Characteristics of Superconducting Magnets

The most outstanding feature of a superconducting magnet is its ability to support a very high current density with a vanishingly small resistance. This characteristic permits magnets to be constructed that generate intense magnetic fields with little or no electrical power input. This feature also permits steep magnetic field gradients to be generated at fields so intense that the use of ferromagnetic materials for field shaping is limited in effectiveness. Since the current densities are high, superconducting magnet systems are quite compact and occupy only a small amount of laboratory space.

Another feature of superconducting magnets is the stability of the magnetic field in the persistent mode of operation. In the persistent mode of operation, the L/R time constant is extremely long and the magnet can be operated for days or even months at a nearly constant field, a feature of great significance where signal averaging must be performed over an extended period of time.

Small superconducting magnets are frequently used to attain field intensities, stabilities or profiles that are not attainable with alternative magnets or because their cost is less than the cost of conventional magnets offering comparable to or inferior performance. In large magnets, the trade-off is frequently made in favor of superconducting magnets based on the relative costs of power for operating the magnets. The cost trade-off becomes more favorable for superconducting magnets as the period of operation increases.

Magnetic field intensities of 1 Tesla or less, without demanding stability requirements, are frequently better generated with water cooled copper coils with or without iron.

Materials and Performance

Most superconducting magnets are wound using conductors which are comprised of many fine filaments of a niobium-titanium (NbTi) alloy embedded in a copper matrix. These conductors have largely replaced the single filament conductors since their magnetic field more readily penetrates the fine filaments, resulting in greater stability and less diamagnetism. Consequently, the linearity of the magnetic field and the magnet current is greatly improved. Another advantage of these conductors is the more rapid rate at which the magnet can be charged and discharged typically a few minutes for most laboratory size magnets.

Although most magnets are wound with multifilamentary niobium-titanium conductors, some are constructed with multifilamentary, niobium-tin (Nb₃Sn) conductors and some with single filaments of niobium-titanium. Nb₃Sn conductors are used when the field experienced by the conductor is in excess of about 9 Tesla (90 kilogauss). Typical magnets of this type are wound with a combination of NbTi windings in the low field region and Nb₃Sn windings in the high field region. Since multifilamentary Nb₃Sn is expensive, brittle, and difficult to wind, these magnets cost more than NbTi magnets.

Single filament NbTi magnets are preferred where the stability of the magnetic field over a long period of time is essential usually in nuclear magnetic resonance measurements. Better persistent mode operation can be obtained with this material, and since the field is held constant for long periods of time, the extra time required to charge the magnet is inconsequential.

During a quench, the magnet generates high internal voltages and locally elevated temperatures. These cause electrical and mechanical stresses in the windings. The consequences of a quench depend on the design of the magnet and its auxiliary equipment. Permanent damage to the magnet can occur. Normal operation of the magnet at the specified temperature and at magnetic fields equal to or less than the rated field are not expected to cause damage to the magnet, and a warranty covering this type of operation accompanies each magnet.

Niobium-titanium magnets are sometimes operated at temperatures below the normal boiling temperature of liquid helium (4.2K) to achieve even higher fields. Typically, an 8 Tesla solenoid will achieve 9.5-10 Tesla when operated at 2K. AMI magnets are rated in terms of their performance at 4.2K. Fields achievable at lower temperatures are only guaranteed if the magnet is designed for lower temperature operation. Some improvement in performance can also be achieved by reducing the temperature of Nb₃Sn magnets, but the increase in field is not as significant as it is in NbTi magnets. When operated at reduced temperatures and higher fields, the energy in the magnet can be increased by 50% or more. Consequently, the magnet might be irreparably damaged if a quench occurs and the magnet is not sufficiently protected. This type of operation should not be attempted without checking with the manufacturer of the magnet you may invalidate your warranty.

Magnetic Field Intensity

Underspecifying the intensity and homogeneity of the magnetic field used in your experiments may seriously impact your experimental results. However, overspecifying these parameters can greatly increase the costs.

An economic compromise occurs in magnets in which the field experienced by the windings exceeds about 9 Tesla, which is the highest field at which NbTi superconducting alloys can conveniently be used at 4.2K. Higher fields can be attained at this temperature using Nb₃Sn conductors, but the increase in cost deserves careful attention. A 10 Tesla magnet operating at 4.2K is substantially more expensive than a 9 Tesla magnet of the same size.

Homogeneity Specifications

Different manufacturers of homogeneous magnets have adopted different standards for specifying the homogeneity of their magnets. Most manufacturers specify the homogeneity in terms of the width of the resonant signal at half the signal height.

AMI uses a more conservative approach that measures the magnetic field at various points in the specified homogeneous volume using a small volume NMR sample. Consequently, small deviations at any point in the volume will be detected. Using an NMR sample equal to the homogeneous volume does not necessarily reveal such small deviations.

If a region of the sample occupying 10% of the specified volume resided in a field having a maximum inhomogeneity five times greater than specified, it is likely that this inhomogeneity would not be noticed. The reason is that the area under this part of the curve is only 10% of the total area and that the resonant line width is five times as broad. Consequently, this inhomogeneous region results in a long shallow tail on the base of the resonant signal.

The cost of homogeneity may be misleading. Homogeneities of ±0.1% in a one-centimeter diameter spherical volume (DSV) are routine. Homogeneities of ±0.001% in the same volume require larger magnets and considerably more effort in their construction. Still more homogeneous magnets require that separately energized superconducting or room temperature trim coils be employed. These coils can escalate the cost quite rapidly. The inside diameter of the magnet scales approximately with the diameter of the homogeneous region specified.

Persistent Switches

Persistent switches are provided on many magnets to increase their stability over long periods of time or to reduce the rate of helium boil-off associated with continually supplying current to the magnet.

A persistent switch is comprised of a short section of superconducting wire connected across the input terminals of a magnet and an integral heater used to drive the wire into the resistive, normal state. When the heater is turned on and the wire is resistive, a voltage can be established across the terminals of the magnet and the magnet can be energized. Once energized, the heater is turned off, the wire becomes superconducting and further changes in the magnet current cannot be made. In this persistent mode of operation, the external power supply can be turned off to reduce the heat input to the helium bath and the current will continue to circulate through the magnet and the persistent switch.

Persistent mode current switches are installed on and become an integral part of the magnet. This is necessary since special care must be taken in making the joints between the switch and the magnet leads. For a typical switch, the electrical heater in the persistent switch has a nominal resistance of 60 Ohms and requires 35 mA of current to drive the superconductor into the resistive state. The superconductive wire typically has 15 to 20 Ohms of resistance in the normal state.

AMI's standard persistent switch is 1.25 inches high and has a maximum outside diameter of 1.0 inches.