Superconducting Materials and Conductors: Fabrication and Limiting Parameters

Abstract

Superconductivity is the technology that enabled the construction of the most recent generation of high-energy particle accelerators, the largest scientific instruments ever built. In this review we trace the evolution of superconducting materials for particle accelerator magnets, from the first steps in the late 1960s, through the rise and glory of Nb-Ti in the 1970s, till the 2010s, and the promises of Nb 3 Sn for the 2020s. We conclude with a perspective on the opportunities for high-temperature superconductors (HTSs). Many such reviews have been written in the past, as witnessed by the long list of references provided. In this review we put particular emphasis on the practical aspects of wire and tape manufacturing, cabling, engineering performance, and potential for use in accelerator magnets, while leaving in the background matters such as the physics of superconductivity and fundamental material issues.

Examination of the trade-off between intrinsic and extrinsic properties in the optimization of a modern internal tin Nb3Sn conductor

“…The parameter λ assumes the value 1/2 for columnar grains and 1/3 for equiaxed grains [31]. In low-Sn Nb 3 Sn strand designs such as the bronze-process, a large portion of the A15 has columnar grains, i.e. a high aspect ratio [16,32] and λ=1/2 can be used.…”

“…the non-Cu fraction) environment and the reaction time and temperature. Thus progress towards a full optimization of Nb 3 Sn conductor design has been slow and empirical, typically proceeding by optimizing one parameter at a time, then followed by finding an acceptable balance of properties in secondary optimizations that maintain as much of the primary optimization as possible. Over the last 15 years the primary accelerator-physics goal has been to raise the non-Cu J c (12 T, 4.2 K) above 3000 A/mm 2 so that efficient quadrupole and dipole magnets can be designed [1,2,3].…”

“…Thus progress towards a full optimization of Nb 3 Sn conductor design has been slow and empirical, typically proceeding by optimizing one parameter at a time, then followed by finding an acceptable balance of properties in secondary optimizations that maintain as much of the primary optimization as possible. Over the last 15 years the primary accelerator-physics goal has been to raise the non-Cu J c (12 T, 4.2 K) above 3000 A/mm 2 so that efficient quadrupole and dipole magnets can be designed [1,2,3]. Since field quality (largely determined by the effective filament diameter, d eff, after reaction) and magnet protection (largely determined by the residual resistivity ratio, RRR, of the Cu stabilizer) are both of great importance, small d eff and high RRR enabled by effective diffusion barriers now provide the two principal constraints on the architecture and the reaction window that can be applied to the conductor.…”

“…Magnetization hysteresis loops were performed in a vibrating sample magnetometer (VSM) in a 14 T superconducting magnet in order to estimate the non-Cu critical current density J c , the pinning force density F p and the extrapolated irreversibility field H Irr at 4.2 K. The specific heat of the wires was measured at 0 and 16 T in a 16 T PPMS with 25-30 mg samples using a relaxation technique. Each wire was measured without removing the external Cu stabilizer in order to prevent changes in Nb 3 Sn properties due to release of the precompression applied to the sub-elements by differential thermal contraction of the Cu matrix.…”

Evidence for length-dependent wire expansion, filament dedensification and consequent degradation of critical current density in Ag-alloy sheathed Bi-2212 wires

“…Interest in the magnet applications of Bi-2212 (Bi 2 Sr 2 CaCu 2 O x ) round wire superconductor has significantly increased [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] since recent successes in fabricating high magnetic field coils from hightemperature superconductors (HTS). These HTS coils have generated fields significantly higher [1,17,18] than the 24 T in solenoids and [16][17][18] T in dipoles possible with any Nb-based conductor [19][20][21][22]. There is much interest in REBa 2 Cu 3 O 7 (RE = rare earth, REBCO) HTS coils because REBCO coated conductor is available and can be wound as-delivered, especially if co-wound stainless steel is acceptable as insulation [23].…”