistribution in a Sub-Scale REBCO Dipole

A sub-scale dipole has been designed, fabricated, and tested to investigate current re-distribution in a non-insulating, non-transposed rare-earth barium copper oxide (REBCO) tape-stack cable operating in a region of magnetic field gradient. The dipole winding is comprised of one turn of cable containing 12 tapes. An H-geometry steel flux return shapes the field distribution in its dipole gap and produces a nearly linear transverse gradient of field magnitude in the winding. The field gradient determines the critical current (I<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub>) in successive tapes in the tape-stack cable. As cable current is increased, the I<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> limit is approached in the innermost tape, and current is naturally driven to redistribute to the next tape, and so forth, so that premature quench is prevented. Experimental results are presented for several modes of operation to determine the distribution of current and the location(s) in the winding where the current is redistributed. The results serve as a proof-of-concept for using a non-insulating, non-transposed REBCO tape-stack cable and a conformal winding strategy for a high-field insert in an 18 T collider dipole.