352
F. Bordry et al.
Superconducting magnet technology relies heavily on the ability to produce
technical superconducting materials in the form of high current cables. Beyond
considerations on the magnetic field that are specific to the arrangements described
above, the design of superconducting magnets must take into account issues related
to the superconductor, such as stability, quench protection, magnetization and AC
loss. The magnet powering requires warm-to-cold transitions capable to transport
the large currents in the range of few to few tens of kA with minimal thermal
loss. The construction of the magnet, and in particular the insulation, coil winding,
assembly in the mechanical structure and yoke, and the placement in a cryostat
necessary for thermal management, all have aspects specific to superconducting
magnet technology. Finally, cooling by helium is a science by itself. In the
following sections we will discuss some of the main principles, without entering
into detail, and provide extensive references on the above matters. The bibliography
in the references directs the reader to excellent books on superconducting magnet
engineering and science, especially [14–18].
8.1.3.1 Superconducting Materials
A superconductor is such only if it operates below the critical surface, a combination of temperature T, magnetic field induction B and current density J that
delimits the boundary between the superconducting and normal-conducting phases.
This surface is best expressed using a function J C (B,T), the critical current density
which is the main engineering characteristic of a superconductor. A good example
is the critical current density for the highly optimised Nb-47%Ti alloy used for the
production of the LHC magnets, shown in Fig. 8.5. When cooled to 4.2 K, this
superconductor can carry a current density up to 3000 A/mm 2 in a background field
of 5 T. Indeed, this is the order of magnitude of current density that is of interest to
make the design of a superconducting accelerator competitive. As we see from Fig.
8.5, higher fields or higher temperatures result in a reduction of the critical current
density, while a decrease of any yields an increase in J C .
Table 8.5 reports a summary of the critical temperature T C and critical field B C
for the technical superconducting materials that have found practical applications
over the past 50 years, as well as materials that are expected to come into use
in a few years. The materials are generally classified as low-temperature superconductors (LTS) and high-temperature superconductors (HTS). This classification
was originally based on the temperature of the superconducting transition T C , but
now it refers rather to the different mechanisms that explain the existence of a
superconducting phase. According to this classification, the alloy of Niobium and
Titanium (Nb-Ti), the inter-metallic compounds of Niobium and Tin or Aluminium
(Nb 3 Sn, Nb 3 Al) and of Magnesium and Boron (MgB 2 ) are LTS materials. On the
other hand, the Perovskites formed by Bismuth, Strontium, Calcium and Copper
oxide (conventionally referred to as BSCCO) and a Rare Earth (e.g. Yttrium),
Barium and Copper oxide (referred to as REBCO) are HTS materials. The details
of the material composition and production route influence the values of T C and
F. Bordry et al.
Superconducting magnet technology relies heavily on the ability to produce
technical superconducting materials in the form of high current cables. Beyond
considerations on the magnetic field that are specific to the arrangements described
above, the design of superconducting magnets must take into account issues related
to the superconductor, such as stability, quench protection, magnetization and AC
loss. The magnet powering requires warm-to-cold transitions capable to transport
the large currents in the range of few to few tens of kA with minimal thermal
loss. The construction of the magnet, and in particular the insulation, coil winding,
assembly in the mechanical structure and yoke, and the placement in a cryostat
necessary for thermal management, all have aspects specific to superconducting
magnet technology. Finally, cooling by helium is a science by itself. In the
following sections we will discuss some of the main principles, without entering
into detail, and provide extensive references on the above matters. The bibliography
in the references directs the reader to excellent books on superconducting magnet
engineering and science, especially [14–18].
8.1.3.1 Superconducting Materials
A superconductor is such only if it operates below the critical surface, a combination of temperature T, magnetic field induction B and current density J that
delimits the boundary between the superconducting and normal-conducting phases.
This surface is best expressed using a function J C (B,T), the critical current density
which is the main engineering characteristic of a superconductor. A good example
is the critical current density for the highly optimised Nb-47%Ti alloy used for the
production of the LHC magnets, shown in Fig. 8.5. When cooled to 4.2 K, this
superconductor can carry a current density up to 3000 A/mm 2 in a background field
of 5 T. Indeed, this is the order of magnitude of current density that is of interest to
make the design of a superconducting accelerator competitive. As we see from Fig.
8.5, higher fields or higher temperatures result in a reduction of the critical current
density, while a decrease of any yields an increase in J C .
Table 8.5 reports a summary of the critical temperature T C and critical field B C
for the technical superconducting materials that have found practical applications
over the past 50 years, as well as materials that are expected to come into use
in a few years. The materials are generally classified as low-temperature superconductors (LTS) and high-temperature superconductors (HTS). This classification
was originally based on the temperature of the superconducting transition T C , but
now it refers rather to the different mechanisms that explain the existence of a
superconducting phase. According to this classification, the alloy of Niobium and
Titanium (Nb-Ti), the inter-metallic compounds of Niobium and Tin or Aluminium
(Nb 3 Sn, Nb 3 Al) and of Magnesium and Boron (MgB 2 ) are LTS materials. On the
other hand, the Perovskites formed by Bismuth, Strontium, Calcium and Copper
oxide (conventionally referred to as BSCCO) and a Rare Earth (e.g. Yttrium),
Barium and Copper oxide (referred to as REBCO) are HTS materials. The details
of the material composition and production route influence the values of T C and