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Superconducting magnets and the LHC for armchair nerds

Posted Oct 01 2008 5:07pm

The Large Hadron Conductor (LHC) experiment requires propagating very high energy charged particles in circular patterns, and magnets with very high magnetic-field strength.  The presence of a magnetic field is what makes the charged particle stream move in a circular path to begin with.  A higher-strength magnetic field is required in cases whenever (1) the "circle" you want to steer these particles inside of is smaller, (2) the particles are heavier, or (3) the particles are faster.  For the LHC particles, condition (3) is definitely the case, so the folks at CERN are almost certainly using superconducting-magnet technology.  Of course, the newer the technology, the more difficult it can be to get it working at the outset.  

For these types of magnets, the high-strength magnetic fields are generated by electrical currents that propagate in a loop, with the direction of the resulting magnetic field being perpendicular to the electric-current loop; this is an effect referred to as Ampere's Law.  To get a stronger magnetic field, you can make the electric current travel multiple times around the same loop.  At normal ambient temperatures, a problem arises in that, as electricity flows through the wires carrying it in the loop, that electric current starts to dissipate; to counteract this, you'd need to keep pumping more electric current into the loop in order to get the magnetic field strength that you'd need.  For the magnetic fields required by the LHC, producing this amount of current would be very expensive, if it was even possible.  

Fortunately, there's a way around this electric-current dissipation.  It turns out that, at very low temperatures, certain materials can pass electric current through them without that current dissipating.  This effect is referred to as superconductivity, and this is what enables the creation of these very high strength magnetic fields using relatively little electric current in the loop (and thus cutting operating costs way down).  MRI machines use superconducting loops; that's part of what's made MRI so readily available in smaller hospitals all over the world.  

The challenge with superconducting current loops is that these current loops must be maintained at these very low temperatures throughout operation of the magnet.  To achieve this, dewars of highly-condensed (and thus very low temperature) helium gas are used.  There are some serious safety issues involved when working with this low-temperature helium, not the least of which being the severe damage to the skin or other tissues if it comes in contact with them.  That may be a large part of the reason why the LHC needs to devote up to two months to safely repair the problem.  

One reason why otherwise grandiose experiments such as the LHC are worthwhile is because such cutting-edge technology is used for the very first time in these experiments.  Indeed, a lot of the technologies that we take for granted today, including the more-stable superconducting magnets used in MRI, but also such things as faster and more-efficient data storage and transfer techniques, have emerged from experiments such as these.

Dr. Kristofer Kainz is a medical oncology physicist and assistant professor at the Medical College of Wisconsin.

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