Graded Field Magnets

Abstract

A lens effect is created at the entry to or the exit from, the airgap of a magnetic system by equipping the terminal faces of the polepieces thereof with ferromagnetic blocks, which are pivotally mounted on the output faces of the magnetic yoke so that the airgap has a variable width and a magnetic field gradient is produced. The upper face of the blocks may be sloped.

Description

The present invention relates to devices intended to guide or focus a beam of particles, such as electrons, which are one of the constituents of the matter. It applies in particular to the field of particle accelerators for nuclear research purposes and to the field of applied and medical gammagraphy. In these fields use is frequently made of a magnetic field developed between polepieces which define an airgap through which the particle beam propagates.

Where the magnetic field concerned is directed perpendicularly to the trajectories of the particles, the effect obtained is essentially that of a deflection, along an arc of a circle.

If the strength of the magnetic field in the airgap is made variable as a function of the distance from the center of curvature of the deviation, the field gradient thus created exhibits a magnetic lens effect upon the beam and this can be utilized in many applications. It may be necessary to create focusing forces in addition to those already produced by the field gradient, by producing magnetic lens effects at the entry to and exit from the airgap.

One of the simplest manners of achieving this lens effect is to incline the entry and exit faces of the polepieces in relation to the plane perpendicular to the beam. However, where the angles are large a drawback is that the size of the polepieces becomes quite substantial.

It is an object of this invention to avoid this drawback.

According to the invention there is provided a magnetic device for deflecting a beam of electrically charged particles, comprising two polepieces defining an airgap; means for creating a magnetic field in said airgap for the propagation of said beam, said polepieces comprising on at least one of their end faces, ferromagnetic metal blocks pivotally mounted about an axis perpendicular to said end faces.

For a better understanding of the invention and to show how the same may be carried into effect reference will be made to the drawings accompanying the ensuing description and in which:

FIG. 1 illustrates an electro-magnet according to the invention; and

FIG. 2 illustrates a variant embodiment of the electro-magnet of the invention.

The electromagnet shown in FIG. 1 comprises a magnetic yoke 1 with two arms 2, two polepieces 3 and coils 4. Between the polepieces, an airgap 5 is formed. The two polepieces have terminal faces 6 and 7 fitted with parallelepiped ferromagnetic blocks 8 which can pivot about axes 9 normal to the faces 6 and 7.

The device described makes it possible to create a magnetic field gradient since the airgap distance between the mutually opposite faces of the parallelepiped blocks varies as a function of the distance from the zone considered, to the center of curvature, and also to control the value of this gradient.

The airgap is bounded by two parts, one having a zero or constant gradient and the other a variable gradient, which leads to the desired magnetic lens effect.

FIG. 2 illustrates a variant embodiment of the device according to the invention in which two separate lens effects are created.

For this purpose, the pivoting ferromagnetic blocks are modified so that their external faces 10 and 11 are inclined in relation to a plane perpendicular to the beam. By modifying the slope of the faces 10 and 11 one may thus modify the length of the path followed by the beam for a given radius curvature, thus providing a further lens effect independent of the former obtained by variation of the airgap width.

The combination of these two means thus makes the rotation of the ferromagnetic blocks result in an adjustment of the lens effect due to the inclination of their terminal faces.

Of course, the invention is not limited to the embodiments described and shown which were given solely by way of example.

Claims

1. A magnetic device for deflecting a beam of electrically charged particles, comprising two pairs of pole pieces defining two successive airgaps for receiving said beam, one airgap being constant and the other variable, means for creating magnetic fields in said airgaps for the propagation of said beam, one of said pairs of pole pieces being pivotally mounted on one of the end faces of the other said pair about an axis perpendicular to said end face, said other airgap being thus selectively variable as a function of the distance of the deflected beam from its center of curvature.