Biology Reference
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electric power to pass the current. Initially, the liquid helium, the coolant,
had to be supplied continuously from outside, which required consider-
able experience and cost. However, this scheme has recently been
replaced with an electrical cooling system which is cryocooler-free or
liquid helium-free. The cylindrical bore diameter can be 100 mm or more.
This allows a temperature regulating tool (e.g. water circulation) to be
installed within the bore. When magnetic fields of more than about 20 T
are needed, a hybrid magnet is used, in which additional fields are sup-
plied by a normal metal wire, in addition to the superconducting wire. In
this case, one needs an enormous amount of electric power to continu-
ously apply electric currents. Only a limited number of facilities in the
world have such hybrid magnets.
Electromagnets have traditionally been used to generate magnetic
fields of up to about 2 T. Masses of iron are used, making these magnets
extremely heavy. Also, electric current must be applied continuously by
using electric power.
With permanent magnets, using alloys containing neodymium and
other metals, increasingly large field strengths have been generated, up to
nearly 2 T. However, the volume of the spatial region of high magnetic
field strength is limited compared with superconducting magnets. The
cost and weight of a permanent magnet is low, and it does not require elec-
tric power. It is challenging to consider to what extent the effects of mag-
netic field and magnetic force can be derived from permanent magnets.
The magnitude of the geomagnetic field by comparison is about
0.06 mT; field strengths of 2-10 T are energetically larger by about 10 10 .
Superconducting magnets for supplying
uniform magnetic force
Regarding the use of the magnetic force, a specified magnetic environ-
ment is required. This is because the magnetic force is proportional to
grad( H 2 ). Often, a superconducting magnet is designed to provide a
uniform H or B , where B is the magnetic induction. That is, at a place
where a sample (e.g. an NMR tube, a long, cylindrical tube) is placed, the
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