HD3 is a dipole magnet which means it has two coils. These coils want to repel each other pretty vigorously when the current is flowing in the magnet. Field strength for HD3 is probably on the order of twelve tesla peak field. To help quantify what a Tesla sized field is one of the little high power neodymium magnets everybody has played with is about 1.25 Tesla right at the surface. If you have messed around with one of these magnets you know just how strong they are. HD3 our big superconducting magnet has a field strength an order of magnitude larger. What's the big deal right? Its only ten times stronger. Well here size and surface area do matter. HD3 weighs over two tons when it goes into the cryostat for testing. The forces inside the magnet are massive. Imagine a refrigerator magnet that weighs two tons and has a field ten times what one of the little finger pinching magnets you played with. In fact it would pull your refrigerator through the wall instead of sticking your shopping list to it.
One of the problems with superconductors is they really have to be coaxed into doing it. We have to treat them nicely and give them a perfect place to sit and a maintain the perfect temperature. The slightest amount of heat or movement and they quit superconducting. We call this a quench. This is when a superconductor quits carrying current without resistance. Its a problem when you have a dozen or so megajoules of energy running around inside the magnet when it quenches. Our test facility has an elaborate system to deal with the large amounts of liberated electricity when one of the test magnets quenches. One of the primary causes of quenches are tiny movements within the magnet itself. Something shifts or settles and these movements cause a tiny local heating which induces a quench. One of the ways we prevent mechanical movement and shifting is by careful preloading of the magnet at room temperature.
When we preload the magnet the idea is we load it to some nominal level slightly higher than what we expect to see when the magnet is under test. This pre-stressing is crucial to preventing movement and resisting the magnetic forces that want to make the coils fly apart from one another. The first step in loading the magnet is to apply some compressive force to the coils. How do we do that?
The final step is to load the magnet in the axial direction using the large threaded rods. We don't actually crank the nuts down to apply the force. This is also done hydraulically and the nuts taken up to maintain the loaded position. With strain gages mounted on the rods we can more accurately preload the rods than by turning or torquing the nuts.
So all of this is just preparation and pre-stressing the magnet before we cool it down to 4.2 Kelvin. Most people know that when you heat something up it expands right? What do you think happens when we cool the magnet down? If you guessed that it shrinks you are correct. In fact we are counting on it here. When the magnet cools it shrinks considerably which is the final part of this operation. The aluminum shell that houses the magnet applies the final loading after cool down. Aluminum expands and contracts at a higher rate than the steel structure inside. This is called the coefficient of thermal expansion shorthand CTE. For each degree of cooling the aluminum contracts almost twice what the steel does causing a constricting force all the way around the magnet. If you do the math the shell shrinks around an eighth of an inch just from the cooling. It is acting like a really expensive hose clamp that holds the two magnet coils from moving when it is energized.