Many factors must be considered when designing a superconducting magnet to assure its proper performance. These factors include the
mechanical structure of the magnet, the magnetic field design and the design of the conductor to be used. It is also important that
the magnet be able to withstand the mechanical stresses caused by the magnetic and thermally induced forces encountered during
normal operation, and the electrical voltages encountered during a quench.
The following paragraphs provide a brief review of some of the more important topics which will assist you in selecting a magnet.
An important phenomenon in superconducting magnets is quenching. Any superconducting magnet can be quenched by increasing the
current and field
indiscriminately. A quench in a well encapsulated magnet typically occurs at the location of the highest field in the magnet.
Resistance is restored to the conductor at this point and heating occurs in the magnet. This heat spreads to adjacent areas and
drives more of the conductor normal, and the normal zone continues to spread until the magnet is completely discharged.
If the resistance across the terminals of the magnet due to the power supply is low, the power supply may be ignored to a first
approximation in analyzing a quench. Thus, the quench may be viewed as the discharge of an inductor into a time varying resistance.
The resistive voltage, iR, is counteracted by an inductive voltage, Ldi/dt. Unlike the few volts used in charging the magnet, the
voltages encountered during a quench discharge can be measured in kilovolts. Initially, the iR voltage is confined to the layers of
windings near the point where the quench initiated and internal arcing can occur between layers if sufficient insulation has not
During a quench a magnet can be damaged by high voltage, high temperature, and high forces. The magnet manufacturer takes all of
these issues into consideration as part of the design. Although the magnet is designed to withstand an occasional accidental quench,
quenches can shorten the useful life of the device.
The photograph above shows an example of a large cold helium gas cloud that
forms quickly when a large magnet quenches.
The heat capacities of the materials in a superconducting magnet at 4K are several orders of magnitude lower than the heat
capacities of the same materials at room temperature. Thus, a small amount of heat dissipated inside the magnet can raise the
temperature of the conductor above its critical temperature at the ambient field and current density.
One source of heating is wire motion caused by the Lorentz force on the conductor in the magnet. Imperceptible motions of the
wire can result in frictional heating sufficient to drive the conductor normal at fields well below the anticipated maximum field of
the magnet. Upon reenergizing the magnet, it is frequently observed that it will "train" to successively higher fields before
quenching, ultimately achieving the design field. In some cases, the wires will remain in their shifted positions and the magnet will
perform well. In other cases, however, retraining is required after the magnet is warmed to room temperature. To avoid this training
effect, it is necessary that the conductors be securely bonded in place to prevent them from moving.
Bonding the conductor in the magnet entails a substantial risk in that the conductor cannot be recovered and reused after it has
been bonded in place.
Since thermal conductivities are very low at these temperatures, the material used to bond the conductor also limits the thermal
conductivity of the magnet. Consequently, the effects of wire motion are amplified in that the heat generated is less effectively
dissipated to the liquid helium. The conductors in most laboratory sized magnets are bonded with epoxy. Most AMI magnets are wet wound
using a filled high thermal conductivity epoxy that is too viscous for vacuum impregnation. Since each turn of the windings is visible
as the magnet is being wound, voids in the epoxy can be avoided. Also, the relatively high thermal conductivity of this epoxy causes
the heat generated during a quench to be better distributed throughout the coil, thereby reducing the thermal stresses caused by the
Premature quenching can also occur if the large forces between coil sections result in the motion of one coil with respect to
another. This is most likely to occur in magnets having coils that are wound in opposition. Such coils are used in bucking coil
magnets and magnets for nuclear demagnetization where a low field region is required close to a high field region.
The alternative to bonded windings is to wind the magnet in such a manner that liquid helium permeates the windings. In this case,
the liquid helium, which does have a high heat capacity at these temperatures, absorbs the heat generated when the windings move. The
conductor used in this type of construction incorporates a much larger amount of normal conductor to limit the electrical resistance
and consequently the temperature increase when the superconductor is driven normal. The operating current density in this type of
magnet is much lower than in the
magnets described earlier.
After it has been energized, a superconducting magnet can be operated in the persistent mode by short circuiting the magnet with a
superconductor. This is accomplished by connecting a section of superconducting wire across the terminals of the magnet. This
section of superconductor can be heated to drive it into the resistive state so a voltage can be established across the terminals
and the magnet can be charged or discharged.
During the persistent mode of operation, the heater is turned off and the switch is permitted to cool into the superconducting
state. In this condition, the power supply may be turned off and the magnet current will circulate through the magnet and the
persistent switch. The decay of the magnet is given by:
H = Hoe-T/t
where T is the usual L/R time constant. The small residual resistance in the magnet occurs either from resistance in the joints or
from the flux motion resistance discussed earlier.
To achieve the best persistence, the magnet must be operated at less than the maximum field to reduce flux flow resistance in the
conductor. High persistence magnets are bulkier, more costly, and require more liquid helium for cooldown than magnets having somewhat
less persistence. Nevertheless, these magnets are quite desirable where great persistence is required, such as in nuclear magnetic
resonance experiments. In addition, it is necessary that the resistance in joints between conductors be as low as possible.