Magnet Phenomena

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.

Quenching

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 been provided.

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.

Training

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 quench.

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 cryostable magnet is much lower than in the intrinsically stabilized magnets described earlier.

Persistent Mode

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.

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