Chapter 9

Principles of Magnet Safety

One may not think of magnets as potentially hazardous objects, but in fact they can be, especially superconducting magnets that achieve very high fields with their coils are cooled to liquid helium temperatures.

9.1 Magnet Safety for Non-technical Staff

Non-technical staff should read the whole of this short chapter, without worrying about the technical details. The basic safety principles can be summarized as follows:

  • Never carry any iron or steel object anywhere near an operational magnet. You may suddenly find the object flying out of your hand (or pocket) with little or no warning. To be safe, avoid carrying any metallic object near operating magnets, unless you know for sure that it is non-magnetic ( i.e., not attracted by a magnetic field).
  • If you have a pacemaker, never go near an operational magnet, as it may interfere with the proper operation of the pacemaker.
  • Be especially careful in the vicinity of superconducting magnets, as these have the largest fields and require extremely cold liquid helium for their operation.
  • Avoid exposing your credit cards to magnetic fields greater than 10-20 G.

9.2 Major Hazards Associated with Magnets

Carrying ferromagnetic objects near an energized magnet can be quite hazardous. The induced force on a ferromagnetic object by a dipole magnetic field goes as r-7, a very steep function of the distance r from the magnet. This means that the force on a steel tool will behave as though it suddenly "turns on" as you approach the magnet; you will have little if any warning before a very strong force acts on the tool. Flying tools can be quite destructive. To prevent this, always use special non-magnetic tools (such as CuBe or plastic tools) in the vicinity of an energized magnet. It is good practice in your lab to mark a border around the magnet within which non-magnetic tools are required. There are many true stories of accidents that have been caused by flying tools near superconducting magnets. One classic involved a flying wrench that trashed a million-dollar multiple-wire chamber at a particle accelerator. Another accident occurred at a magnetic resonance imaging (MRI) facility at a major hospital, in which a lab tech was wheeling a metal cart past an active superconducting magnet. When told not to bring the cart into the room, the tech replied that he had wheeled carts past the magnet many times, and knew what he was doing. What he did not know, apparently, was the r-7 force law, and on this specific occasion the cart was suddenly pulled from his grasp by the magnet and smashed into the magnet at high velocity, destroying the cart. Had a patient been in the imaging magnet at the time (fortunately, there was none), he or she might very well have been killed by the collision.

The biological effects of strong magnetic fields on humans are not well known, but are potential hazards that should not be ignored. There are a few unofficial regulation standards for human exposure that have been set up at national accelerator laboratories, that are listed here:

Unofficial National Accelerator Laboratory Standards (dc fields)

  • > 10 kG (> 1.0 T): to be avoided for even short periods
  • 5 - 10 kG (0.5-1.0 T): whole body exposure up to one hour max
  • 100 G - 5 kG (0.01-0.5 T): 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 from 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 liquified 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 a) mechanical failure of magnet; fortunately, this is a very rare event, b) suffocation from the displacement of air by gaseous helium (a much more likely hazard).

9.3 References

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

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