The Magnet

 

Motivation

Magnet_Deflection.jpgFor each particle there is an associated antiparticle with the same mass but opposite charge. For example, the antiparticle of the negatively-charged electron is the positron, which has the same properties as the electron, but a positive charge.

The easiest way to seperate particles and antiparticles is with a magnetic field. When a particle passes through a uniform magnetic field, its trajectory is bent according to its charge sign. Hence, particles and antiparticles are bent in opposite directions. It is then possible to distiguish electrons and positron from the particle curvature. Additionally, the momentum of the particle can be deduced from the radius of the curvature.

The Two Magnets

Two different magnet types were developed for AMS-02. A Permanent Magnet and a Superconducting Magnet. The Permanent Magnet was developed first and used in the AMS-01 prototype which was flown in the STS-91 shuttle mission. After the AMS-01 flight, a new superconducting magnet was developed, with an expected life time of 3 years, coinciding with the duration of the original AMS-02 mission after which the experiment was to be return to Earth. However, in the aftermath of the Columbia disaster, AMS-02 was changed into a permanent installation on board the International Space Station. In order to maximize the physics output of the experiment by measuring till 2025, it was decided to change the magnet back to the permanent magnet.

The Permanent Magnet

Permanent_Scheme01_3001.jpgThe AMS permanent magnet has the form of a cylinder. It is 1 m high and has a diameter of 1 m. It is made up of over 6000 Nd-Fe-B blocks. The picture on the left shows a section of the magnet with 64 such blocks. The whole magnet is composed of 100 such rings.

Nd-Fe-B magnets are the strongest permanent magnets and result in a total field strength of 0.15 Tesla. Despite of its cylindrical form, the magnet produces a uniform magnetic field along the x-axis by using the so-called Hallbach design.

The magnet has been tested for mechanical stresses with vibration tests, centrifugal tests and space qualification tests and was deamed fully qualified to fly into space.

After the extension of the AMS-02 mission, the question arose, if the permanent magnet would hold its strength over time. Therefore, in 2010 the magnetic field was measured in Aachen, having previously been measured in 1997 before the AMS-01 flight. The two measurements coincided within 1%, even with 13 years and a Space Shuttle flight inbetween,  showing the stability of the magnetic field.