Magnet Power Supply Systems

Power supply systems for superconducting magnets are usually low voltage, high current systems. Since most standard magnets have inductances of ten Henries or less, only one volt across the magnet will provide a minimum charge rate of 0.1 amperes per second or six amperes per minute. Operating currents for standard magnets are typically in the 60-90 ampere range so they can be charged to their rated fields in ten to fifteen minutes or less at one volt.

Due to the large reactive impedance of the magnet, especially at higher frequencies, the high frequency noise of the power supply is of secondary importance compared with the stability, or long term drift of the current. Too much noise or voltage spikes, however, will affect persistent switches used with the magnet. Since the persistent switch has a very low resistance and it is in parallel with a high impedance inductor, AC ripple can lead to heating of the switch and prevent it from cooling into the superconducting state required for proper operation.

Transient voltages, which can occur when the supply is turned off, can also affect the persistent switch and must be avoided. Within these limits, the characteristics of the power supply are not critical to the performance of the magnet. Where extreme regulation and stability are required, the magnet is operated in the persistent mode (via use of the persistent switch) and the power supply may be removed from the system during the experiment.

Very high voltages can be generated across the input terminals of a superconducting magnet during a quench unless some form of limiting device is incorporated. These voltages can cause internal arcs in the magnet, damage to the power supply, and present significant risks to operating personnel. AMI power supply systems prevent these high voltages by including a large rectifier and heat sink rated for continuous operation at the full output current of the supply. Also, the AMI quench detection circuit provides visual and optoisolated electrical indication of a magnet quench and automatically ramps the power supply system and magnet current to zero. The system is prevented from initiating a recharge of the magnet current until the front panel quench detection switch is reset.

AMI power supply systems provide user programmable charge rate, current limit and voltage limit settings. The charge rate is typically adjustable between 0 and 10 amps per second with a resolution as small as 0.0001 amps per second, while the current limit and voltage limit settings provide better than 0.001% resolution. Along with charging and discharging the magnet current, the user can also select the pause feature to temporarily suspend ramping of the magnet current.

Superconducting magnets can store significant amounts of energy and this energy must be dissipated when the magnet is discharged. With a simple power supply system, energy is dissipated in the power supply cabling and current leads according to P=I ² R. This configuration usually limits the discharge voltage to less than one volt.

In systems employing magnets with high inductances, or where frequent decreases of the magnetic field are anticipated, purchase of the fast rampdown option should be considered. The fast rampdown option permits the magnet to be discharged at 5 volts. This is a particularly important addition to large systems with high inductances which store hundreds of kilojoules of energy. In such cases, the time required to discharge the magnet at the end of the test might be a few minutes with the fast rampdown option, or exceed one hour without it.

For experiments which require the polarity of the magnetic field to be changed, a four-quadrant power supply system should be purchased.