8 Accelerator Engineering and Technology: Accelerator Technology
355
the precursor elements Nb and Sn into large size billets that are extruded and/or
drawn to the final diameter wire. The Nb 3 Sn is then formed by a chemical reaction
induced by a heat treatment to temperatures in the range of 650 ◦ C for durations of
few tens to few hundreds hours. Various manufacturing routes have been established
industrially, resulting in wires with different properties of critical current density
and filament size. Most common fabrication methods are based on the so-called
“bronze-route”, or “internal-tin”, which distinguish themselves by the way that the
relatively mobile Sn is made available for reaction with Nb. The interest in Nb 3 Sn
comes from the improved T C and B C with respect to Nb-Ti, resulting in record
critical current density exceeding 1500 A/mm 2 at 15 T and 4.2 K. This potential is
commonly exploited in commercial solenoids built for NMR spectroscopy or other
laboratory applications, but to date found no accelerator applications due to the
technical difficulties associated with the heat treatment and the handling of a fragile
magnet coil. In addition, high J C Nb 3 Sn has presently larger filament size than NbTi, in the range of 50 to 100 μm. This material is nonetheless very relevant to extend
the reach of present accelerators such as the LHC beyond the limit of Nb-Ti (e.g.
the High-Luminosity upgrade of the LHC and the R&D on magnet technology for a
Future Circular Collider), or to build compact accelerators that could be of interest
for industrial and medical applications (e.g. high-field compact cyclotrons for proton
therapy).
8.1.3.1.3 MgB 2
The most recent of the technical superconductors [22], the inter-metallic compound
MgB 2 is a relatively inexpensive material obtained from readily available precursor
elements. Superconducting MgB 2 wire can be produced through the powder-in-tube
(PIT) process. Variants of this process exist, depending on whether the MgB 2 is
formed at the end of the wire processing from Mg and B precursors (in-situ variant),
or rather powders of pre-reacted MgB 2 are sintered in the finished wire (ex-situ
variant). In both cases, a heat treatment is required in the range of 600 ◦ C to 1000 ◦ C.
As for Nb 3 Sn, the wires and tapes of MgB 2 are fragile and require careful handling
to limit deformation. In spite of a high critical field, which in thin films has reached
values above 70 T, the bulk material becomes irreversible at much smaller applied
fields. MgB 2 is hence of interest in the range of low to medium field applications
(presently up to a maximum 5 T range), but for operating temperatures up to 20 K,
i.e. well above boiling helium conditions. The main technical realizations presently
based on MgB 2 are open MRI systems at modest magnetic field (0.5 T), and cables
for power transmission planned for the High Luminosity upgrade of the LHC (the
so called SC links) or electric energy distribution. This material is to date still in the
developmental state, with specific interest in enlarging the field range for magnet
applications. While the main motivation remains helium-free MRI magnets, MgB 2
could be an option for accelerator magnets subjected to radiation loads that require
high operating temperature and energy margin (see later).
355
the precursor elements Nb and Sn into large size billets that are extruded and/or
drawn to the final diameter wire. The Nb 3 Sn is then formed by a chemical reaction
induced by a heat treatment to temperatures in the range of 650 ◦ C for durations of
few tens to few hundreds hours. Various manufacturing routes have been established
industrially, resulting in wires with different properties of critical current density
and filament size. Most common fabrication methods are based on the so-called
“bronze-route”, or “internal-tin”, which distinguish themselves by the way that the
relatively mobile Sn is made available for reaction with Nb. The interest in Nb 3 Sn
comes from the improved T C and B C with respect to Nb-Ti, resulting in record
critical current density exceeding 1500 A/mm 2 at 15 T and 4.2 K. This potential is
commonly exploited in commercial solenoids built for NMR spectroscopy or other
laboratory applications, but to date found no accelerator applications due to the
technical difficulties associated with the heat treatment and the handling of a fragile
magnet coil. In addition, high J C Nb 3 Sn has presently larger filament size than NbTi, in the range of 50 to 100 μm. This material is nonetheless very relevant to extend
the reach of present accelerators such as the LHC beyond the limit of Nb-Ti (e.g.
the High-Luminosity upgrade of the LHC and the R&D on magnet technology for a
Future Circular Collider), or to build compact accelerators that could be of interest
for industrial and medical applications (e.g. high-field compact cyclotrons for proton
therapy).
8.1.3.1.3 MgB 2
The most recent of the technical superconductors [22], the inter-metallic compound
MgB 2 is a relatively inexpensive material obtained from readily available precursor
elements. Superconducting MgB 2 wire can be produced through the powder-in-tube
(PIT) process. Variants of this process exist, depending on whether the MgB 2 is
formed at the end of the wire processing from Mg and B precursors (in-situ variant),
or rather powders of pre-reacted MgB 2 are sintered in the finished wire (ex-situ
variant). In both cases, a heat treatment is required in the range of 600 ◦ C to 1000 ◦ C.
As for Nb 3 Sn, the wires and tapes of MgB 2 are fragile and require careful handling
to limit deformation. In spite of a high critical field, which in thin films has reached
values above 70 T, the bulk material becomes irreversible at much smaller applied
fields. MgB 2 is hence of interest in the range of low to medium field applications
(presently up to a maximum 5 T range), but for operating temperatures up to 20 K,
i.e. well above boiling helium conditions. The main technical realizations presently
based on MgB 2 are open MRI systems at modest magnetic field (0.5 T), and cables
for power transmission planned for the High Luminosity upgrade of the LHC (the
so called SC links) or electric energy distribution. This material is to date still in the
developmental state, with specific interest in enlarging the field range for magnet
applications. While the main motivation remains helium-free MRI magnets, MgB 2
could be an option for accelerator magnets subjected to radiation loads that require
high operating temperature and energy margin (see later).