Investigation of Ceramic Matrix Composites for Applications in Superconducting Magnets

 A.D. LaForge and S.A. Gourlay, Lawrence Berkeley National Laboratory

Abstract 

As superconducting magnets are being developed to meet the demands of new accelerator designs, existing systems are being scrutinized to reach optimal performance. Ideally, each component of the magnet is being pushed to the intrinsic limits of its constituent material. Ceramic matrix composites have been investigated for possible use in the fabrication of magnet coil end spacers. Several fabrication methods have been examined, and we see that favorable properties in handling, resistivity, and compatibility can be gained without making a large sacrifice in elastic modulus.

Superconducting Magnets in High-Energy Physics 

-Magnets are used to accelerate, bend, and focus beams of particles in accelerators. -New accelerator designs call for higher fields, prompting investigation of new materials and coil geometries. -Cosine-theta geometries using NbTi superconducting cables reach maximum dipole fields of 10.2 Tesla. -With common-coil racetrack geometry and Nb3Sn superconducting material, however, magnets can reach dipole fields approaching 15 Tesla.

End Parts: Importance and Properties -The magnetic field that a magnet creates is determined by the geometry of the current density, i.e., the position of the cable. -End-spacers are inserted into the coil at calculated locations to optimize the current distribution to obtain the highest quality field. -These spacers must meet certain criteria:

-High mechanical strength: elastic modulus, compressive strength, shear strength-Low electrical resistance-Ability to be formed into precise shapes

-Currently, end parts are machined from aluminum and plasma-coated, an expensive and multi-step process. -Composite materials may provide a simpler solution.

 

Conclusions We have demonstrated the fabrication of solid parts using both ceramic fabric and chopped ceramic fiber. Using elementary wet-lay methods, the material strength has approached that of Nb3Sn superconducting cable stacks. With future implementation of vaccum resin impregnation, we anticipate the strength to improve. Prototype chopped fiber molds show that single-step end part formation is a possibility, simplifying elements of magnet construction. Areas to be tested in the future include thermal and electrical properties. A possible concern is the outgassing that has been measured during heat-treatment, as hydrogen and hydrocarbons can degrade and weaken the Niobium conductor. In matters of strength and ease of formation, this composite material has met the necessary criteria for end part fabrication, and we anticipate that it will play a useful role in future magnet construction.

AcknowledgementsThe author would like to thank the Department of Energy and Lawrence Berkeley National Laboratory for this research opportunity, Steve Gourlay for his guidance and Laurel Egenberger, Susan Aberg, and Kathleen for their assistance. The project could not have been completed without the technical assitance of Phil Bach, Paul Bish, Ken Chow, Steve Dellinges, Dan Dieterich, Roy Hannaford, Hugh Higley, Nate Liggins, Tom Miller, Jim Oneill, Evan Palmerston, and Jim Swanson.

Composite Fabrication

Ceramic Fabric

One fabrication method utilized 3M Nextel Ceramic Fiber. A ceramic matrix liquid was applied before curing the

sample at 150C, then 650C.

Chopped Ceramic Fiber

Complex pieces can be formed by filling a mold with chopped ceramic fiber.

Elastic Modulus Testing

0

10

20

30

40

50

60

ChoppedCeramic

Fiber

CeramicFabric, 50%

f.v.

CeramicFabric, 60%

f.v.

Nb3Sn(Published)

CeramicFabric, 75%

f.v.(Published)

Mo

du

lus

(GP

a)

X

Y

Z

Residual Gas Analysis

Methane Partial Pressure vs. Time

0.00E+00

2.00E-08

4.00E-08

6.00E-08

8.00E-08

1.00E-07

1.20E-07

11:00 12:12 13:24 14:36

Time (hours)

Par

tial

Pre

ssu

re (

To

rr)