Environmental Engineering Reference
In-Depth Information
enters the sample and forms individual
uxoids) [
21
,
22
]. Most basic
materials and some alloy superconductors show type-I behaviour. These will loose
their superconductivity above the critical magnetic
fl
ux lines (
fl
eld and will behave as normal
electric conductors. Therefore, type-II superconductors are of interest for engi-
neering applications, since they are able to carry high current densities and large
magnetic
elds.
Figure
3.24
(top) has been produced based on data from Refs. [
23
25
]. It shows
the historical development of superconducting materials according to their critical
temperature. Since these temperatures relate to cryogenics, accordingly, super-
conducting magnets can be divided into different groups:
-
Low-temperature superconducting magnets (LTS Magnets), which apply
liquid helium or are cooled by a cryocooler (please see subsequent text relating
to different technologies for cryocoolers),
High-temperature superconducting magnets (HTS Magnets), which apply
liquid helium, a cryocooler or liquid nitrogen,
Hybrid magnets, which combine a copper magnet in an inner section with the
superconducting magnetic in an outer section.
In Fig.
3.24
(bottom) a review on HTS materials with respect to their critical
temperature is shown. This
gure has been produced based on data from Refs. [
26
,
27
]. According to the design, most superconducting magnets fall into the following
groupings:
Solenoids: Represent cylindrical structures and are most broadly applied
(Fig.
3.25
),
Dipoles: These magnets generate a uniform
eld transverse to their longer axis
and can be found in particle accelerators and magnetohydrodynamic (MHD)
applications,
Quadrupoles: Generate a linear gradient
eld transverse to their axis over the
central region of their bore and can also be seen in particle accelerators,
Racetracks: Racetracks are wound in a plane where each turn consists of two
parallel sides and two semi-circles at each end, where a pair is assembled to
approximate the
eld of a dipole. These magnets can be found in supercon-
ducting motors, generators, as well as in train applications (Maglev),
Toroids: Generate magnetic
elds in the azimuth direction along the toroid.
They can be found in fusion reactors (Tokamak) or in superconducting magnetic
energy-storage systems (SMES).
For more information on the design of superconducting magnets, the reader is
referred to the work of Yuan [
20
] and Iwasa [
28
].
A cryogenic cooling system for a superconducting magnet is a must. Despite of
the fact that some may use an expression like
“
”
superconducting
magnets, this of course does not mean that superconductors are cooled at temper-
ature above cryogenic temperatures. Any existing superconducting magnet will
require cooling of its coils to cryogenic temperatures, whether this is applied using
liquid (cryogenic) refrigerants or a cryogenic cooler (cryocooler). If we focus on the
cryogenic-free
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