High temperature superconductors for fusion magnets - influence of neutron irradiation Michal Chudý M.Eisterer, H.W.Weber
Outline 1. Superconductors in thermonuclear fusion 2. High temperature superconductors - Coated conductors 3. Experimental results
1. Superconductors in thermonuclear fusion
Why do we need superconductors in fusion? Generate high magnetic fields up to 13 T Superconducting machine is significantly smaller Resistive losses would consume too much energy Why high temperature superconductors? ITER conventional (low temperature superconductors) will be used Higher operating temperature o Save energy for cooling o Replace 4 He (expensive, rare and maybe not available in the future) by another coolant (nitrogen, hydrogen) ITER magnet system
Superconductors in fusion magnets n n n n n blanket coils Superconductors in the magnets have to withstand high neutron fluences
Radiation affects all magnet components
TRIGA Mark Reactor II in Vienna (Atominstitut) Max. Power: 250 kw 80 fuel elements (8% uranium, 1% hydrogen, 91% zirconium) maximum neutron flux density is 2,1 x10 17 m -2 s -1 Fast neutron flux density (ZBR): 7,6 x 10 16 m -2 s -1 Temperature of the sample: < 50 C
2. High temperature superconductors - Coated conductors
Coated conductors Most promising conductors for many applications 2 nd generation of high temperature superconducting wires YBCO (Yttrium-Barium-Copper-Oxygen) (Re)BCO high T c good properties in high magnetic fields Layered structure complicated and expensive fabrication
Flux lines Magnetic flux is penetrating to the superconductor
Flux pinning I YBCO layer Pinning sites: Intrinsic: ab planes additional: mis-oriented grains, twin boundaries, dislocations,voids, precipipates S. R. Foltyn at al.
Defects induced by fast neutrons Point defects, E n < 0.1 MeV o weak pinning centres High energy displacement cascades, E n > 0.1 MeV o Effective pinning centers o Random distribution o Amorphous regions in the crystal structure ( 5nm) o Possible recrystalization Neutron induced defect in YBCO lattice
3. Experimental results
M(10 4 A/m 2 ) Voltage (V) Characterization of coated conductors Transport measurements 3,0x10-6 2,0x10-6 1,0x10-6 0,0 Magnetic measurements -1,0x10-6 2 0 20 40 Current (A) 1 VSM (Vibrating sample magnetometer) 0 SQUID (Superconducting quantum interference device) -1-2 -6-4 -2 0 2 4 6 H(T)
J c (A.m -2 ) Anisotropy measurements H c H ab 10 ab plane: intrinsic pinning 5 Anisotropic scaling approach Blatter at al. (1992), =5 c-axis - corelated pinning: -twin boundaries -grain boundaries (dislocations) I 0 0 45 90 135 (deg) 6 T Helium gas flow cryostat
I I I c c c (A) (A) Anisotropic transport measurements after irradiation H c H ab 35 30 25 1x10 22 m -2 4x10 21 m -2 2x10 21 m -2 unirr 64 K, 6 T Irradiated : Irradiated Fast neutrons: : 1x10 22 m -2 Fast ZBR Irradiated neutrons: (250 W) : ~ 4x10 36.5 21 h m -2 unirradiated ZBR Fast (250 neutrons: kw) ~ 2x10 14.6 21 h m -2 ZBR (250 kw) ~ 7.3 h 20 15 0 20 40 60 80 100 120 140 angle
I c (A) 77 K 1 T H c H ab 1x10 22 m -2 4x10 21 m -2 2x10 21 m -2 unirr 20 10 0 45 90 135 angle
J c (A.m -2 ) J c (A.m -2 ) Performance in 2 main field orientations µ 0 H c µ 0 H ab 10 10 un 2x10 21 m -2 4x10 21 m -2 1x10 21 m -2 DEMO 10 10 64 K DEMO 10 9 64 K ITER ITER 77 K 10 9 77 K 10 8 0 2 4 6 8 10 12 0 H (T) 0 2 4 6 8 10 12 14 16 0 H (T) toroidal field (on axis) maximal field (toroidal, c.solenoid)
H (T) Critical temperature (T c ) 16 14 12 10 8 6 µ 0 H = 0 T T c (unirradiated) = 89,1 K T c (2x10 21 m -2 ) = 88,32 K T c (1x10 22 m -2 )= 86,56 K 4 2 0 µ 0 H c µ 0 H ab un irr 2x10 21 m -2 irr 1x10-22 m -2 ΔT = 2,54 K -2 68 70 72 74 76 78 80 82 84 86 88 90 T (K) Critical temperature is reduced due to induced lattice disorder
Conclusions High temperature superconductors are strong candidates to be applied in the next step of fusion devices Defects induced by fast neutrons are effective pinning centres, which can significantly improve critical current densities and reduce J c anisotropy Due to induced lattice disorder, T c is reduced Requirements for ITER (DEMO) are partially achieved at 64 K
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