Creating the perfect magnetic field

Permanent magnets are of course very handy to hold a shopping list to the fridge but they are also used in many devices, from tiny magnets in mobile phones and computer drives to much larger magnets in vehicle motors and wind turbines.
 
At Diamond we use permanent magnets to produce high intensity X-rays. Fridge magnets are made from powdered iron oxide mixed in a plastic, here we use extremely powerful magnets made from an alloy of Neodymium, Iron and Boron.
     

Tens or even hundreds of magnets are clamped very precisely along an aluminium alloy beam, two of these magnet filled beams are then placed opposite each other and as the electron beam travels along the gap between them high intensity X-rays are produced. (see animation below)

Sounds simple, but dealing with such powerful magnets has its problems. These magnets produce huge forces, even a magnet on its own must always be clamped down and never handled by hand. The largest magnet currently used is about the size of a hardback novel and two of these 22mm apart will lift 200kg.

The quality of the X-rays produced also depends on how uniform the magnets are. The magnets have some slight variations and so from each batch manufactured only the best ones are chosen, in addition each magnet has to be adjusted within a few microns to create the perfect magnetic field.

Holding the magnets apart requires some very strong and stiff structures to ensure that the magnets keep their position with a few microns even when subjected to up to 5 Tonnes of attractive force. The gap between the magnets also needs to be varied. By changing the gap the energy of the X-rays changes so this enables the beamline team to tune the peak X-ray energy to match the needs of the experiment. To adjust the gap large servo motors drive a set of heavy duty precision ball screws that pulls the magnets apart. 

This picture shows one of these cold magnet systems during assembly. The vacuum vessel is not fitted yet and will go around the two beams, each holding 230 magnets. The large shafts go through the vessel and hold the magnets apart. They need to be strong but poor conductors of heat.
This picture shows one of these cold magnet systems during assembly. The vacuum vessel is not fitted yet and will go around the two beams, each holding 230 magnets. The large shafts go through the vessel and hold the magnets apart. They need to be strong but poor conductors of heat.

Many of these magnet systems operate in air with a thin walled vacuum vessel located between the magnets. The electron beam has to operate in vacuum and so to get even more X-rays produced we would like to get the magnets even closer together. So how about putting the magnets inside the vacuum vessel?

Diamond Engineers are working on the next generation of devices which not only have the magnets inside the vacuum vessel but use even stronger magnets. These magnets are made from Neodymium Praseodymium Iron Boron but to get the high strengths they need to be cooled to -200°C.
 
So now the challenge is to produce structures that can hold these very powerful magnets very precisely inside a vacuum vessel whilst at -200°C.