A cabled superconductor of current-carrying capacity of 10 kA at 6 T has been designed and fabricated to meet the requirements of the fast ramp-rate superconducting poloidal coils. The basic strand was a Cu/CuNi mixed matrix NbTi multifilamentary composite. The a.c. loss and the stability of subcables have been investigated under pulsing fields with a maximum of 9 T and ramp rates in excess of 200 T/sec. The temperature rise caused by a.c. losses was so small that the quench current in pulsing fields was close to the critical current at 4.2 K and considerably larger than the cold-end recovery current in static fields. These results confirm the scientific feasiblity of cryostable, low loss poloidal coils of pulsing rates of 200 T/sec.