Dipole magnet

A dipole magnet is a magnet with a positive or a negative or North and South Pole ( see magnetism ). Most magnets ( permanent magnets and electromagnets ) are dipole magnets.

The term is mainly used in the field of particle accelerators, which other magnet configurations such as quadrupole magnets may be used. Dipole magnets in accelerators are electromagnets made ​​of a U-shaped iron yoke. The ends of the yoke coils are wound. In the gap between the ends created by the current flow, a variable magnetic field.

Basics

In particle accelerators, dipole magnets are used to generate a magnetic field for beam deflection, it is therefore also called deflection. The field can be homogeneous ( ie spatially constant ) or to focus purposes be inhomogeneous; decisive for the surface of the pole pieces with parallel or non-parallel planes. The particle of the Lorentz force is performed on a path whose curvature is perpendicular to the field. If the field is homogeneous, such as in a classical cyclotron, the path is a circular arc.

In accelerators for high particle energies such as synchrotrons and storage rings not a single large magnet is due to the technical feasibility used, but many smaller magnets, so-called sector magnets. In such systems, there is no circular path, but there is between the magnets straight field-free routes. These have a capacity for acceleration elements, interaction zones in Colliding -beam experiments or for wigglers or undulators to produce synchrotron radiation.

The iron cores of the magnets move in a magnetic flux density of about 2 tesla in saturation. If higher magnetic flux densities are needed, such as reasons of space no larger radius of curvature is possible superconducting magnets must be used without seeds. The current densities in superconducting magnets can reach values ​​of several kA/mm2. Although a part of the conductor cross section of copper ( for stabilization of the superconductor ), and thermal insulation is required, the net current density averaged over the entire cross section of the coil is substantially higher than in the conventional copper windings. The ohmic losses (electricity heat, copper losses) fall to zero.

In superconducting magnets, virtually no set by pole pieces or yoke ends equipotential surfaces form the field. Instead, the superconductor in the coil must be arranged so that the average power distribution is proportional to the cosine of the angle to the beam axis in it.

Relationships

The key for the deflection angle and thus the orbital radius of charged particle beams magnetic flux density in the air gap results in a constant cross section of the magnetic path ( yoke air gap) approximately to:

With   - Magnetic field constant   - Permeability of the yoke   - An electric current through the coil   - Number of coil windings   - Air gap of the yoke   - Iron path of the yoke

It can be seen that

• The yoke must be as compact as possible ( short iron path length )
• The permeability of the yoke material should be as high as possible.

It is also apparent the great influence of the air gap. The air gap can not be made arbitrarily small since it iA must take up the vacuum pipe for the particle beam. The (increasing with the square of the electric current ) heat loss often requires water cooling.

To fill the winding cross-section of the yoke as effectively as possible, so that the iron path can stay as short as possible, copper strips or rectangular conductors are used instead of round wires often.

The deflection angle of a beam of charged particles is proportional to the flux density and the length of the field through which the air - one reason that such magnets often weigh many tons and the air gaps develop huge forces that must be intercepted.