368
F. Bordry et al.
earlier. With the exception of internally cooled cables, the temperature gradient is
directly proportional to the thermal resistance of the coil. Low thermal resistance
insulation schemes can be obtained by proper choice of insulation thickness and
overlap, as demonstrated by recent work [32] devoted to the upgrade of the
quadrupoles for the inner triplet of the LHC.
The fabrication of a superconducting magnet resorts on the standard techniques,
tools and instruments used in electrical and heavy industries, adapted to specificities
of the materials used. Particular attention is devoted to preserve the physical
properties of the conductor throughout the coil fabrication and handling, as these
can be degraded by excessive strain during the winding or by heat treatments
(e.g. curing of the resin). The case of Nb 3 Sn magnets fabricated by the windand-react technique poses additional constraints on the structural and insulating
materials. These need to withstand the high temperature heat treatment required for
the formation of the superconducting phase (approximately 700 ◦ C for a few hours
to a few days). When compared to standard electrical equipment, superconducting
coils have very tight manufacturing tolerances originating from demands of field
quality (recall that the field is dominated by the current distribution) and to avoid
movements during energization that could trigger quenches (recall the discussion
on stability). Indeed, the level of accuracy demanded is often beyond the standard
experience in electromechanical constructions. Which is why trained personnel and
field experience are often critical to the successful performance of a superconducting
magnet.
8.1.3.8 Super-Ferric Magnets
Super-ferric magnets are a special case of electromagnets, where the ferro-magnetic
yoke, producing the dominant portion of the magnetic field in the useful aperture, is
magnetized by superconducting coils. These electromagnets resemble the normal
conducting magnets described earlier, apart for their cryogenic features. The
prime interest of super-ferric magnets is to profit from the power advantage of
superconductors (no ohmic loss). Provided the design is well optimized, the cost of
cryogenic cooling is smaller than the cost of powering the resistive coils, resulting
in a net gain. Part of this optimization is the trade-off between a cold or warm
yoke, affecting the amount of required cooling power at cryogenic temperatures. In
addition, super-ferric magnets provide operational flexibility. In particular, steady
operation at nominal field is possible with no significant overhead. By contrast,
high field operation can be very power-intensive, and costly, in normal conducting
magnets. Finally, the high current density which can be achieved in superconducting
coils is more than one order of magnitude larger than in normal conducting coils.
This can bring additional benefits to the reduction of the overall magnet dimension,
also thanks to a reduced yoke reluctance.
The limitation of super-ferric magnets is similar to normal conducting electromagnets, namely that the field is limited by the saturation of the ferro-magnetic
material of the yoke. In fact, super-ferric magnets for accelerators, such as the
F. Bordry et al.
earlier. With the exception of internally cooled cables, the temperature gradient is
directly proportional to the thermal resistance of the coil. Low thermal resistance
insulation schemes can be obtained by proper choice of insulation thickness and
overlap, as demonstrated by recent work [32] devoted to the upgrade of the
quadrupoles for the inner triplet of the LHC.
The fabrication of a superconducting magnet resorts on the standard techniques,
tools and instruments used in electrical and heavy industries, adapted to specificities
of the materials used. Particular attention is devoted to preserve the physical
properties of the conductor throughout the coil fabrication and handling, as these
can be degraded by excessive strain during the winding or by heat treatments
(e.g. curing of the resin). The case of Nb 3 Sn magnets fabricated by the windand-react technique poses additional constraints on the structural and insulating
materials. These need to withstand the high temperature heat treatment required for
the formation of the superconducting phase (approximately 700 ◦ C for a few hours
to a few days). When compared to standard electrical equipment, superconducting
coils have very tight manufacturing tolerances originating from demands of field
quality (recall that the field is dominated by the current distribution) and to avoid
movements during energization that could trigger quenches (recall the discussion
on stability). Indeed, the level of accuracy demanded is often beyond the standard
experience in electromechanical constructions. Which is why trained personnel and
field experience are often critical to the successful performance of a superconducting
magnet.
8.1.3.8 Super-Ferric Magnets
Super-ferric magnets are a special case of electromagnets, where the ferro-magnetic
yoke, producing the dominant portion of the magnetic field in the useful aperture, is
magnetized by superconducting coils. These electromagnets resemble the normal
conducting magnets described earlier, apart for their cryogenic features. The
prime interest of super-ferric magnets is to profit from the power advantage of
superconductors (no ohmic loss). Provided the design is well optimized, the cost of
cryogenic cooling is smaller than the cost of powering the resistive coils, resulting
in a net gain. Part of this optimization is the trade-off between a cold or warm
yoke, affecting the amount of required cooling power at cryogenic temperatures. In
addition, super-ferric magnets provide operational flexibility. In particular, steady
operation at nominal field is possible with no significant overhead. By contrast,
high field operation can be very power-intensive, and costly, in normal conducting
magnets. Finally, the high current density which can be achieved in superconducting
coils is more than one order of magnitude larger than in normal conducting coils.
This can bring additional benefits to the reduction of the overall magnet dimension,
also thanks to a reduced yoke reluctance.
The limitation of super-ferric magnets is similar to normal conducting electromagnets, namely that the field is limited by the saturation of the ferro-magnetic
material of the yoke. In fact, super-ferric magnets for accelerators, such as the