2.1 LTS materials Ë 23Fig. 2.2:Critical current density of Nb-Ti, at 4.2 and 1.9 K, compared with that of Nb 3 Sn at 4.2 K [11].
This demonstrates that Nb 3 Al superconducting wires can be used in accelerator
magnets.
In 2001, Nagamatsu et al. [14] discovered superconductivity in a magnesium
diboride (MgB 2 ), which had a transition temperature of 39 K. This is the highest critical
temperature so far achieved in a conventional superconductor. The material has
attracted considerable interest because of its potential for low cost, higher stability,
and relatively simple deposition techniques, which enable MgB 2 to be comparative
with Nb-Ti in the future. The operating temperature of the MgB 2 can be kept around
20 K by a cryocooler, but Nb-Ti requires 4.2 K and even lower. However, in comparison
to standard Nb-Ti, MgB 2 has some disadvantages in the current density, mechanics,
and uniformity.
The properties of major LTSC materials are shown in Tab. 2.1.
HTSC wires of Bi-2212 or YBCO are interesting for magnetic fields above 16 T, since
this field is barely achievable with Nb 3 Sn at 1.9 K. The limits of the upper critical field
Hc2of Nb 3 Sn drive the development of HTSC materials with higherHc2values and
greater critical current density (Jc) for ultra-high field applications.
Tab. 2.1:Characteristic parameters of LTSC materials [15–18].
Materials Tc(K) 휇 0 Hc1(T) 휇 0 Hc(T) 휇 0 Hc2(T) 휆L(Å) 휉(Å)
Nb 9. 3 0.181 0. 199 2 850 400
Nb-Ti 9. 5 0. 253 15 3000 40
Nb 3 Sn 18 0.035 0. 53 32 650 30
Nb 3 Al 18. 7 34
Nb 3 Ge 23. 2 38 900 30
MgB 2 39 0. 66 74 1850 50