Lake Shore 625, User Manual

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Lake Shore Model 625 Superconducting MPS User’s Manual
Magnet System Design 2-1
CHAPTER 2
MAGNET SYSTEM DESIGN
2.0 GENERAL
This chapter provides information on general magnet system design. It is intended to give the user insight into
superconducting materials, superconducting magnets, persistent switches, dewars, and cabling issues. For information on
how to install the Model 625 please refer to Chapter 3. Instrument operation information is contained in Chapter 4.
2.1 SUPERCONDUCTING MATERIALS
Superconducting materials have a very special property, that when cooled to very low temperatures, they become perfect
conductors of electricity. The transition to the superconducting state happens abruptly as the critical temperature is
reached. When the material is in its superconducting state, it has absolutely zero resistance. Such materials have a variety
of applications, one of which is for the construction of high field magnets.
The unique properties of superconductors make them ideal for use in high field magnets. Since a superconductor has no
resistance it requires no voltage to maintain a current through it. Magnet grade superconductors also have a very high
current density allowing a relatively small wire to carry a large amount of current that can be used to generate large
magnetic fields.
There are three properties that determine if a material is in its superconducting state. The first property is critical
temperature. A superconductor needs to be cooled in order to transition to a superconducting state. This temperature is
called its critical temperature. Most materials need to be cooled with liquid helium in order to reach their critical
temperatures although some ceramics have shown to have a critical temperature as high as 125 K which would be
suitable for nitrogen cooling.
The second property is critical current density. A superconducting wire can only carry a certain amount of current in its
superconducting state. The current density of a typical superconducting wire made from niobium-titanium is on the order
of 10
10
A/m
2
, about three orders of magnitude greater than normal house wiring. If the critical current density is
exceeded, the wire will return to its normal resistive state.
The last property is critical field. A superconductor will return to its normal resistive state if it is exposed to a magnetic
field larger than its critical field. Superconducting wire such as niobium-titanium and niobium-tin have critical fields in
excess of 10 T and 20 T respectively. Elemental superconductors, such as lead, have very low critical fields, in this case
0.08 T, and are not suited for creating superconducting magnets.
All three of these properties are related to one another. For instance, a superconducting wire is able to carry more current
and withstand a higher magnetic field as it is cooled to a lower temperature. In the case of niobium-titanium, a common
superconducting wire, the critical temperature is 9.3 K but at that temperature both the critical field and critical current
density are both zero. At a temperature of 6 K, the critical field increases to approximately 7 T and at 4 K it is
approximately 11 T.
2.2 SUPERCONDUCTING MAGNETS
Superconducting magnets are wound from many turns of superconducting wire. They are used to generate magnetic
fields that are larger than can be achieved with permanent magnets or electromagnets or when field stability is important.
They can also be more economical to run than electromagnets since the power needed to maintain the charge is minimal.
2.2.1 Superconducting Magnet Construction
The magnetic field (B) that can be generated by a solenoid is given by the equation B=μ
0
In/l, where μ
0
is the
permeability of air, I is the current in the wire, n is the number of turns, and l is the length of the solenoid. Most
superconducting magnets are wound using a conductor made from many fine strands of niobium-titanium (NbTi) or
niobium-tin (Nb
3
Sn) embedded in a copper matrix. The copper matrix is used for mechanical stability and to provide a
path for large currents in the case of a magnet quench. Typically, niobium-tin is only used in magnets that can generate
fields in excess of 9 Tesla because it is more expensive and harder to work with than niobium-titanium.