Ramp rate instability of multifilamentary superconductors due to longitudinal magnetic field

Ramp rate instability due to an external longitudinal magnetic field of Cu/NbTi multifilamentary superconductor was studied experimentally and theoretically. Quench current was measured when transport current and external magnetic field with transverse and longitudinal component were ramped up simultaneously. At increasing ramp rate, quench current of S-twisted (anticlockwise-twisted) multifilamentary superconductor was degraded in the positive longitudinal magnetic field parallel to the transport current. The calculated stability limit agreed with the experimental quench current within an error of 17%. Furthermore, we numerically estimated the dependence of stability limit on ramp rate and longitudinal magnetic field for a multifilamentary superconductor in practical large scale devices.