NMR Magnet Shielding
The seat of the pants guide to understanding the problems of shielding NMR magnets
One of the problems associated with working with NMR instruments is the stray magnet field surrounding the magnet. This stray magnetic field has grown to be a larger problem as the magnets had moved to higher fields and wider bores. The problems with stray magnet fields are multiple and include:
- Small metal objects can be attracted to the magnet. These objects can move at speeds which can harm objects which get in the way.
- Credit Cards with magnetic strips can be erased.
- Watches and heart pace makers can cease operation in the magnet field. They may not start working when removed from the magnetic field.
- Televisions and computer monitors are distorted in the fringe magnetic fields. These items can be affected for a considerable distance in the horizontal direction and for one or two floors above and below the magnet.
It should be noted that if the NMR magnet can affect objects at these distances, then movement of magnetic objects at these distances can affect the NMR magnet. This can cause field shifts larger than the NMR system’s lock can correct and/or the loss of magnet homogeneity.
The problems these stray magnetic fields can cause are often made worse by the reactions of the people working in these stray fields. To my knowledge, no scientific studies have shown any harm to biological objects caused by static stray magnetic fields, even very strong fields like inside an MRI unit. However, the emotional reaction of many people to finding out they are in a stray magnetic field can be “non-linear”. Often these are the same people strapping magnets on their own or their horse’s body (Magnetic Therapy for Horses) for the “beneficial” and therapeutic effects of magnetic fields. Take this “non-linear” attitude and combine the litigious nature of today’s society and people and organizations can tend to overreact.
The first approach many groups make to addressing the logical and illogical issues that the stray magnetic field can cause is to put a steel plate between the magnet and the area where they do not want any stray magnetic field. This can often make the stray magnetic field in the target area stronger. Below we will attempt to explain without rigorous mathematical treatment why this is. While making this explanation, we will cover topics which will lead the reader to a more complete understanding of the problems associated with magnetic shielding.
When talking about magnetic field strength, it is helpful to think in the terms of magnetic flux lines. The more flux lines, the stronger the magnetic field. The straighter the flux lines, the more homogeneous the magnetic field. When we have a magnet, we have two magnetic poles — the north pole and the south pole. With a super-conducting magnet, we will define of the top of the magnet as being the north pole and the bottom of the magnet as being the south pole. When we have a magnet, we have flux lines coming out of the north pole (top of the magnet) and moving through the channel of least resistance to the south pole of the magnet (bottom of the magnet). The path of least resistance can be a vacuum or in our case usually air. If the magnet is isolated with only air surrounding it, then the flux lines make large sweeping curves out the top of the magnet, arcing around the side of the magnet and back to the bottom of the magnet. All flux lines leaving the top of the magnet need to return to the bottom of the magnet.
Magnetic metals like steel have a lower resistance to the passage of flux lines than does air. Therefore, all the flux lines on their way from the top of the magnet will prefer to move through steel rather than through air. This means that any flux line which intersects a steel object will proceed through the steel object until it needs to exit the steel object to return to the bottom of the magnet. Since the flux lines like to move through the steel more than through the air, all the flux lines possible tend to collect in the steel object for as long as possible. This will be true as long as the steel remains out of magnetic saturation. The thicker the steel the more flux lines can be captured before the steel object becomes saturated. The steel object acts like a “flux channel”, where it grabs all the flux lines it can and channels them down to where they need to exit to get back to the magnet.
Another factor complicating this process is somewhat counter intuitive. A piece of steel is attracted to the area where the magnetic field has the strongest gradient (most inhomogeneous), not the strongest absolute field value. An interesting example (don’t try this at home!) is to drop a nail into a super-conducting magnet. The nail will be strongly pulled into the top of the magnet and will fall freely through the homogeneous region of the magnet and stick near the bottom exit port of the magnet. If you take a wooden stick and push up on the nail it will move freely back through the homogeneous region of the magnet but will be very difficult to push out the top. This is because, even though the field in stronger in the center of the magnet, it has less magnetic gradient in the homogeneous center of the magnet than near the ends of the magnet where the flux lines spread out on their individual return path arcs.
So, in our example of placing a steel plate between the magnet and the area we desire to shield from the magnetic field, the ends of the steel plate will have all the flux lines possible trying to get into it, traveling down to where they must exit to get back to the magnet (the flux channel) and exiting in a big bunch. Remembering that the more flux lines, the stronger the field, what we have accomplished with our actions is to make the magnetic field near the ends of the steel plate stronger (collected flux lines) and more inhomogeneous (bent more). Both of these points make the stray magnetic field more of a problem near the ends of the steel plate. One way to overcome this problem is to make the steel plate much larger that the desired area to shield. As the dimensions of the steel plate get larger, the larger the area of the lowest magnetic field behind the steel plate.
A more practical way to address this issue is to make a steel box totally surrounding the magnet. Then the flux lines going out the top of the magnet would enter the steel flux channel on the top and move through the flux channel on the side to the flux channel on the bottom and back to the magnet. The area outside the steel box would have a very low residual magnetic field. One problem with this approach is that the smaller the box you build around the magnet, the thicker the steel has to be to stay out of magnetic saturation. Very close in, this can take tons of steel to shield a small magnet and lots more to shield a big magnet. So to avoid having to use such thick steel, you move farther away from the magnet and make the walls of the room into a magnetic shield. Of course, the smaller the room, the thicker the steel required and the more effect opening the steel door in such a room would have on the magnet itself.
Forces on the Magnet
At this point, it is appropriate to talk about the forces on the magnet itself. When a steel object is attracted to a magnet, we need to remember that the magnet itself is being attracted to the steel object with the same force. The force of this mutual attraction is proportional to the size of the steel object and its distance from the magnet. If a big steel object gets near a super-conducting magnet, it can pull the magnet over. Even if this is avoided, the magnet inside the dewar can have a lot of force on it. With many super-conducting magnets, the internal supports in the dewar are not very strong. The magnet is often hung from the top with the minimal amount of materials and held in vertical alignment with small struts. Both the material to hang the magnet and the struts are as small as possible to avoid thermal leaks to the outside world. These thermal leaks would lead to higher helium boil-off. Since the internal supporting material is so weak, a metal object nearby can cause enough force to alter or break the magnet supports. This is an irreversible process leading to much higher boil-off and the need to disassemble the magnet for repair.
So when shielding a magnet, do two things; 1) keep the steel as far from the magnet as practical and 2) keep the shielding as symmetrical around the magnet as possible. The reason for the symmetry is that a force from one side can be balanced by a force from the other side. This symmetry also makes it easier to shim the main magnetic field. When thinking about symmetry, don’t forget the distance from the top of the magnet to steel objects above and the distance from the bottom of the magnet to similar steel objects below.
If one of the problems you are trying to deal with in an NMR facility is the distortion of televisions and computer monitors, then a much more cost effective way to deal with this issue than shielding the entire magnet room is to build a shielding box for the monitor. This can be a sheet metal box with five sides where the front of the monitor faces the open side of the box. Low cost steel plates can be bolted to angle iron and painted to construct the box (keep such a box from getting so close to the magnet that it gets pulled in). Keep at least one inch of space between the box and the monitor for ventilation. With some larger monitors a small ventilation fan is required to keep the monitor cool enough for operation. Boxes such as these are amazingly effective at keeping monitors working without distortion.
Active Magnet Shielding
What we have discussed here so far has been passive shielding of magnetic fields. Today many NMR and magnet vendors are providing active magnet shielding. This is done by building a second magnet on the outside of the main magnet with a field of the opposite sign, thereby canceling the external magnetic field. Here the outside magnet acts as the other pole with the top of the outside magnet being the south pole and the bottom of the magnet being the north pole. This is a really good flux channel! Obviously, if the shielding magnet and the main magnet were of the same size and strength, the net field inside and outside the magnet would be zero. To avoid this problem the outside magnet is bigger in size and therefore can operate at a smaller field strength. This is because the field at some distance from the magnet is less than the maximum field strength and it is this field which needs to be cancelled. Using active shielding always means that some of the main magnet’s field is being cancelled, so this technique can only be used when you have field strength to burn.
Last updated: 01/22/03