Persistent-current magnetizations of Nb3Sn Rutherford cables and extracted strands

Abstract

The magnetizations of eight high-gradient quadrupole cables designated HQ and QXF and a pair of strands, identical in architecture but with different effective strand diameters extracted from an HQ and a related QXF cable, were measured. In the service of field quality assessment, the cable magnetizations and losses were measured by pickup coil magnetometry at 4.2 K in face-on fields, Bm, of ± 400 mT at frequencies, f, of up to 60 mHz. Based on the coupling component of loss, Qcoup, the coupling magnetization Mcoup = Qcoup/4Bm was derived for a ramp rate of 7.5 mT/s. Persistent current (shielding) magnetization and loss (Msh and Qh,strand) were measured on short pieces of extracted strand by vibrating sample magnetometry at 4.2 K. Unpenetrated M-B loops to ± 400 mT and fully penetrated loops to ± 14 T were obtained. Mcoup can be easily controlled and reduced to relatively small values by introducing cores and adjusting the preparation conditions. But in low fields near injection Nb3Sn’s high Jc and correspondingly high Msh,cable may call for magnetic compensation to preserve field quality. The suitably adjusted cable and strand fully penetrated M-B loops were in reasonable accord leading to the conclusion that strand magnetization is a useful measure of cable magnetization, and that when suitably manipulated can provide input to magnet field error calculations.