Magnetization, Low Field Instability and Quench of RHQT Nb₃Al Strands

Ryuji Yamada, Masayoshi Wake,
Akihiro Kikuchi, and Vesselin Velev

Abstract—Since 2005 we made and tested three RHQT Nb₃Al strands, one Nb matrixed and two Ta matrixed, which are fully stabilized with Cu electroplating. We observed anomalously large magnetization curves extending beyond 1 to 1.5 Tesla with the F1 Nb matrixed strand at 4.5 K, when we measured its magnetization with a balanced coil magnetometer. This problem was eliminated with the Ta matrixed strands operation at 4.5 K. But with these strands a similar anomalous magnetization was observed at 1.9 K. We studied these phenomena with FEM. It is explained that at low external field, interfilamentary coupling currents in the outer layers of subelements make a shielding current, reducing the inside field, keeping the inside Nb matrix in superconducting state and standing against higher outside field beyond H_c of Nb. At further higher external field eventually the superconductivity of the Nb matrix elements collapses and releases a large energy, causing the quench at that point. Depending on the size of the energy the strand or the cable starts the quench operation, causing the low field instability.

Index Terms—FEM, Instability, Magnetization, Nb, Nb₃Al Strand, Ta.

Manuscript received 26 August 2008.
This work was supported by the U.S. Department of Energy.
R. Yamada and V. Velev are with Fermi National Accelerator Laboratory, Batavia Illinois, 60510 USA (phone: 630-840-3660; fax: 630-840-3369; e-mail: yamada@ fnal.gov).
A. Kikuchi is with National Institute for Material Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan.
M. Wake is with High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan.

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