The Permanent Magnet


Permanent Magnet Scheme (right resolution)

Magnetic field orientation and intensity of the AMS Permanent Magnet.

The AMS Permanent Magnet is a very robust and reliable magnet, already space qualified during the STS-91 flight. Its magnetic field is 3 000 more intense than the magnetic field of the Earth.

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How does the Permanent Magnet works?

The AMS Permanent magnet is made of Neodimium-Iron-Boron. The Neodium is a rare-earth element (lanthanoid). Do not be misled by the term rare-earth, Neodimium is a quite abundant element. Indeed the Neodimium magnets are very common in normal life. They are used in many applications, from toys to hard-disks, from the magnetic resonance instruments to headphones.

The Nd-Fe-B is a ferromagnetic magnet. In a simplified picture, ferromagnetic materials are composed by a large number of “tiny magnets”. Each one of them is generated by the spinning of an electron. These spinning electrons magnetic moments tend to orient all in the same direction giving rise to a macroscopic magnetic field. [A more realistic description of ferromagnetism involves quantum-mechanical quantities as the electron spin, and the exchange interaction able to orient the magnetic moments spontaneously].

The rare-earth element’s main advantage is that it tends to magnetize along a preferred direction of the crystal, resistent to magnetization in other directions. This is an important property in the case of the construction of a big, permanent magnet.

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The AMS PM, already flown in 1998, currently (April 2010) at Aachen (D).

How is the Permanent Magnet built?

The AMS Permanent Magnet has dimensions of a cylinder of 1 m diameter and 1 m height. It consists of over 6,000 Nd-Fe-B blocks (2⨯2⨯1 inches³). The picture shows a section of the magnet with 64 blocablks. The Magnet is composed by 100 of such sections. The blocks are glued together with epoxy that is also able to protect the magnet from corrosion.

The Nd-Fe-B magnets are the strongest permanent magnets, and the AMS permanent magnet is able to develop a magnetic field of 0.15 T. Despite the toroidal form the field is designed to be uniform along the X axis in the section plane.

The AMS Permanent Magnet has been tested for mechanical stresses such as vibration, centrifugal test and space qualification tests. The PM proved high reliability and resistance.

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PM Field map intensity as measured in 1997 compared with the 2010 measurement. No changes in the B-field intensity are observed within the experimental accuracy (about 1%).

How much Permanent is Permanent?

In view of the extended ISS lifetime to 2020 or even 2028, the Permanent Magnet becomes a very attractive solution for the AMS-02 spectrometer.

With present technology is not possible to operate a Superconducting Magnet on the ISS for more than 2-3 years, while a Permanent Magnet would allow operating the experiment for the entire lifetime of the ISS, collecting much larger statistics and extending the sensitivity of the instrument to very rare events. For this, it is crucial to verify the stability in time of the field of the Permanent Magnet.

The field had been measured in 1997 – before the AMS-01 mission on STS-91 – and measured again in April 2010 in Aachen. The 1997 and 2010 measurements coincide within 1%, the accuracy of the measuring device, despite the fact that 13 years and a flight on a Shuttle have passed in the meantime!

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If you want to know more:

» Ferromagnetism (Wikipedia)

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