Nb3Al
Prototype Conductor for the Transmission Line Magnet
E. Malamud, E. Barzi, G. William Foster, P. O. Mazur,
H. Piekarz, Fermilab
and
M. Wake, KEK
and
K. Hayashi and M. Koganeya, Sumitomo Electric Inc.
Presented at the "Magnets for a
Very Large Hadron Collider" Workshop,
Port Jefferson, L.I., N.Y., Nov. 16-18, 1998
Introduction
The Very Large Hadron Collider (vlhc), under consideration for construction at Fermilab in the next 1-2 decades, is a 100 TeV cm pp collider [1]. The single most costly item in the vlhc is the magnet. R&D is underway in several laboratories on several possible magnet designs.
A low-field (2T) superferric magnet, sometimes called a transmission line magnet [2], may be the most cost-effective route to the vlhc. Although NbTi is now the cheapest superconductor measured in cost/kA-meter, Nb3Al has the potential advantage of higher operating temperature. It may be particularly suited to the single "turn" and long straight lengths of the transmission line design. The combination of the simple magnet design and the higher strain tolerance (than Nb3Sn) allows a simple process of cable fabrication, reaction, and magnet assembly.
Sumitomo Electric Inc. is producing Nb3Al conductor for the low-field magnet program. We have ordered three 4-meter lengths of transmission line from them.
Strand specifications
The conductor is jelly roll [2] material produced in prototype quantities for the ITER program.
| Diameter (one strand) | 0.809 mm |
| Cu/non-Cu ratio | 1.4 |
| Number of filaments | 96 |
| Filament Diameter | 53 microns |
| Twist pitch | 30 mm |
| Critical current dens. @ 1T, 6.5 K | >9000 A/mm2 |
Cable specifications
The figure shows how the Nb3Al strand is enclosed with 6 copper strands to form one sub-cable. The effect of contact between the Nb3Al and the Invar during reaction is not known, so this sub-cable configuration is used to prevent any such contact.
| Number of sub-cables | 41 |
| Sub-cable dimensions | 1.85 x 1.92 mm2 |
| Sub-cable topology | 1 Nb3Al, 6 Cu |
| Cabling pitch | ~ 4 mm |
| OD of assembly (ID of Invar) | 35.7 mm |
| OD of Invar pipe | 38.1 mm (1.5 in) |
| Thickness of invar | 1.2 mm (0.047 inch) |
| Copper (OFHC) RRR | >100 |
| Copper stabilizer width | 85 mm |
| Copper stabilizer ID | 29 mm |
| Copper stabilizer thickness | 1 mm |
| Critical current @ 1T, 6.5 K | >75 kA |
To form the cable the 41 sub-cables are arranged on a copper strip as shown below, then helically formed into the finished assembly and inserted into the invar pipe.
Sumitomo’s R&D plan
Sumitomo will investigate several issues before finalizing the manufacturing process:
Photographs below show Sumitomo's initial test pieces.
Conductor properties
The graph below compares the performance of NbTi and Nb3Al, both at 1 T.
Short Sample Tests
R&D and production schedule for the 4-m lengths is shown below.
Test Program
A loop measuring approximately 3 m by 6 m is being assembled in the MW-9 building at Fermilab. It will be used as a test facility for development work on the transmission line magnet. This loop will be excited with a flux transformer. Supercritical helium is circulated using a cryogenic pump and heat exchanger system, with refrigeration supplied by liquid helium from dewars.
Most of the loop is made of left over SSC outer conductor as shown in section 1 of the drawing below. Opposite the drive transformer is a 4-meter removable section that will be used as a test bed to evaluate different NbTi transmission line designs as well as the Nb3Al sections. Since measuring the performance as a function of temperature is an important objective of the test program, a "heat bump" [4] will be installed within the 4-meter section. This permits operation of the specimen under test at a temperature significantly higher than that of the rest of the loop.
The drawing and the sketch show how we plan to make the joint between the transformer conductor portion and the test section.
References
