8 Accelerator Engineering and Technology: Accelerator Technology
367
that mates with the inner surface of the iron yoke. The iron yoke, assembled from
packs, is held by an outer shell that takes part of the mechanical load.
Force transfer from cold to warm parts is usually kept to a minimum, and possibly
reduced to the bare support of gravity loads. This is because any massive mechanical
component also acts as a thermal bridge, affecting heat loads and cooling efficiency.
Due to differential thermal contraction, any warm-to-cold transition is also subjected
to relative movements, which shall be accomodated by adjusted kinematics or
flexibility.
One of the recurrent issues in the manufacture of a superconducting magnet is
the choice of electrical insulation between coil turns and between coil and ground.
Most important is to include in the consideration of insulation all coil discontinuities
(e.g. terminations, contact surfaces, coil heaters) as well as the instrumentation. The
insulation of accelerator magnets is submitted to moderate dielectric stresses (in the
range of few hundred V to a kV), but extremely high mechanical stresses under
cryogenic temperatures (in the range of few ten to hundred MPa), and possibly
in a radiation environment. Good coil insulation must rely in materials capable
of retaining their dielectric and mechanical properties at cryogenic temperatures,
such as polyimide tapes, cryogenic-grade epoxy resins and relevant glass-fiber
composites. The dielectric strength of impregnated coils can be as good as for
resistive magnets. In the case of insulation porous to the coolant, such as obtained by
dry-winding the coil, the dielectric strength depends on the properties of the coolant
itself. Liquid helium has a high breakdown strength, about 30 kV/mm which is one
order of magnitude larger than that of dry air. Gaseous helium, on the contrary, has
a much lower breakdown voltage, typically one tenth than that of air at the same
pressure, while at sub-atmospheric pressures it decreases to a Paschen minimum
of about 150 V [31]. In this case it is important to consider all possible operating
conditions (e.g. the decrease of helium density during a magnet quench).
A further issue in the design of superconducting magnets is cooling. Any heat
load from internal or external origin (e.g. particle energy deposition, heating at
resistive splices, heat conduction and radiation to the cold mass, AC loss), of
both steady state and transient nature, must be removed to a suitable cryogenic
installation that provides the heat sink at the lowest temperature of the system.
Magnets subjected to small heat loads (in the range of few mW) can be indirectly
cooled by thermal conduction. This is the case of small-size magnets, operated
at low current, and well shielded. The recent advance in cryo-coolers has made
cryogen-free operation a convenient solution for this class of instrumentation
magnets. At the other extreme, large cold masses, and high current cables, subjected
to much larger heat load (few W to few tens of W) require the direct use of the
coolant as a thermal vector. The magnet can then be cooled by immersion in a bath of
liquid, either normal or superfluid helium, or by force-flow cooling. A further option
in case of force-flow is to either cool the magnet as a whole, or to distribute the
cooling channel within the coil using, as an example, internally cooled cables. An
important aspect of any cooling method, either direct or indirect, is the temperature
gradient that is established under the heat load between the superconductor and
the heat sink. This temperature gradient affects the temperature margin discussed
367
that mates with the inner surface of the iron yoke. The iron yoke, assembled from
packs, is held by an outer shell that takes part of the mechanical load.
Force transfer from cold to warm parts is usually kept to a minimum, and possibly
reduced to the bare support of gravity loads. This is because any massive mechanical
component also acts as a thermal bridge, affecting heat loads and cooling efficiency.
Due to differential thermal contraction, any warm-to-cold transition is also subjected
to relative movements, which shall be accomodated by adjusted kinematics or
flexibility.
One of the recurrent issues in the manufacture of a superconducting magnet is
the choice of electrical insulation between coil turns and between coil and ground.
Most important is to include in the consideration of insulation all coil discontinuities
(e.g. terminations, contact surfaces, coil heaters) as well as the instrumentation. The
insulation of accelerator magnets is submitted to moderate dielectric stresses (in the
range of few hundred V to a kV), but extremely high mechanical stresses under
cryogenic temperatures (in the range of few ten to hundred MPa), and possibly
in a radiation environment. Good coil insulation must rely in materials capable
of retaining their dielectric and mechanical properties at cryogenic temperatures,
such as polyimide tapes, cryogenic-grade epoxy resins and relevant glass-fiber
composites. The dielectric strength of impregnated coils can be as good as for
resistive magnets. In the case of insulation porous to the coolant, such as obtained by
dry-winding the coil, the dielectric strength depends on the properties of the coolant
itself. Liquid helium has a high breakdown strength, about 30 kV/mm which is one
order of magnitude larger than that of dry air. Gaseous helium, on the contrary, has
a much lower breakdown voltage, typically one tenth than that of air at the same
pressure, while at sub-atmospheric pressures it decreases to a Paschen minimum
of about 150 V [31]. In this case it is important to consider all possible operating
conditions (e.g. the decrease of helium density during a magnet quench).
A further issue in the design of superconducting magnets is cooling. Any heat
load from internal or external origin (e.g. particle energy deposition, heating at
resistive splices, heat conduction and radiation to the cold mass, AC loss), of
both steady state and transient nature, must be removed to a suitable cryogenic
installation that provides the heat sink at the lowest temperature of the system.
Magnets subjected to small heat loads (in the range of few mW) can be indirectly
cooled by thermal conduction. This is the case of small-size magnets, operated
at low current, and well shielded. The recent advance in cryo-coolers has made
cryogen-free operation a convenient solution for this class of instrumentation
magnets. At the other extreme, large cold masses, and high current cables, subjected
to much larger heat load (few W to few tens of W) require the direct use of the
coolant as a thermal vector. The magnet can then be cooled by immersion in a bath of
liquid, either normal or superfluid helium, or by force-flow cooling. A further option
in case of force-flow is to either cool the magnet as a whole, or to distribute the
cooling channel within the coil using, as an example, internally cooled cables. An
important aspect of any cooling method, either direct or indirect, is the temperature
gradient that is established under the heat load between the superconductor and
the heat sink. This temperature gradient affects the temperature margin discussed