Division of accelerators, cryogenics and magnetism

In the process of being installed at the CEA Paris-Saclay site since July 2017, the colossal magnet weighing 132 tons is now fully operational. To do this, the teams had to meet the many challenges involved in setting up such equipment:

• Connecting the magnet to auxiliary equipment already installed and tested (cryogenic plant, electrical power supplies, instrumentation and control system);
• Reaching a stable temperature of 1.8 K: the magnet must be cooled to 1.8 K (- 271°C), at which temperature helium, its "cooling liquid”, is in a special physical state known as superfluid. At this temperature level, the conductor that forms the magnet (niobium-titanium alloy) creates no resistance to the electric current passing through it, thereby preventing any power dissipation or temperature rise: it is in a superconducting state;
• Ramping-up to reach 11.7 T[1]: once the conductor had cooled to its nominal temperature, current was gradually injected into the magnet to reach the target magnetic field of 11.7 T. This “ramp-up” was performed in several stages, with numerous electrical and magnetic tests, as well as tests of emergency shutdown procedures. In total, 1,300 procedures intended to detect any potential defects were tested.

To reach its nominal magnetic field, the magnet is supplied with a current of 1,500 amperes and the conductor coils are cooled constantly by 7,000 liters of superfluid helium. With a magnetic field of 11.7 T, this is a world record in the field of MRI and an all-time record with this type of superconducting material. Manufacturing this 132-tonne prototype – measuring 5 m in length with an outer diameter of 5 m and an inner diameter of 90 cm – will have taken six years of development in Alstom (now General Electric, GE) factories in Belfort and nearly two years of installation and testing before it is commissioned.

Over the following months, numerous items of equipment will be added to the magnet to transform it into an MRI machine capable of producing images of volunteers’ brains: gradient coils, radiofrequency coils, patient table, outer casing, etc.

[1] The Earth’s magnetic field: 5.10-5 T; standard hospital MRIs: 1.5 to 3 T; Large Hadron Collider magnets: 8.5 T. There are also small-diameter MRI machines, used only in preclinical research, capable of reaching magnetic fields of 17 T.