The Magnet is the heart of the AMS experiment. Thanks to the magnetic field AMS is able to separate matter from anti-matter. In addition, from the radius of curvature it is possible to estimate the particle momentum.
Why do we need a Magnet?
To each particle is associated an antiparticle with the same mass but opposite charge. As an example the partner of the negatively charged electron (e-) is the positron (e+) that has the same properties of the electron but a positive charge.
The simplest way to separate particles and antiparticles makes use of a magnetic field. Passing through a uniform magnetic field particles and anti-particles, i.e., particles with opposite charges, are bent in opposite directions. From the particle curvature (positive or negative) it is possible to distinguish among electrons and positrons. From the radius of curvature we can measure the particle momentum.
The two AMS Magnets
Two types of magnets have been developed by the AMS Collaboration to be used in space: a Permanent Magnet (PM) and a Superconducting Magnet (SCM). During the AMS precursor flight AMS-01 a Permanent Magnet was used. In preparation for AMS-02 the Collaboration has developed a Superconducting Magnet having the same inner diameter as the AMS-01 Permanent Magnet. Since the AMS-02 magnet is a technological challenge, all the AMS-02 detector subsystems have been designed with fully compatible interfaces to the Permanent Magnet, in case this will be used on the ISS. In the following links you can find some information’s about the two type of magnets developed by the AMS Collaboration:
In Depth: Which is the best magnet system for a mission on the ISS?
The two types of magnets have a very different construction techniques as well as operating conditions. The SCM is much more complex than PM; it has a higher field but a finite endurance. From the point of view of the Cosmic-Rays Physics two main aspects are the most important:
1) The Magnetic Field Strength: a higher magnetic field allows for a higher bending power, and can separate between antiparticles and particles at higher energies.
2) The Experiment Endurance: an extended lifetime increases the probability of observing rare signals like candidates of primordial antimatter. In addition, high energy Cosmic-Rays have a much lower flux than low energy ones. Increasing the exposure time extends the energy reachable by the experiment.
In order to reach the AMS-02 scientific goals two scenarios have been considered:
The Superconducting Magnet Scenario: the strength of the magnetic field is 5 times greater than for the PM. In combination with a compact Silicon Tracker matter/antimatter separation can be achieved up to about 1 TeV. The SCM will have a lifetime not exceeding 3 years: during this time the number of particles expected above or around 1 TeV is quite small. Thus the AMS-02 SCM spectrometer configuration has been optimized considering an exposure limit of 3 years and a maximum energy of few TeV.
The Permanent Magnet Scenario: it has a 5 times weaker magnetic field but an unlimited lifetime. The reduced bending power can be compensated by a different Tracker planes configuration which significantly extends the lever arm of the curvature measurement outside the magnet bore. In this configuration the charge sign determination can be performed at 1 TeV using the PM with the same accuracy as with the SCM. Although this configuration decreases the AMS acceptance for high energy particles by about 40%, the statistical accuracy is recovered by the much longer exposure time. Similarly such a configuration will not be so useful in case of the SCM Scenario due to its limited lifetime (< 3 years).
Both SCM and PM Scenarios are then rather equivalent from the point of view of the AMS Project Physics Goals.
AMS-02 was initially planned to operate on the ISS for three years with a Superconducting Magnet: after the Columbia disaster the number of Shuttle flights has been drastically reduced. After installation AMS-02 will then not return to earth and it will operate on the ISS for 10 to 15 years. The decision on which magnet to chose depended on the results of the TVT test performed at ESTEC in April 2010: depending on the SCM endurance in the space simulator, it was possible to decide which would have been the AMS-02 experimental configuration which will maximize the physics output on the ISS.