Magnetization, Low Field Instability and Quench of RHQT ${\rm Nb}_{3}{\rm Al}$ Strands

Abstract

Abstract-Since 2005, we made and tested three RHQT Nb 3 Al strands, one with Nb matrix and two with Ta matrix, which are fully stabilized with Cu electroplating. We observed anomalously large magnetization curves extending beyond 1 to 1.5 Tesla with the F1 Nb matrix strand at 4.2 K, when we measured its magnetization with a balanced coil magnetometer. This problem was eliminated with the Ta matrix strands operating at 4.2 K. But with these strands a similar but smaller anomalous magnetization was observed at 1.9 K. We studied these phenomena with FEM. With the F1 Nb matrix strand, it is explained that at low external field, inter-filamentary coupling currents in the outer layers of sub-elements create a shielding effect. It reduces the inside field, keeps the inside Nb matrix superconductive, and stands against a higher outside field beyond the Hc of Nb. At an even higher external field, the superconductivity of the whole Nb matrix collapses and releases a large amount of energy, which may cause a big quench. Depending on the size of the energy in the strand or the cable, a magnet could quench, causing the low field instability. Some attempt to analyze the anomaly with FEM is presented.

Comparison Between ${\hbox {Nb}}_{3}{\hbox {Al}}$ and ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Strands and Cables for High Field Accelerator Magnets

“…6. As the precursor of Nb 3 Al is very hard, we observe hardly any deformation in it, while quite extensive deformation is observed in the copper stabilizer part. As bonding of the copper to the Nb surface is strong, there are no deboned regions nor any observable voids.…”

“…They are all copper stabilized 1 mm diameter Nb 3 Al strands. The differences are in the material of their central cores and the matrix between the subelements.…”

“…The differences are in the material of their central cores and the matrix between the subelements. F1 strand was made with Nb for both elements, and F1 was shown to be unstable at low field due to its huge magnetization caused by Nb components [3].…”

“…For the last decade, several different types of Nb 3 Sn strands were manufactured for, and have been successfully used for, high field accelerator magnets [5]. But it seems the Nb 3 Sn Rutherford cables are still in an R&D stage for the application to high field accelerator magnets, and not all magnets made with Nb 3 Sn are completely trouble free [6].…”

“…But it seems the Nb 3 Sn Rutherford cables are still in an R&D stage for the application to high field accelerator magnets, and not all magnets made with Nb 3 Sn are completely trouble free [6].…”