The AMS Superconducting Magnet


  • Why we need a magnet The magnet enables AMS to measure a particle's momentum and the sign of its charge. In free space, every particle tends to travel in a straight line; in a magnetic field, a charged particle prefers to move in a circle. So AMS will watch particles enter our strong magnetic field, and using the precision silicon tracker we measure the curvature of the trajectory. The curvature tells you the momentum divided by the charge, including the sign (positive or negative) of the charge. The magnet is the only tool we have to separate positive from negative charges; it is the only tool for identifying antimatter.

  • The design of the magnetic field The AMS magnet is a state-of-the-art superconducting magnet, and really one of the greatest challenges of the experiment. It is being designed and constructed by Space Cryomagnetics, Ltd.. Its core is a set of niobium-titanium wires, cooled to 1.8K, carrying current. Two big coils of this wire - dipole coils we call them - set up the main magnetic field. These coils, however, set up what is called a "magnetic dipole". What does that mean? Well, the magnet in a compass also has a magnetic dipole; your compass-needle tends to rotate due to Earth's magnetic field. If AMS has a dipole field, it would make the ISS rotate due to Earth's magnetic field! That would be bad! So, our magnet has a set of racetrack coils to cancel out the dipole field outside of AMS.


  • The magnet is cryogenic Superconductors only work at low temperatures. AMS has to keep cool, then, for three years in space. It carries 360 kg of superfluid, ultra-cold liquid helium. Over three years, the helium will bleed off slowly and help keep the superconductor cold. This is "evaporative cooling", the same physical process by which sweating cools you down on a hot day.

    There's a lot of technology that keeps the helium as cool as possible in space: vapor cooled shields, multilayer insulation, mechanical refrigeration pumps. All of this technology, however, is useless in the atmosphere; if our superfluid helium tank had to sit in the warm Florida atmosphere while we filled it up, since the helium is so cold, the air itself would freeze and stick to the outside of the tank, no matter how much insulation you wrapped around it. The only insulator for liquid helium, the only thing that will let it stay cold in warm surroundings, is a vacuum. So we have surrounded the entire helium/magnet system with a huge vacuum case. This case, since it's rather big and stiff, has become part of the support structure.

    (back to top) (back to AMS tour) (Magnet images (c) Space Cryomagnetics Ltd.)

    Technical details

  • Weight: 2350 kg
  • Cold mass: 2200 kg
  • Helium: 360 kg
  • Operating current: 459 A.
  • Operating temperature: 1.7 K
  • Superconductor: NiTi, sheathed in ultrapure Al for quench protection.
  • Max. B field: 0.865 Tesla
  • Bending power: 0.782 T m^2
  • Stored energy: 5.15 MJ
  • a paper on the magnet cryogenic system
  • Space Cryomagnetics, Ltd. Superconducting magnet coils superfluid helium storage tank superfluid helium storage tank superfluid helium storage tank vacuum case flange vacuum case wall AMS Magnet (c) Space Cryomagnetics Ltd. Primary magnet coil Primary magnet coil Primary magnet coil racetrack coil racetrack coil magnet bore, where the detectors go support frame support frame support frame racetrack coils primary magnet coil racetrack coils primary magnet coil superfliud helium tank vacuum case vacuum case Support ring magnet support point magnet support point Magnet support point Magnet support point magnet support point magnet support point magnet bore (where the detectors go) magnet structural element