The Superconducting Magnet
The original AMS-02 design involved a powerful, complex Cryogenic Superconducting Magnet having the same mechanical dimensions as the Permanent Magnet flown in 1998 but able to develop a 5 times larger magnetic field.
What is a Conducting Magnet?
A current passing through a wire coil is able to produce a magnetic field. More turns of the wire in the coil correspond to stronger magnetic fields. This is a consequence of the Ampere’s Law. Charged particles traversing a magnetic field experience a force, called Lorenz force, which bends the particle trajectory along circular paths. The curvature radius depends on the particle energy while the curvature direction depends on the sign of the electric charge.
Why would we need a Superconducting Magnet?
The higher is the magnetic field the larger is the curvature of the charged particle trajectory. Energetic, fast particles are said to be rigid because their trajectories are difficult to be bent significantly from the magnetic fields developed by coils built with normal wires. A superconducting wire is able to carry large currents without dissipating energy: a superconducting magnet is then able to produce a field several times more intense than a normal or a permanent magnet, providing the possibility of separating particles from antiparticles up to high energies (the TeV scale).
From the point of view of space applications, the principal advantage of a superconducting magnet is the absence of dissipation of the current circulating in the coils because superconductors have zero resistance! Once the magnet is charged the coils can be closed – this is the so called persistent mode – and, since the resistance of the system is nearly zero, the current circulates and the magnetic field remains alive for years. The main disadvantage is that, with present technology, it cannot be operated indefinitely, since it consumes cryogenic fluids (low temperature liquid Helium).
After the success of the STS-91 mission based on a Permanent Magnet, the AMS Collaboration started to develop a SCM designed to operate for the anticipated duration of the AMS-02 mission on the ISS, namely three years. The two magnets are identical from the point of view of their mechanical interfaces to the particles detector systems, so they can be used to configure two different versions of the AMS-02 experiment: a SCM version optimized for a shorter operation on the ISS and a PM version optimized for a longer operation on the the ISS.
Why a Superconducting Magnet must be Cryogenic?
Superconductors work at temperatures near absolute zero (4 K in the case of AMS Niobium-Titanium wires)! A cryogenic system is required to keep the temperature under the critical threshold, to manage the possibility of a quench and to recover after a quench occurrence.
In Depth: What is a Superconducting Magnet Quench?
When a portion of the superconductor reaches a temperature exceeding the superconductivity critical threshold (for the AMS-02 superconducting wire, T > 4K), suddenly becomes a normal resistive wire. This piece of wire, like the filament of tungsten of a light bulb, begins to heat the surrounding material. This local transition from the superconducting to the normal state can be very fast and ends with the complete conversion of the magnetic energy stored in the coils in to heat which can destroy the wire if not properly dissipated. This process is called a Magnet quench.
The AMS Magnet electronics is able to detect the occurrence of a quench and activate a series of heaters in order to force all the magnet superconducting wires into the normal resistive mode at the same time. In this way the magnetic energy can be dissipated on a large volume of material without damage for the magnet coils. At the beginning of the life of a Superconducting Magnet quenches happens quite frequently (training quenches); during the operational lifetime of the SCM quenches are normally not present anymore. The AMS-02 Superconducting magnet is designed to withstand several training quenches during ground testing and to recover from a few quenches also in orbit, at the cost of about 25 % of the liquid Helium initial storage.
How is the Superconducting Magnet System built?
The AMS Magnet system consists of 14 superconducting coils, a superfluid helium vessel and a cryogenic system, all enclosed in a vacuum tank.
While the two largest coils – dipole coils – provide the main field, the racetrack coils close the field minimizing the stray field outside the magnet, cancelling the overall dipole moment of the magnetic system. This will avoid undesirable torques on the ISS resulting from the interaction with the Earth’s magnetic field. The coils are made by tiny (22.4 micron diameter) filaments of Niobium-Titanium which carry the current without resistivity. Each of the two dipole coils has 3,360 turns. When the Magnet is charged the two large dipole feel a net attraction to each other of around 250 tons. The mechanical structure of the magnet is designed to support these large forces.
The Magnet operates at a temperature of 1.8 K, using the cryogenic power provided by 2,500 L of superfluid Helium stored in a toroidal vessel. Because of parasitic heat loads, the Helium will gradually boil off throughout the lifetime of the experiment, expected to be around three years. After this time the SCM will warm up and it will no longer be operational.
A complex cryogenic system has been developed in order to maximize the life of the SCM by keeping the superfluid Helium cold (Passive Phase Separator, Vessel Cooling System, Cryocoolers, Thermo-mechanical pumps, and so on).
A big toroidal vacuum case encloses the SCM and the Helium Vessel. This vacuum case has the same purpose of a thermos, it keeps the system thermally insulated from the surrounding environment. This is most important in the case of the SCM operation on the ground with an environmental nulling temperature of 20 °C.