Static Devices with New Permanent Magnets
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STATIC DEVICES WITH NEW PERMANENT MAGNETS J. CHAVANNE, J. LAFOREST and R. PAUTHENET Laboratoire Louis N4el, C.N.R.S., 166X, 38042 Grenoble cedex, France ABSTRACT The high remanence and coercivity of the new permanent magnet materials are of special interest in the static applications. High ordering temperature and large uniaxial anisotropy at the origin of their good permanent magnet properties are obtained in rare earth-transition metal compounds. Binary SmCo 5 and Sm2 Co17 and ternary Nd2Fe 1 4 B compounds are the basis materials of the best permanent magnets. New concepts of calculations of static devices with these magnets can be applied : the magnetization can be considered as rigid, the density of the surface Amperian current is constant, the relative permeability is approximately 1 and the induction calculations are linear. Examples of hexapoles with Sm-Co and NdFeB magnets are described and the performances are compared. The problems of temperature behaviour and corrosion resistance are underlined. INTRODUCTION New static devices with specific magnetic field configurations can be designed and constructed with new permanent magnets. The permanent magnets are ferromagnets or ferrimagnets in which ordering temperature above room temperature, e.g. large exchange interactions and high spontaneous magnetization are quested. The specific property of a permanent magnet is the ability to stay in a metastable state of saturated magnetization. This can be obtained through the phenomenon of coercivity which is intrinsically linked to magnetic anisotropy of uniaxial character. In rare earthtransition metal rich compounds, Curie temperatures above 300 K are observed.
Some have large uniaxial anisotropy associated with a uniaxial crystallographic structure. New permanent magnets such as SmCo 5 , Sm2 Co17 and Nd-Fe--types with very high characteristics are now manufactured. This has led to an important development of static devices for charged particle beam applications using these permanent magnets, such as in travelling wave tubes, undulators/wigglers and in Electron Cyclotron Resonance (ECR) multicharged ion sources. We summarize in the first part some physical aspects of rare earthtransition metal alloys and the second part emphasizes the interest of the rare earth magnets. In the third part we present a method of calculation of
the magnetic induction in static devices made with these magnets. The fourth part is devoted to the hexapolar magnetic field configuration with these magnets describing the evolution during the last few years and new improvements due to the use of Nd-Fe-B magnets. RARE EARTH-TRANSITION METAL COMPOUNDS FOR PERMANENT MAGNETS A permanent magnet material requires simultaneously, at room temperature, a large atomic magnetic moment and a large anisotropy. The first of these properties is characteristic of the 3d elements : Fe, Co or Ni and the second one is characteristic of the 4f elements : rare earths. Alloys between these two series of elements allow the combination of large values of both magn
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