Chapter Twelve - Magnetic Fields

Stray Field Guidelines

Unofficial National Accelerator Laboratory Standards (dc fields)

• >10 kG (>1.0T): to be avoided for even short periods

• 5-10 kG (0.5-1.0T): whole body exposure up to one hour max

• 100 G - 5 kG (0.01-0.5T): work in area should be minimized

Unofficial Stanford Linear Accelerator Standards (dc fields)

• whole body or head, extended periods, <200 G

• arms and hands, extended periods, <2000 G

• whole body or head, short periods (minutes), <2000 G

• arms and hands, short periods (minutes), <20,000 G

Persons with cardiac pacemakers must not go near energized magnets; the high magnetic field can interfere with the operation of the pacemaker.

Remove all ferromagnetic items on your person before approaching an energized magnet, for the same reasons discussed above.  Also, remove all magnetized credit cards from your wallet if you wish to ever use them again.

Quenching of superconducting magnet coils can also be hazardous.

This occurs when (for some reason) the current-carrying coil goes from the superconducting to the normal state, thereby suddenly changing its resistance form zero to a finite value.  The tremendous I²R energy generated in the coils vaporizes the liquid helium, creating large internal pressures within the magnet dewar (the section that holds the liquefied helium coolant) and a large He gas outflow from the magnet's pressure relief valves.  Superconducting magnets are designed to safely weather a quench, but accidents sometimes happen.  At the time of this writing, a colleague's magnet had just experienced its fourth quench (one tries to avoid quenches!); the first three were uneventful, but the fourth literally blew apart the magnet!  Not only is there a mechanical danger from such mishaps, but one also has to worry about the possibility of suffocation due to the sudden displacement of air by the large volume of gaseous helium generated in such an event. 

In summary, the dangers of a quench are:

• mechanical failure of magnet - fortunately this is a very rare event.

• suffocation from the displacement of air by gaseous helium (a much more likely hazard).

References:

1. T.S. Tenforde, ed., "Magnetic Field Effects on Biological Systems", Plenum Press (1979).

2. Above information from: Cryomagnetics

Last Update: 2006-09-21