Neutron irradiation of Nb3Sn and NbTi multifilamentary composites

NbTi and Nb
3
Sn multifilamentary composites have been irradiated with fast-neutrons at 60 ± 5°C to fluences of 1.2×10
²⁰
n/cm
²
(E > 1 MeV). The NbTi samples show only a moderate reduction of I
c
as a function of neutron fluence in an applied field of 40 kG. Reductions in I
c
were observed for fluences greater than 3 × 10
¹⁷
n/cm
²
and saturate at 18% for fluences greater than 3-4 × 10
¹⁹
n/cm
²
. The Nb
3
Sn composites showed large neutron radiation induced changes in T
c
, I
c
and H
c2
. Reductions in T
c
were observed for fluences greater than 7 × 10
¹⁷
n/cm
²
. No measurable changes in I
c
(40 kG) were observed below 10
¹⁸
n/cm
²
. Between 2 and 3×10
¹⁸
n/cm
²
, however, there is an apparent threshold where a very rapid reduction in I
c
(40 kG) is initiated. At the threshold the decrease in T
c
is 13%. Between the threshold and 1.1 × 10
¹⁹
n/cm
²
, I
2
(40 kG) has been reduced to 4% of the unirradiated value. These changes in superconducting properties in NbTi and Nb
3
Sn are analyzed in terms of the radiation induced defects. The impact of the response to irradiation of both materials on their applications in fusion reactor magnets is discussed.