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F. Bordry et al.
the electromagnetic force in situations of both large forces and large force density,
at cryogenic conditions, maintaining the conductors in their nominal position, and
avoiding excessive stress on superconducting cables, insulating materials and the
structure itself. This simple task can be addressed using different strategies that
depend on the level of field and electromagnetic force, on space and other operation
constraints, on material selection, and, not last, on the specific choice of the magnet
designer.
A few general lessons have been learned in the past 40 years of development,
and are common practice in superconducting magnet engineering. It is important
to constrain the winding pack in all directions, with a force that is greater than
the Lorentz load, including an appropriate safety factor. This is done to reduce
the energy inputs of mechanical origins that can trigger instabilities. For simple
magnetic configurations (e.g. a solenoid) the coil itself can be the bearing structure.
In the more complex situations encountered in accelerator magnets, the winding
must always be in contact with a force-bearing surface. An initial load is applied
on this surface, sometimes just a few MPa sufficient to remove the fluff. Caution
must be used to avoid cracking or tearing at the interfaces, and in particular those
bonded or glued during an impregnation process. In some cases, it may be of
advantage to intentionally remove the bond between the windings and the surfaces
that are not supporting the Lorentz load, especially at the surfaces that tend to
separate. In this case the coil would be allowed to move as much as required, e.g.
by field quality considerations. These principles may be difficult to apply in cases
where the force distribution is complex, e.g. in high order field configurations such
as nested multipole corrector magnets of particle accelerators. These magnets are
then designed with larger operating margin to cope with the increased perturbation
energy spectrum.
Coils can be dry-wound, in which case the conductor has free surfaces and
can be permeated by the cryogen. Alternatively, they can be impregnated with a
polymer resin that fills the coil spaces and once cured provides mechanical strength
but prevents direct contact to the coolant. Common resins do not have sufficient
mechanical strength to withstand thermal and mechanical stresses, and are loaded
with fibers (e.g. glass). It is important to avoid volumes of unloaded resin, as these
tend to crack and release energy that can lead to magnet quenches. Nb-Ti based
conductors, ductile and strain tolerant, are well adapted to both techniques, while
impregnation is favoured in the case of Nb 3 Sn or HTS based conductors that are
strain sensitive and require stress homogenization in the winding. Coil support is
usually achieved using a stiff clamping system. For magnets working at moderate
fields (up to about 5 T) a simple structure acting on the coil (referred to as collars),
and locked by dowels or keys, may be adequate. This is the type of structure used for
the Tevatron dipole (see Fig. 8.4), in which the collared coil assembly is enclosed in
a cryostat (for thermal management) and centreed in the warm iron yoke by means
of spring-loaded bolts. Higher fields require additional force transfer structures, for
example the collared coil can be further clamped inside the magnetic yoke, thus
increasing rigidity, as in the case of the RHIC, HERA and LHC dipoles, also shown
in Fig. 8.4. In this case the collared coil assembly has a well defined outer surface
F. Bordry et al.
the electromagnetic force in situations of both large forces and large force density,
at cryogenic conditions, maintaining the conductors in their nominal position, and
avoiding excessive stress on superconducting cables, insulating materials and the
structure itself. This simple task can be addressed using different strategies that
depend on the level of field and electromagnetic force, on space and other operation
constraints, on material selection, and, not last, on the specific choice of the magnet
designer.
A few general lessons have been learned in the past 40 years of development,
and are common practice in superconducting magnet engineering. It is important
to constrain the winding pack in all directions, with a force that is greater than
the Lorentz load, including an appropriate safety factor. This is done to reduce
the energy inputs of mechanical origins that can trigger instabilities. For simple
magnetic configurations (e.g. a solenoid) the coil itself can be the bearing structure.
In the more complex situations encountered in accelerator magnets, the winding
must always be in contact with a force-bearing surface. An initial load is applied
on this surface, sometimes just a few MPa sufficient to remove the fluff. Caution
must be used to avoid cracking or tearing at the interfaces, and in particular those
bonded or glued during an impregnation process. In some cases, it may be of
advantage to intentionally remove the bond between the windings and the surfaces
that are not supporting the Lorentz load, especially at the surfaces that tend to
separate. In this case the coil would be allowed to move as much as required, e.g.
by field quality considerations. These principles may be difficult to apply in cases
where the force distribution is complex, e.g. in high order field configurations such
as nested multipole corrector magnets of particle accelerators. These magnets are
then designed with larger operating margin to cope with the increased perturbation
energy spectrum.
Coils can be dry-wound, in which case the conductor has free surfaces and
can be permeated by the cryogen. Alternatively, they can be impregnated with a
polymer resin that fills the coil spaces and once cured provides mechanical strength
but prevents direct contact to the coolant. Common resins do not have sufficient
mechanical strength to withstand thermal and mechanical stresses, and are loaded
with fibers (e.g. glass). It is important to avoid volumes of unloaded resin, as these
tend to crack and release energy that can lead to magnet quenches. Nb-Ti based
conductors, ductile and strain tolerant, are well adapted to both techniques, while
impregnation is favoured in the case of Nb 3 Sn or HTS based conductors that are
strain sensitive and require stress homogenization in the winding. Coil support is
usually achieved using a stiff clamping system. For magnets working at moderate
fields (up to about 5 T) a simple structure acting on the coil (referred to as collars),
and locked by dowels or keys, may be adequate. This is the type of structure used for
the Tevatron dipole (see Fig. 8.4), in which the collared coil assembly is enclosed in
a cryostat (for thermal management) and centreed in the warm iron yoke by means
of spring-loaded bolts. Higher fields require additional force transfer structures, for
example the collared coil can be further clamped inside the magnetic yoke, thus
increasing rigidity, as in the case of the RHIC, HERA and LHC dipoles, also shown
in Fig. 8.4. In this case the collared coil assembly has a well defined outer surface