Radiation Effects on Nb3Sn, copper and inorganic insulation Al Zeller NSCL/ MSU

Need to examine all components that go into a magnet: Superconductor (Nb3Sn) Insulation Copper Structural materials 3

Nb3Sn Unlike NbTi need to anneal at 700 C to regain critical current

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Possible problem with initial rise: If radiation field is not uniform over the whole conductor, get spatial   variation in Jc, which might affect field harmonics.   Note that radiation damage is like critical current in the peak field area: It is where coil will fail. 10

General limits for Nb3Sn: 5 X 108 Gy (500MGy) end of life Tc goes to 5 K – 5 X 1023 n/m2 Ic goes to 0.9 Ic0 at 14T – 1 X 1023 n/m2 Bc2 goes to 14T - 3 X 1022 n/m2 NOTE: En < 14 MeV Damage increases as neutron energy increases 11

Radiation increases resistance Copper Radiation increases resistance 15

Quench protection becomes an issue – lower allowed current in the copper or higher temperatures during a quench   Can anneal out 80-90% of the increased resistance by going to 300K 16

From the Wiedemann-Franz-Lorenz law at a constant temperature λρ = constant   Thermal conductivity decreases   Minimum propagating zone decreases: Lmpz = ((Tc-To)/j2)   So Lmpz -> λ 17

Problem: Gas evolution Ranges from 0.09 for Kapton to >1 cm3/g/MGy for other epoxies Gas is released upon heating to room temperature 19   Can cause swelling, rupture of containment vessel or fracturing of epoxy

Ceramic slurry that is fired? Warming to room temperature may be required to keep conductivity of copper high, but conflicts with gas evolution problems.   Would like space for helium, so strong porous material would be nice. 20 Ceramic slurry that is fired? Differential contraction?

Structural materials Crystal structure affect radiation sensitivity FCC materials (e.g. copper) less affected than BCC materials (e.g. iron)

Radiation causes dislocation pinning Structural materials Radiation causes dislocation pinning Ø    Decreases thermal conductivity Increases ratio of yield-to-tensile strength (brittle) Generally makes material harder Ø Decreases density (swelling)

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What do we want from inorganic insulation? Electrical insulation Structural strength Radiation resistance Dimensional stability Thermal conductivity (possibly) Low radiation absorption Flexible Low cost Thermal contraction matched to conductor

A few general points of organic vs inorganic: Inorganics should be last resort in high rad areas Inorganics not favored in areas where the major radiation is not neutrons Inorganics useful for high-temperature processes Inorganics good in compression, poor in shear

Volume swelling of MgO 1% at 1 x 1025 N/m2 At 5 x 1023 n/m2 Nb3Sn has Jc = 0 for reactor spectra neutrons.

Aluminum oxide swelling as a function of fluence 1% at 2 x 1025 N/m2

Spinel has lower expansion rate than alumina.

Density change in alumina as a function of fluence. Note 1024 n/m2 Nb3Sn is resistive at 5 x 1023.

Dose rate for SLHC Cos θ D1 calculated by Mokhov Resistance decreases by a factor of 1000.

Neutron fluence: 6 x 1022 n/m2 At 4 K, thermal cond. reduced by about a factor of 250.

Big caution: Damage in inorganic materials is temperature dependent. Damage at 4 K, for some properties, is 100 times more than the same dose or fluence absorbed at room temperature. Since Nb3Sn has a useful fluence limit of 1023 n/m2, critical properties of inorganic insulators should be stable to 1025 n/m2 at 4 K. Note that electrical insulation properties are 10 times less sensitive than mechanical ones.

Volume changes after 1024 n/m2 Material ΔV/V (%) notes ΔV/V (%)crit Al2O3 0.35 3.45 MgO 0.75 1.46 SiO2 3.0 Vitreous SiO2 11.0 Quartz 0.82 MgAl2O4 0.04 2 x 1022 3.06 MgAl2O4 0.8 2 x 1026 ΔV/V (%)crit is the strain limit

Diamond: density decreases by 3.5% Ceramics Exposure to ~ 1024 – 1025 n/m2 Alumina: λ decreases by a factor of 4, density increased by 6.8%   BeO: Linear expansion of ~0.13%, Young’s modulus decreases by 64% 24 Diamond: density decreases by 3.5%

Important Note   All of the radiation studies on Nb3Sn are 15-25 years old and we have lots of new materials.

Need new studies But I may be able to help. Have funding for HTS irradiation, so may be able to irradiate Nb3Sn Need place to test samples